50 years of experimental particle physics in bonn a personal recollection

Eur. Phys. J. H 38, 471–506 (2013)DOI: 10.1140/epjh/e2013-30028-7

Personal recollection

THE EUROPEANPHYSICAL JOURNAL H

50 Years of experimental particle physicsin BonnA personal recollection�

Ewald Paula

Physikalisches Institut der Universitat Bonn, D53012 Bonn, Germany

Received 16 May 2012 / Received in final form 5 June 2013Published online 30 July 2013c© EDP Sciences, Springer-Verlag 2013

Abstract. The first synchrotron for electrons in Europe was built at thePhysikalisches Institut Bonn in the fifties, and two electron acceleratorswere built in the following decades. For fifty years, accelerators in Bonnhave been in use for studying particle physics in scattering experimentswith electron and photon beams, and for research and development ofnew detector components in both accelerator and experimental tech-nology. Also, for fifty years, experimental groups in Bonn have workedon external experiments at accelerators and storage rings in the largeresearch centres CERN and DESY. In this article, the long history ofexperimental particle physics in Bonn and at external accelerators isreviewed. It is shown that the interplay between an institute at a uni-versity and research centres can be very fruitful. Running acceleratorsat the institute supported by well equipped workshops were the basisfor a wide range of technical developments. Most of the work was car-ried out in the hands of students. This was successful and guaranteedoptimal possibilities for their education. The article is based on thepersonal recollection of the author.

1 Introduction

After the second world war, science and in particular the research activities in physicsdeclined in Germany. Many experienced people had left: after 1933, leading Jewishscientists were emigrated to the United States of America, and, after the war, quitea few distinguished scientists in Germany were relocated to the USA and the SovietUnion. This subsequently opened great chances for excellent young scientists to startresearch in new fields at German universities. In experimental physics, such peoplewere Heinz Maier-Leibnitz in Munich, Hans Kopfermann in Heidelberg, WillebaldJentschke in Hamburg and Wolfgang Paul (Fig. 1) in Bonn. First priority was to setup a substantial spectrum of research programs learning from the science activitiesin the USA at this time.

Particle physics at accelerators was one of the prospering new fields of experi-mental physics. Since Rutherford’s famous experiment in 1908, where he detected the

� Dedicated to Wolfgang Paul, on the occasion of his hundredth birthdaya e-mail: [email protected]

http://www.epj.org

http://dx.doi.org/10.1140/epjh/e2013-30028-7

472 The European Physical Journal H

Fig. 1. Wolfgang Paul 1913-1993.

structure of atoms when he scattered α particles off gold foil, it was established thatthe structure of matter can be analysed by scattering particles off it. The resolutionof the Rutherford experiment was limited by the energy of the α particles. It can beimproved by accelerating the particles to be scattered.

First particle accelerators were developed in the 40’s. A significant breakthrough inaccelerator technology was achieved by a discovery made in the early fifties. Physisistsat the Brookhaven National Laboratory (USA) had found a new principle for accel-erating charged particles to high energies [Courant 1952]. They proposed to focusaccelerated particles in a ring structure equipped with small sectors of light mag-nets with special field gradients. With this technology, which provides the so-calledStrong Focussing, particles could be accelerated to higher energies than with anyother accelerator concept. In particular, Strong Focussing was superior to the WeakFocussing technology used previously, which required larger magnets - the larger themagnets the higher the energy, and which was less efficient in focussing beam crosssections. With smaller magnets, both the costs for their production and for runningthem in accelerators could be kept within an acceptable range. In five decades, manygenerations of accelerators based on the concept of Strong Focussing were built andsuccessfully used for scattering experiments. Large accelerators were built which, overthe past fifty years, pushed up the energy available in the scattering process by fourorders of magnitude, presently up to several TeV.

Before Wolfgang Paul accepted a call on a physics chair in Bonn, he worked as post-doc under the guidance of Hans Kopfermann in Gottingen on an electron acceleratorwith Weak Focussing, a 6 MeV Betatron. They studied medical applications. Withthe experience of accelerating electrons and focussing electron beams by means ofmagnetic lenses, Paul was well prepared for his scientific program later on in Bonn.

After Paul had accepted a call on a physics chair in Bonn1, he envisaged to build anelectron synchrotron based on the concept of Strong Focussing. In a letter to WernerHeisenberg who was the head of the Atomkomission at this time, he asked for moneyin order to build a 100MeV synchrotron with the option to increase the energy ata later stage. After Heisenberg and Paul had talked to each other on the phone, adecision was achieved which was somewhat surprising. The committee granted money

1 He was professor of Physics in Bonn from 1952 to 1981

Ewald Paul: 50 Years of Experimental Particle Physics... 473

for building a machine with an end energy of 500MeV. This energy was now highenough that a pion could be produced in the process of scattering an electron offa nucleon. A door was opened for the new field of meson physics. In 1958, after afew very exciting years of pioneering work by a group of physicists, technicians andstudents (Section 2.1), the 500MeV synchrotron successfully accelerated electrons andthe preparation of experiments could begin.

Paul was also interested to try other accelerator concepts. A prototype of a plasmabetatron for accelerating heavy ions was built and came into operation [Drees 1964].However, work in this research field was not continued in Bonn. Heavy ion acceleratorswere built in other places.

Moreover Paul pursued other applications of particle focussing with magnets. Themost famous project was the development of an ion trap, for which he was honouredwith a Nobel Prize in physics in 1989. Another team, which was also set up by Paul,developed and built mass filters which allow to separate particles which differ in massusing the principle that particles of equal energy follow different trajectories throughmagnetic fields. Mass filters built in Bonn were used for measuring particle flows inthe higher atmosphere. This program was also very successful and carried on in Bonnfor several decades.

The 500MeV synchrotron was the first of three electron accelerators which weredesigned and built in Bonn. The second accelerator was an electron synchrotron whichaccelerated electrons up to 2.5GeV and delivered, for the first time in Bonn, externalelectron beams to experiments. It came into use for experiments in 1967. The thirdaccelerator is the Electron Stretcher Accelerator (ELSA), which has been in use since1987. It delivers an almost continuous current of polarised electrons (with nearly 100%duty cycle) to experiments with an electron energy of up to 3.5GeV.

The technology of Strong Focussing was suitable to build larger accelerators forhigh-energy beams. Large accellerators were built in two international research centresin Europe: at the European Organisation for Nuclear Research, CERN, at Geneva inSwitzerland, founded in 1954; and at the Deutsches Elektronen Synchrotron, DESY,in Hamburg, founded in 1959. For each new generation of an accelerator, the beamenergy grew typically by an order of magnitude. The high-energy accelerators weremade available for scattering experiments to be carried out in collaboration withuniversities and other external institutions. The Physics Institut Bonn was one ofthe first external users at CERN and DESY and continues with experiments in bothplaces to date.

The list of highest-energy accelerators at CERN is long. The first machine withStrong Focussing was the proton synchrotron (PS), which delivers protons with anend energy of 28GeV and is, with significant upgrades [Brianti 2004], in use todate2. It came into operation in the fifties at about the same time as the electronsynchrotron in Bonn. The following high-energy machines were the world-wide firstproton-proton collider called Intersecting Storage Ring (ISR), then the Super ProtonSynchrotron (SPS), which was upgraded to the Proton-Antiproton collider, SppS,and the Large Electron-Positron Collider (LEP). The current high-energy machine isthe Large Hadron Collider (LHC), where proton beams of an energies up to 8TeV arescattered against each other. The LHC is at present the collider with the world-widehighest centre-of-mass energy3.

The first accelerator built in Hamburg was an electron synchrotron, which de-livered an electron beam of 5.3GeV to experiments. It was called DESY and this

2 The famous history of the PS has been documented in an article written by G. Plass[Plass 2011].

3 Data taking of experiments began in 2008.


name was also choosen for the whole laboratory4. The accelerator DESY came inoperation in 1963. The following higher-energy machines at DESY were the electron-positron colliders DORIS and PETRA and the world-first electron-proton collidercalled HERA. At HERA, data were taken from 1994 to 2007.

In this article, I consider developments in the field of particle physics based on thehistory of experiments. Preference is given to experiments carried out by groups fromthe Physics Institut Bonn at accelerators in Bonn, CERN and DESY.

I begin in 1956, when both the 500MeV electron synchrotron in Bonn and the28 GeV Proton Synchrotron at CERN were installed, and I cover the following fiftyyears finishing in 2006 i.e. before experiments at the LHC started data acquisition.This time period was full of discoveries and has led to the development of the so-calledStandard Model of particle physics (SM). So far all existing data are consistent withthe SM.

This article is based on the personal perception of the author and does not pretendto deliver a complete picture. For the development of the SM, theoretical milestoneswere as important as experimental results. However, an adequate appreciation of thetheory is not within the scope of this article.

2 The Bonn 500MeV electron synchrotron

2.1 The maschine: from planning to data taking

The first electron accelerators with Strong Focussing were built in the fifties: first the1.1GeV synchrotron at the Laboratory of Nuclear Studies at the University of Cornell(USA) and then the 500MeV synchrotron in Bonn. The accelerator at Cornell wasoriginally designed with Weak Focussing, but then, after the discovery of StrongFocussing, the magnets were modified in order to achieve Strong Focussing. The500MeV accelerator in Bonn was directly designed with Strong Focussing. Bonn couldnot benefit very much from experience made in Cornell.

The 500MeV machine was built with manpower having little or no experience inaccelerator technology, and in the absence of Monte-Carlo techniques to simulate theperformance of the accelerator. This was a great challenge. The steps from the designof the machine to finally achieving a running machine required many single victories!

The exciting history of the 500MeV synchrotron has been documented in an excel-lent book written by R. Burmester in close cooperation with contemporary witnesses[Burmester 2010]. For the basic design of the 500MeV synchrotron, Paul formed afirst group of physisists5. For the long journey from the construction of the vari-ous components to the finally operational accelerator, many young people joined theproject: physisists6, technicians and students7. Money always had to be found, and theproduction of the magnets for the machine, the pre-accelerator and other larger hard-ware components required negotiations with companies and a well-equipped workshopwhich was installed in the institute. The magnets had to be adjusted without laser

4 The history of DESY as laboratory for particle physics has been documented in a bookwritten by E. Lohrmann and P. Soding [Lohrmann 2009].

5 Companions in the design phase were E. Bodenstedt, H. Ehrenberg, O. Gildemeister, O.Osberghaus and the theoretician H. Steinwedel.

6 Two of them, K.-H. Althoff and G. Knop, became later professors for physics in Bonn.7 Students were the manpower for developing and building hardware components and, after

computers had come in use, also for software developments around accelerator technology,data taking of experiments and finally physics analyses on the data.


Fig. 2. The 500 MeV synchrotron in Bonn.

technology, which did not exist, and it was a major puzzle to find the right arrange-ment in which electrons were accelerated as wanted. Finally, in 1958, Bonn had arunning machine. In the next step, experiments were prepared.

A picture of the electron synchrotron is shown in Fig. 2. The accelerator com-prised of nine magnetic sectors and six high frequency resonators which formed aring of 5.30m diameter. The electrons were kept in a circular path by the magneticsectors, while in the resonators, the electrons were accelerated. The magnetic sectorshad strong inhomogeneous field gradients of opposite signs by which particles werealternately focussed and defocussed in a radial direction.

The 500MeV synchrotron was in use for experiments from 1958 to 1984. Stablerunning conditions and long running time were a solid basis for the experimentalprogram. The synchrotron was available for data aquisition by experiments in totalfor about 100 000 hours.

In 1995, parts of the 500MeV synchrotron were moved to the Deutsches Museumin Bonn where it is presently one of the highlights.

2.2 The experimental program

Enrico Fermi and his collaborators had discovered in the early fifties that the crosssection for the scattering of pions on protons rises monotonously with energy up to apion energy of about 300MeV; then it falls monotonously [Anderson 1952]. The bumpin the cross section was interpreted as the occurrence of a resonance excitation of theproton. This resonance was called Δ(1232) resonance. However, the interpretation ofthe bump as a resonance was not clear: It could have been a dynamically generatedstate formed by a nucleon and an orbiting pion, or something more fundamental. Thecorrect interpretation was derived later from considerations of the quark content ofΔ(1232) and SU(3) symmetry: the Δ(1232) is as fundamental as the proton.

The Δ(1232) was now studied in Bonn. At the beginning, two experiments wereset up in order to measure the one-pion production by scattering a photon beam ona liquid hydrogen target i.e. by measuring the reaction γp → nπ+. The photon beamwas obtained by bremsstrahlung radiation produced by electrons on a wire of tungstenplaced in the path of the electrons inside of the synchrotron. It was scattered off atarget placed inside an experimental arrangement. The target was filled with coldhydrogen.

A group had been set up by K.-H. Althoff in order to build cold targets in Bonn.The cold hydrogen target was built first. A cold deuterium target followed. Later on,polarised targets were built which came into use in experiments at the next accel-erators in Bonn (Sections 5 and 10.2) and also in experiments carried out in otherlaboratories (Sections 10.2 and 12).


Experimental arrangements for particle experiments, developed at this time,comprised of magnets to bend charged particle tracks, scintillation counters andCherenkov counters. Scintillation counters served to localise the path of the tracks us-ing the property that charged particles deposited light in plastic materials which wasmeasured in photomultipliers. Cherenkov counters were used to distinguish chargedpions from heavier charged particles. Charged particles emit so-called Cherenkovlight in materials where they travel faster than light does in the same material.Since pions are faster than heavier particles of equal momentum, they were recog-nised by the emission of light while kaons and protons were still too slow to give offlight.

A first experimental set-up8 at the 500MeV synchrotron contained a range tele-scope for measuring the range of the pions in absorber materials, and a second set-up9

contained a magnetic spectrometer. In both set-ups, the signals in scintillation coun-ters were used to trigger on events to be recorded using first generation electronicdevices which were either built in the institute or already bought.

Both experiments measured cross section of π+ production in the Δ(1230) massrange, after several improvements and upgrades of the experimental set-ups, withthe highest precision worldwide. Figure 3 shows, as an example, the measurementobtained with the range telescope.

Further experiments at the 500MeV synchroton measured the photoproductionof π0 i.e. the reaction γp → π0p, and extended the measurements of single-pionphotoproduction by scattering photons off a liquid deuterium target and off solidtargets made of heavy nuclei10.

A theoretical group at the Physics Institut Bonn, led by H. Rollnik, developeda phenomenological description for photoproduction of single pions. It was based onso-called dispersion relations which relate real and imaginary parts of the scatteringamplitude. The data were analysed in terms of multipole amplitudes [Schwela 1967].

Another interesting process which was measured at the 500MeV synchrotron wasthe so-called Compton scattering i.e. the elastic scattering of photons off protons,γp → γp. This process had been discovered by A.H. Compton in 1922. He scat-tered Roentgen radiation off graphite for probing the structure of the surface. Withbremsstahlung photons from 500 MeV electrons, Compton scattering was used to re-solve scattering processes on protons. At higher photon energies obtained at the epcollider HERA (Section 11), Compton scattering was used to resolve the substructureof the proton.

A considerable part of the experimental activities at the 500MeV synchrotronwas devoted to research and development work for new particle detectors. One suchdevelopment was a new sort of tracking detector: the so-called acoustic spark chamber.A spark chamber consists of a bunch of thin metal foils in a gas mixture, connectedto high voltage, so that ionisation deposited by a charged particle along its path inthe gas induces a discharge between the foils, resulting in a spark. The ionisationimpact of a charged particle was read out in two different modes: either by takingpictures on film (optical sparc chambers) or by using the bang accompanying theformation of the sparks for an acoustic position finding. Bonn developed acousticspark chambers and carried out tests at the 500MeV synchrotron. They were used inthe next generation of experiments carried out at the 2.5GeV synchrotron in Bonn(Section 5).

Another interesting project was the building and testing of a small heavy liquidbubble chamber at the 500MeV accelerator in Bonn. The concept of bubble chambers

8 D. Freytag, K. Lubelsmeyer, R. Wedemeyer and students.9 K.-H. Althoff, H.-M. Fischer, W. Paul and students.

10 G. Noldeke, B. Mecking and students.


Fig. 3. Cross section of the photoproduction of π+ on hydrogen in the range of the deltaresonance measured at the 500 MeV synchrotron in Bonn [Freytag 1965].

had been introduced by D. Glaser in 1952. He had shown that tracks of chargedparticles produced in a scattering process can be made visible in a superheated liquidas chains of bubbles. While the bubbles exist for milliseconds, stereo-pictures of thebubble chamber filling can be taken through windows of the chamber and stored onfilm. If bubble chambers are placed in a magnetic field, charged particle tracks arebent according to their momenta.

The small bubble chamber was built in collaboration with the TechnischeHochschule Aachen and successfully tested, without a magnetic field, at the 500MeVsynchrotron. The chamber worked fine: straight tracks of charged particles were vis-ible in the chamber. No further chamber was built in Bonn, since to build a largerchamber with a magnetic field, which could be used for particle scattering experi-ments, was beyond the possibilities at the Physics Institut Bonn. Such were built atlarger laboratories11 and used for experiments at CERN and DESY.

3 The era of bubble chambers

The bubble chamber era at CERN began soon after the foundation of the organisationin the late fifties. For more than two decades, bubble chamber experiments played animportant role for the discovery of new physics. At the CERN-PS, bubble chambers,filled with hydrogen or deuterium, were exposed to hadron beams, and a heavy-liquid(freon) bubble chamber, called Gargamelle, was exposed to νµ/νµ beams12. Later on,

11 Bubble chambers were built in Saclay in France and at CERN.12 Before Gargamelle, a spark chamber and a 1.2m heavy liquid bubble chamber (NPARamm chamber) were already exposed to νμ/νμ beams at CERN.


at the SPS, the Big European Bubble Chamber (BEBC) was exposed to both hadronand νµ/νµ beams.

In bubble chamber experiments, pictures were taken on film in three projectionsfrom each burst of beam particles passing the chamber. So bubble chamber experi-ments could give a complete overview on the scattering processes. At this time, theywere the ideal detector for exploring physics in a new range of high energy.

The films were scanned by eye on scanning tables where interesting events weremarked. Such events were measured on special measuring devices. Scanning and mea-suring of the events on the films was time consuming. Between five and ten eventsper hour were scanned and about the same time was needed to measure their tracks.Later on when half-automic measuring devices had been developed the rate of mea-sured events was somewhat increased.

Collaborations were founded where films were shared and then scanned and mea-sured in the home laboratories. The measurements were stored, and local computerswere used to reconstruct the events in space and to identify scattering processes bytheir kinematics. The measured and reconstructed events were combined in one sam-ple, which then was shared with respect to local physics analyses. Finally, the physicsresults were published by the whole collaboration.

At the beginning, the use of computers was a major challenge. For many years,very restricted space for the storage of data and the slow speed of the computers forreconstruction and analyses of the events, were serious limitations for volume andtime consumption of the experiments. However, computers improved continuously,and bubble chamber experiments achieved basic progress in several fields of software,in particular in storage and handling of large amounts of data, Monte-Carlo simu-lation techniques and the application of statistical tools in data analyses. Softwaredevelopments achieved in bubble chamber experiments played the role of pioneeringwork of software developments for multipurpose experiments of later generations.

The Physics Institut Bonn was one of the first institutions that joined a bubblechamber experiment at CERN13. An 81cm bubble chamber built in Saclay was filledwith liquid hydrogen and exposed to a beam of 10GeV π− which was produced withprotons from the PS. The physics aim was to investigate the production of strangeparticles.

W. Paul set up a group in Bonn who built a scanning table and installed a firstmachine for the measurement of the events on film. A Ph.D. student14 built parts ofthe scanning table, using the technical infrastructure of the workshop in the PhysicsInstitut Bonn. Then he developed software packages for the reconstruction of theevents in topology and kinematics on a tube computer which was available at theInstitut of Applied Mathematics in Bonn15. One of the events which he analysedis shown in Fig. 4. Besides tracks of non-interacting pions, an event is seen withthe typical topology: there is a primary interaction point of a beam particle witha proton yielding charged tracks and, in some distance to the primary interactionpoint, the decay point of a neutral kaon, K0, with two charged particles from thedecay into π+π−. The neutral kaon itself is not visible because only charged particlesproduce a visible track. In this event, tracks of a charged kaon, K+, and a decay pion,π+, are visible, too. All tracks are curved because they are bent in a homogeneousmagnetic field covering the sensitive volume of the chamber. Particle momenta weredetermined via the measurement of the tracks. The total kinematics of events was

13 The experiment was carried out by a group at CERN led by Ch. Peyrou. Physicists andstudents from Bonn and other places joined the group.14 This student was Siegmund Brandt, who was later professor for physics in Siegen.15 This computer was a Standard-Elektric ER 56 which was, as all computers of this gen-eration, very slow and had very little storage space.


Fig. 4. Event with kaon pair production seen in the 81cm hydrogen bubble chamber[Brandt 1963].

probed by testing energy and momentum conservation. For the event shown in Fig. 4it was concluded that invisible π0’s were produced in addition. This first analysiscontained about hundred events with K0 decay [Brandt 1963].

Strange particles, which had been discovered and studied in emulsion experiments,became a domain of bubble chamber experiments. Most famous was the discovery ofa strangeness-3 particle, called Ω−, in an experiment carried out at the 80-in. bubblechamber at the Brookhaven AGS (USA) [Barnes 1964]. A similar event which wasseen in the CERN-2m chamber is shown in Fig. 5. An Ω− is produced where all stepsof the decay chain are visible and a full reconstruction was obtained. A negativelycharged kaon is scattered on a proton. An Ω− is produced together with two K+ sothat the Ω− must have strangeness -3. The Ω− decays to a pair of of particles withstrangness -1 (Λ and K−), followed by the decay of the Λ into proton and π−.


Fig. 5. Event of the CERN-experiment WA21 (taken from CERN Archive, Courtesy ofCERN).

Bubble chambers were suitable to measure the decay length of strange particlesin order to determine their lifetime. A precision measurement of the lifetime for K0

s

decays was obtained in an experiment scattering a K+ beam from the PS at theCERN-2m chamber [E. Paul 1976]. This was interesting because the previously dis-covered CP violation in K0 decays was still a puzzle and the lifetime acted as one ofthe parameters.

The bubble chamber group in Bonn16 grew fast. More scanning tables and mea-suring machines were installed. As a member in several international collaborations,Bonn carried out a long sequence of experiments at bubble chambers which were ex-posed to hadron beams from the PS and later to hadron and νµ/νµ beams from theSPS.

The central issue of the experiments scattering hadron beams in bubble chambers,was to study the dynamics of strong interactions, in particular, to reconstruct unstable(resonant) mesonic and baryonic states from a measurement of multiparticle finalstates. Such resonances decay, by strong interaction, immediately at the productionpoint, i.e their decay point coincides with the event vertex. They were made visible aspeaks in mass distribution calculated from combined four-momenta of the producedparticles.

The search for new resonances marked a a very exciting time-period. Within afew years, a whole zoo of unstable hadronic states was discovered and analysed withrespect to spin and parity. In quite a few theses written in Bonn, it is documentedthat Bonn contributed to the long list of discoveries. I remember e.g. the discussionsabout the so-called A1/A2 meson puzzle. The states A1/A2, today called a1(1260)and a2(1320), were excited mesonic states which overlapped in the ρπ mass spectrum.

16 K. Boeckmann, B. Nellen, technicians and students.


They were recognised as two states by studying the decay angular distributions whichdiffer according to different spin-parity of the states (JP = 1+ and 2+).

At the end of the sixties, the zoo of excited resonances contained more than 100states. The large ensemble of states filled multiplets derived from considerations ofSU(3) group symmetry17 [Gell-Mann 1964a]. Each hadronic state could be assigned,on a one-by-one basis, a predicted state in a multiplet predicted by symmetry groupSU(3); and also vice versa: all states predicted in the multiplets were afterwards dis-covered in experiments. The most famous discovery was the Ω− which was predictedas a state in the spin-3/2 decuplet representation of SU(3) before a careful search wascarried out where it was detected (see above).

M. Gell-Mann [Gell-Mann 1964b] and G. Zweig [Zweig 1964] recognised in 1964that all states filling the multiplets can be built up by three substates which werecalled quarks. A baryon contains three “valence” quarks (u, d, s) and a meson aquark-antiquark pair. The existence of substates in the proton was discovered in asequence of experiments, which were carried out at SLAC in the late sixties: it wasshown that, in deep inelastic scattering, electrons are not scattered off the protonas a whole, but off point-like constituents, called partons, contained in the proton[Breidenbach 1969]. Moreover it was discovered that the proton participates in thescattering process not only through the quark content predicted by SU(3) (called“valence quarks”), but also through other components, later identified with gluonsand “sea quarks”.

“The most all-time greatest impact on the field of elementary particles” (J.Steinberger in [Tsemelis 2011]) was the discovery of Neutral Currents in νµ and νµ

scattering in experiments at the Gargamelle heavy liquid bubble chamber. A firstNeutral Current event was discovered in 1972, about at Chrismas time, a second onea year later. The discovery of these events was the cornerstone for the unification ofelectromagnetic and weak interactions to electroweak interactions.

Later on, Neutral Current events were studied in scattering processes on hydrogen,exposing BEBC to high intensity νµ/νµ beams from the SPS. BEBC was made upby an external detector called External Picket Fance, and an external muon detector.Neutral Current (NC) and Charged Current (CC) were analysed on an event-by-eventbasis. One of the contributions made by Bonn was the development of a method toimprove the separation of NC and CC events.

When the SppS collider came into use, a streamer chamber18 was installed inorder to measure the flow of charged particles produced in pp collisions. Pictureswere scanned and measured in Bonn on the devices used before for bubble chamberexperiments. The knowledge of the particle flow was used to prepare the experimentsUA1 and UA2, which were designed to search for the heavy vector bosons W andZ. The discovery of both particles in pp collisions by both experiments is one of thehighlights in the history of CERN. It was honoured by the first Noble Prize for CERN.

Bubble chamber experiments lost their importance when technical developmentshad reached a level that multipurpose detectors with large spatial acceptance couldbe built. Unlike bubble chambers, new detector components could be triggered oninteresting events and read out electronically. This opened the door to experiments

17 SU(3) symmetry is based on the assumption of invariance of strong interactions withrespect to electric charge and strangeness. Each multiplet contains states of the same spinand parity, but differs in isospin or strangeness.18 Streamer chambers consist of metal plates placed in a sealed box filled with gas. Chargedparticles which travel through the chamber, ionise the gas and can, in case of an interestingevent, trigger the high voltage on the plates so that sparks are caused along the track images.Pictures were taken and recorded on film.


which can accumulate high statistics samples of events and so also study scatteringprocesses with low cross sections.

4 Start of particle physics at DESY

The first accelerator which was built at DESY was a 6GeV electron synchrotron calledDeutsches Electronen-Synchrotron DESY19. Electrons were delivered to experimentssince 1963. Photon beams were made using the bremsstrahlung of the electrons inthin materials.

A broad experimental program was started. German universities and other insti-tutions were invited to carry out experiments at the electron synchrotron. The firstexperimental groups who came to DESY, were from Hamburg, Karlsruhe and Bonn.

A group from Bonn20 exported an experiment for single pion photoproduction tothe 6GeV machine DESY in order to extend measurements made in Bonn to higherenergies. In this experiment, π0 mesons were measured via the decay π0 → γγ. Thephotons were detected by means of two total absorbing Cerenkov counters built inBonn. Another experiment which was carried out in collaboration with the universitiesof Pisa and Rome, measured the photoproduction of η mesons using the Primakoffeffect where η mesons were produced through the reverse process of the decay intotwo photons. One of the main results was the precise measurement of the lifetime ofthe η meson.

In the middle of the sixties, the 85cm bubble chamber was installed at DESY.It was built at Saclay (France) and was a copy of the 81cm hydrogen bubble cham-ber used at CERN. The chamber was filled with hydrogen and later with deuteriumand exposed to a photon beam obtained as bremsstrahlung spectrum of the electronbeam. Several experiments were carried out in collaborations between DESY andbubble chamber groups at German universities including Bonn. It was beneficial thatscanning tables and measuring machines were already available from bubble chamberexperiments carried out at CERN. In total about 1.7 million pictures were taken. Thephysics results covered an overview on photoproduction reactions contributing in thisenergy range, and detailed measurements of cross sections of the photoproduction ofthe light vector mesons ρ, ω and φ. The studies of vector mesons led to the devel-opment of the Vector Meson Dominance Model. It assumes that, in the scatteringprocess, the initial state photon behaves like a superposition of vector meson states21.Moreover, the total cross section of the photoproduction of hadrons on hydrogen wasmeasured, using a special method to tag the photon energy: the electrons in a burstwere distributed horizontally so that a photon interacting in the chamber could berelated to an electron, measured in a tagging system.

5 A 2.5 GeV electron synchrotron for Bonn

The second accelerator which was built in Bonn, was the 2.5GeV electron syn-chrotron22. The machine was planned in 1963 and was ready for experiments in 1968.A picture of the 2.5 GeV synchrotron is shown in Fig. 6. The electrons were accel-erated in a ring structure of 22.20m diameter comprising twelve combined-functionbending magnets and sextupole components for chromatic corrections in focussing and

19 A nice overview on the history of particle physics at DESY is given in [Lohrmann 2009].20 D. Husmann, K. Lubelsmeyer and D. Schmitz and students.21 A nice review is given in [Schildknecht 2006].22 K.-H. Althoff, G. Knop, W. Paul, staff members, technicians and students.


Fig. 6. The 2.5 GeV electron synchrotron in Bonn.

defocussing sectors. Experiments were carried out with electron and photon beams.The latter were obtained by bremsstrahlung of the electrons in matter.

Experiments were designed to study the intermediate energy range between500MeV synchrotron in Bonn and the 6 GeV synchrotron at DESY i.e. the massrange of the so-called second nucleon resonance. A central issue of the experimen-tal program was to measure the production cross sections of single pions with bothelectron and photon beams at proton and deuterium targets. Additional detectorswere built and used for measuring the polarisation of recoiling protons and neutrons[Althoff 1969]. Photoproduction experiments showed that, in the mass range abovethe Δ(1232) resonance, several other resonances start to contribute in addition to thesecond resonance, and that a separation of the single contributions was with littlesuccess. The idea was born to reduce the number of contributing resonance states byrestricting the spin configuration of the initial state.

In the next step, experiments were carried out with an unpolarised photon beam,produced by scattering electrons extracted from the 2.5GeV synchrotron and po-larised solid state targets of protons and deuterons. Interesting results were obtained,but the data were still not sufficient to resolve the resonance structure in the consid-ered mass region [Althoff 1977].

Later on, in the eighties, spin dependent form factors of proton and deuteron weremeasured by scattering electrons on polarised NH3 and ND3 targets developed andbuilt in Bonn23. An excellent result was the simultaneous measurement of the threeformfactors of the deuteron obtained in an experimental arrangement, where the ND3

target was kept in in a transverse magnetic field of a superconducting Helmholtz coilof 3.5Tesla [Boden 1991].

Since 1987, the 2.5GeV synchrotron served as pre-accelerator and injector forELSA.

6 Gluon jets at the e+e−-storage ring PETRA

The first e+e− collider at DESY was DORIS which was designed for beam energiesup to 3 GeV. Experiments at DORIS began with data taking in the seventies. Theywere carried out to test the validity of Quantenelektrodynamics at high energies and

23 K.-H. Althoff, W. Meyer and students.


Fig. 7. The two-arm spectrometer solenoid TASSO..

to study e+e− annihilation into hadronic final states. After the discoveries of J/Ψand Υ , experiments at DORIS explored the mass spectrum of charmonium, and afterupgrade of the beam energies, the mass spectrum of bottonium [Lohrmann 2009].

Bonn24 and a group of Mainz built a detector, called BONANZA, which wasdesigned to measure the production of nucleon-antinucleon pairs produced in thereaction e+e− → nn, pp. However, the event rates at DORIS were very low. After thediscovery of the J/Ψ , the detector was used to measure nn and pp production ratesin the mass range of this resonance.

The second e+e− collider at DESY was PETRA, built for beam energies of up to23.4 GeV. Four multipurpose detectors, CELLO, MARKJ, JADE and TASSO, werebuilt by international collaborations for the four interaction zones25. Data were takenat PETRA in the late seventies and in the eighties.

Bonn26 was a member of the TASSO collaboration. A schematic drawing of theTASSO detector is shown in Fig. 7. It consisted of three major components: theinner detector, the outer detector and the forward detector. A large central driftchamber was the heart of the inner detector. A liquid argon calorimeter was thecentral component in the outer detector. The forward detector served to measurecharged particles scattered at small angles, in particular scattered beam particles, e±,

24 H.J. Besch, G. Noldeke, M. Tonutti and students.25 MARKJ, JADE and TASSO were installed at the beginning, together with PLUTOwhich had been used at DORIS before. Later PLUTO was replaced by CELLO.26 H.M. Fischer, E. Hilger, G. Knop, H. Kolanoski, R. Wedemeyer and students.


Fig. 8. TASSO events: with three jets (above) and two jets (below).

which gave access to γγ reactions (see below). The Bonn group made considerablecontributions to building and running of forward detector, central drift chamber,magnetic coil and luminosity monitor.

In the new high energy region, which became accessible at PETRA, exciting newphysics was observed straight away by looking at the event topology of hadronic finalstates. Events were seen where hadrons were produced in bundles called jets. Manyevents had two jets. The jets showed the hadronisation of a quark-antiquark pairproduced in the annihilation process e+e− → qq. An example of a two-jet event seenin the TASSO experiment27 is shown in the lower part of Fig. 8.

The experiments at PETRA made another, even more surprising, discovery: eventswere seen with three bundles of particles indicating three jets in the final state. Inthe framework of QCD, a hard gluon can be emitted as bremsstrahlung in the strongfield of the qq pair and hadronise into a third jet. An example of a three-jet eventis shown in the upper part of Fig. 8. Each of the three jets has four or five chargedparticles and, with some probability, also neutral particles (not visible).

Detailed studies of three-jet events established the interpretation to be the hadro-nisation of a qqg intermediate state [Soeding 2010]. This was the discovery of thegluon and a major breakthrough of the Quantenchromodynamics (QCD) as micro-scopic theory of strong interactions and thus of the SM.

Data taken at PETRA were a rich testing ground for various other predictionsof QCD. Bonn studied photon-photon scattering based on events of the reactione+e− → e+e− + hadrons where a scattered beam particle, e+ or e−, was measured

27 I would like to thank Guenter Wolf, DESY, for allowing to me to use the figure from hisprivate file.


in the forward detector [Kolanoski 1984]. Evidence was seen that, in deep inelasticscattering, the photon interacts via two components: as a point-like (electromagnetic)particle and as a hadronic state of large mass. Such photon interactions were foundto be well described by predictions obtained in the framework of the Standard Model.

7 Deep inelastic photoproduction at the OMEGA spectrometer

CERN’s OMEGA spectrometer was the first set-up that combined large spatial ac-ceptance of a bubble chamber with the possibility to trigger on interesting events.The set-up comprised, inside of a big magnet, a target which was filled with liq-uid hydrogen or deuterium, surrounded, at the beginning, by optical spark chambersread out with Plumbicon cameras28, which were replaced by drift chambers, whenthis technique was available. Digitised pictures were analysed on computers relyingon software which had been developed for bubble chamber experiments.

In experiments organised in collaborations, the OMEGA spectrometer was exposedto beams from the PS and later from the SPS throughout a time period of 25 yearsfrom 1971 to 1996.

At the PS, a group in Bonn29 carried out an experiment where the OMEGAspectrometer was exposed to π− beams. The aim was to search for the production ofresonant hadronic states with low cross section, in particular for so-called baryonium(pp and pn) states. Such states were predicted from confining gauge theories and inquark models (see e.g. [Maharana 1979]). No such states were seen in the experiment,and also not in later experiments.

In another experiment, an electron beam from the PS was used to study the pho-toproduction of vector mesons by scattering linearly polarised photons on hydrogen.The polarised photons were produced by bremsstrahlung of the electrons on a crystal.The energy of the photons was determined in a tagging system. The Bonn group30

built large counters filled with liquid scintillator which were needed to veto on theproduction of e+e− pairs.

Bonn continued with an experiment using beams from the SPS. New downstreamdetectors were added to the OMEGA set-up. Two detectors, for identifying pions,kaons and protons, were built by the OMEGA-Photon collaboration: a big ring-imaging Cherenkov counter31 and a transition radiation detector32 which coveredthe identification of charged hadrons with large momenta.

The OMEGA-Photon collaboration measured, covering the same beam energyrange, photon-, K±- and π±-induced inclusive one-pion cross sections as a function ofthe pion transverse momentum, pT [Apsimon 1989]. A hadron-induced cross sectionwas computed by combining pion- and kaon-induced cross sections according to theassumptions of the Vector Meson Dominance model [Schildknecht 2006], so that itrepresented the same quark content as the superposition of the light neutral vectormesons ρ, ω and φ. The ratio of photon- to hadron-induced cross sections is shown

28 This type of camera read out an event on a screen in 20 msec.29 E. Paul and students as member of an international collaboration.30 B. Diekmann, K. Heinloth, E. Paul and students, as member of the OMEGA-Photoncollaboration.31 This detector was built at the Rutherford lab under the guidance of P. Sharp. It was thefirst large ring-imaging Cherenkov counter for identifying pions, kaons and protons used inan experiment.32 Transition radiation is produced when charged particles pass through a sandwich ofmedia of different dielectric constants. The transition radiation detector was developed andbuilt in Bonn.


Fig. 9. Ratio of cross sections for photon- and hadron-induced inclusive one-pion production.It was normalised to one at small tranverse momentum pT .

in Fig. 9. It rises with increasing transverse momentum. This was the first directobservation that a photon beam interacts differently from hadron beams, and showedthat contributions from the direct (bare) photon-induced component becomes moreimportant if inelasticity (pT ) is increased. Moreover this result indicated that deepinelastic scattering (DIS) is the domain to study direct photon scattering processes.This was entirely used at HERA (Section 11).

8 The triumph of the Standard Model at the e+e− collider LEP

The next higher-energy e+e− collider in Europe was LEP built at CERN. Initially theenergy of the beams was chosen such that the centre-of-mass energy of collision wasequal to the mass of the Z0 boson e.g. at 91 GeV. LEP was operated as Z0 factory.Later the energy was increased to approximately 200 GeV.

Four collaborations built four multipurpose detectors called ALEPH, DELPHI,L3 and OPAL, which were placed in the four interaction zones. A large group in thePhysics Institut Bonn participated in the OPAL collaboration33. The OPAL detectoris shown in Fig. 10. The inner part comprised, in a magnetic field of a solenoid, track-ing detectors with the central Jet chamber. The outer part was instrumented withelectromagnetic and hadronic calorimeters. A forward part contained a detector formeasuring the absolute luminosity. Bonn contributed to the building of several detec-tor components, so to the central Jet chamber and the silicium-tungsten calorimeterof the forward set-up.

Experiments collected data between 1989 and 2000. They were optimised to mea-sure charged and neutral particles from decays of Z0’s i.e. all particles except neutri-nos. Data were used to test a broad spectrum of SM predictions and to determine free33 H.M. Fischer, P. Fischer, G. Knop, M. Kobel, B. Nellen, A. Stahl, N. Wermes, techniciansand students.


Fig. 10. The OPAL detector. It was a cylinder of 12 m diameter and 12 m length.

parameters in this model, in both the electroweak and the strong interaction sector.It was verified that SM predictions are valid on a high level of precision and in alldetails at LEP energies. The SM was definitively established by LEP experiments!

The number of neutrino generations is not predicted by the SM. An early high-light was the precise measurement of the total decay width of the Z0 which allowed toconclude on the number of light neutrino generations. The total width is constrainedby the sum of partial decay widths of all possible decay modes. Large contributionswere expected from Z0 decays into light neutrino pairs, Z0 → νν which escaped frommeasurement. However, since the total width is constrained by all decay modes to-gether, the measurement of the total decay width can tell how many light neutrinogenerations contribute to the Z0 decay34. A precise determination of the total widthwas achieved by combining all available measurements, those of the four LEP ex-periments and a corresponding measurement, obtained with the SLD detector at thee+e− collider SLC at SLAC (USA) [ALEPH 2006]. The resulting Z0 peak, compiledby the DELPHI collaboration, is shown in Fig. 11. The width of the Z0 peak is bestdescribed with three generations of light neutrinos. This measurement established animportant cornerstone of the SM: the number of lepton generations to be taken intoaccount in the SM is three.

The heaviest charged lepton, τ±, had been discovered at SLAC [Perl 1975]. AtLEP it was studied with about 200 decays Z0 → τ+τ− per experiment. A very rareevent of a Z0 decay into τ+τ−γ was observed in the L3 experiment (Fig. 12). Suchevents deliver information about the electromagnetic dipole moments of the τ .

34 Heavy neutrinos cannot be excluded because they could contribute with small partialwidths.


Fig. 11. Cross section as a function of the energy in comparison to SM predictions for 2, 3and 4 neutrino generations (taken from the DELPHI homepage at CERN).

Fig. 12. An event of the reaction e+e− → τ τγ obsereved in the L3 experiment (takenfrom [Stahl 1999]).


0

1

2

3

4

5

6

10030 300

mH [GeV]

Δχ2

Excluded

Δαhad =Δα(5)

0.02758±0.00035

0.02749±0.00012

incl. low Q2 data

Theory uncertainty

Fig. 13. Prediction of the h mass (taken from LEP Electroweak Working Group Homepage,status July 2006).

The Bonn group studied τ± decays in the OPAL experiment [Stahl 1999]. It wasshown that the weak coupling strength in leptonic τ decays is consistent with that inthe decay of muons which implied that the leptonic coupling strength is universal asit is predicted in the SM. The system of hadrons in τ decays is attractive as testingground for QCD on lattice (see also Section 10), since it represents an isolated systemin a soft kinematic range35. At LEP, such studies were still limited by large statisticaluncertainties.

Another triumph of LEP experiments and the SM was the correct prediction ofthe mass of the top quark, t, which was not observed directly in production at LEPenergies. An estimate of the t mass was derived, within the framework of the SM,from precise measurements of electroweak processes at LEP experiments. The 1-loopcorrections depend on several known SM parameters and the “unknown” t mass. Theprediction pointed to a mass value in a range around 170 GeV, and the t with a mass inthis range was discovered at the proton-proton collider TEVATRON at FERMILAB(USA) in the nineties.

From sixteen elementary particles36 building up the SM up to 2006, seven werepredicted before they were observed. Another one, the so-called Higgs boson, wasalready introduced in 1964, in context with spontaneous symmetry breaking and thecreation of the mass of gauge bosons [Higgs 1964,Englert 1964]. It was supposed tobe a fundamental ingredient of the SM. However, for a long time, no candidate wasseen in scattering experiments, and it was an open question whether it exists as a freeparticle.

Experiments at LEP were devoted to search for it, but the energy was too low tosee it in production. However, an estimate of the Higgs particle mass was obtainedindirectly, again from precision measurements of electroweak processes at LEP. An

35 The strong coupling strength is constrained by the hadronic mass.36 As such are counted three doublets of quarks and three doublets of leptons, gauge bosonsmoderating weak, electromagnetic and strong interactions: γ, W±, Z0 and the gluon.


example of such an estimate is shown in Fig. 13. The minimum of the Δχ2 distributionpoints to a Higgs particle mass around 100GeV. Finally, via a more comprehensiveanalysis, a lower limit was obtained: the mass of the Higgs particle should be largerthan 114 GeV. Upper limits were also derived, from experiments at LEP, SLC andTEVATRON and theoretical considerations, so that a small mass window between114 and 150GeV was finally left to search for the Higgs particle.

To see the Higgs particle in production, required an accelerator at higher energy.This was realised with the Large Hadron Collider, LHC, at CERN. It was completedin 2006 and started with data acquisition in 2007. After the time period covered bythis article, a resonance, consistent with the Higgs particle and having a mass in thepredicted mass range, was discovered in LHC data (see Section 13.2). This discoveryis a great triumph of LHC experiments, but also of LEP experiments.

9 The lifetime of the neutron: an experiment at Grenoble

A precise knowledge of the neutron lifetime is relevant in astrophysics as well as inparticle physics. In astrophysics, it governs the almost equilibrium of protons andneutrons in the primordial phase of nucleosynthesis. In particle physics, it constrainsthe first element of the CKM quark mixing matrix describing the transition from dto u quarks.

In the late eighties, W. Paul and a small team from Bonn37 carried out a precisionmeasurement of the lifetime of free propagating neutrons. The experiment was carriedout at the Laue-Langevin Institute in Grenoble (France). Cold neutrons were storedin a magnetic bottle [W. Paul 1989]. The lifetime was extracted by counting survivalneutrons after a well defined holding time. The result of the experiment was themost precise measurement of the lifetime of the neutron at this time. The neutrondecay is, also beyond the lifetime measurement, in the focus of current experiments[S. Paul 2009].

10 Non-perturbative QCD: a challenge for both theoryand experiment

Potentials simulating QCD were successfully applied to calculate mass spectra ofhadrons made of the heavy quarks c and b. The total mass of such hadronic statesis mainly due to the rest mass of the heavy quarks, while binding energy is small.Normal hadronic matter, consisting of nucleons, nuclei and light mesons, is quitedifferent. Such states contain light quarks, and the hadronic masses are mainly thebinding energy of fluctuating gluon fields created by coloured quarks. For instance, incase of the proton, which contains the light quarks u u d, the binding energy accountsfor 98% of the proton mass. In the framework of QCD, large binding energies implylong-range hadronic interactions and large coupling strengths so that perturbativeQCD is not applicable.

QCD inspired models were developed which described special features of hadronicstates made of light quarks and gluons. The so-called Bonn model which was developedby a group38 at the Helmholtz Institut fuer Strahlen- und Kernphysik (HISKP) inBonn. Baryon masses were treated within a relativistic covariant quark model basedon the three-fermion Bethe-Salpeter equation with instantaneous two- and three-body

37 F. Anton, L. Paul, S. Paul, W. Paul and W. Mampe.38 B.C. Metsch, H.R. Petry and students.


Fig. 14. Calculated spin-parity states of Δ-resonances (left part of the columns) in compar-ison to the experimetental spectrum. The experimental resonance positions are indicated bya bar, the corresponding uncertainty by the shaded box.

forces [Loring 2001]. Predicted mass spectra of excited nucleon states are in reasonableagreement with measured states. An example is shown in Fig. 14.

Using a more general ansatz, calculations of non-perturbative strong interactionswere based on so-called Lattice QCD. In this approach, the continuous space-timedependence of perturbative QCD is replaced by discrete values located on a gridor lattice. The choice of the lattice spacing allows to regularise the theory (see e.g.[Gattringer 2010]). First applications are encouraging. A prediction of the mass spec-trum of excited nucleons has been obtained in this framework [Edwards 2011].

Another promising theory describes long-range hadronic interactions on the ba-sis of Chiral QCD dynamics. It takes into account that the degrees of freedom areno longer those of quarks and gluons, but those of hadrons as confined particles[Leutwyler 1994].

A new generation of accelerators and experiments is in progress where electronsof energies in the range of a few GeV are scattered off nuclear matter. The goal is toresolve the production of nucleon excitations in the mass range above the Δ(1232)resonance. Since many broad excited nucleon states contribute in the relatively narrowmass range between one and two GeV, there are strong overlaps between such states,and interference effects have to be taken into account.

The desintegration into excited nucleon states is tried applying the techniques ofpartial wave analysis to the data. From earlier experiments carried out at the 2.5 GeVsynchrotron (Section 5), it was already known that such analyses suffer from the largenumber of amplitudes to be determined. A reduction can be obtained by scatteringinitial particles in well defined spin states i.e. polarised beams on polarised targets,and using different combinations of spin orientations.

A new generation of accelerators and experiments are dedicated to make use ofpolarisation of both beams and targets. In Bonn, the stretcher ring, ELSA, was built


Tran

sfor

mat

oren

,Tr

ansf

orm

ator

en,

10 k

V10

kV

NHV1NHV1

Filte

rFi

lter

Mag

nets

trom

ver-

Mag

nets

trom

ver-

Syn

chro

tron

Syn

chro

tron

sorg

ung

sorg

ung

Traf

oTr

afo

M

T

Crystal Barrel

Tagger

M Q BPM

DORIS-Resonator

PETRA-Resonator

Extraktionssepta

Labor des FZK

supraleitendesSolenoid

Sprung-Quadrupol

Sprung-Quadrupol

Skew-Quadrupole

Messplatz zumDetektor-Test

(26 MeV)

LINAC 1

DESY-Resonator

(20 MeV)

Elektronen-kanone

Injektionssepta

BN3 BN2

BN1

BN0

KA1

KA2

LINAC 2

0,5 – 1,6 GeV

Booster-Synchrotron

0,5 - 3,5 GeV Synchrotronlicht-Experimente

Stretcherring

Quadrupol

Combined-Function-Magnet

Dipol (horizontal)Dipol (vertikal)

HochfrequenzSolenoid

SextupolSkew-Quadrupol

Halbzelle des Stretcherrings

pol. eQuelle

(50 keV)

-

Mott-Polarimeter

Elektronen-kanone

Hadronenphysik-Experimente

EKS

0 m 5 m 10 m 15 m

Elektronen-Stretcher-Anlage (ELSA)

Compton-Polarimeter

Tagger(im Aufbau)

PolarisiertesTarget

CB-Detektor

Flugzeit-wände

Fig. 15. The Electron Stretcher Accelerator, ELSA, in Bonn.

which delivers a continuous beam of polarised electrons into the experimental setup,called CB-ELSA/TAPS, where it is scattered on polarised targets.

10.1 The electron stretcher accelerator, ELSA, for Bonn

The Electron Stretcher Accelerator, called ELSA, was built for an electron energy ofup to 3.5 GeV39 and is in use for experiments since 1987. A sketch of the ELSA facilityand the environment with the 2.5 GeV synchrotron as injector (Booster synchtrotron)and the setup of the experiments, as it was in 2006, is shown in Fig. 15. ELSA is aseperated function machine used to stretch the short electron bunches delivered fromthe Booster synchrotron (with a duty factor of 5%) to an electron beam of constantintensity [Husmann 1988,Hillert 2006] at a slightly higher energy. The stretching ofthe electron beam allowed to reduce the dead time effects in the readout electronicsof the experiments, so that higher event rates could be taken. In the late nineties asource of polarised electrons was implemented and ELSA was upgraded so that thepolarisation of the electrons was conserved when being transferred from the sourcevia pre-accelerators, the booster synchrotron and ELSA to the experiments.

The development of sources for polarised electrons has a long tradition in Bonn.The first polarised electrons were produced already in 1969 by W. Paul and studentswho used the photoionisation of polarised Lithium atoms [Baum 1969]. About thirtyyears later, a source of polarised electrons had been developed in Bonn which camein use for experiments at ELSA: spin-polarised electrons were produced using thephotoemission of a GaAS-like crystal pumped with circularly polarised laser light[Hillert 2006]. It was prevented that the electrons lost their polarisation when pass-ing the 2.5GeV Synchrotron and ELSA. For being transferred through the circularaccelerators, the longitudinal polarisation of the electrons was rotated in order topoint perpendicular to the acceleration plane. After beam extraction out of ELSA,

39 K.-H. Althoff, D. Husmann, H. Hillert, technicians and students.


Fig. 16. Measurement of polarisation of the electron beam extracted from ELSA.

a solenoid rotated the spin back into the accelerator plane. The extracted polarisedelectron beam was first used for the GDH experiment (see Section 8.2). The degree ofpolarisation of the electron beam, as measured in this experiment, is shown in Fig. 16.

10.2 The experimental program at ELSA

In the first decade of ELSA running, several experiments were carried out using un-polarised electrons and unpolarised targets. With these experiments, the former ex-perimental program (see Section 5) was improved and extended [Hillert 2006]. A newdetector, SAPHIR, was built for scattering a tagged photon beam on a hydrogen tar-get40. The photons were produced by bremsstrahlung of beam electrons on a radiatortarget, and their energy was determined in a tagging system by measuring the en-ergy of the electron which had emitted the photon. The SAPHIR detector (Fig. 17)comprised, in a magnetic dipole field, a liquid hydrogen target and a drift chambersystem around the target for measuring charged tracks, followed by an electromag-netic calorimeter. In total about 200 millions of events were taken with photons in theenergy range up to 2.6GeV. The data were analysed with respect to the formation ofbaryon resonances which decay into strange particle pairs or a nucleon accompaniedby a light vector meson i.e. ρ, ω and φ. Baryon resonances coupling preferentially tosuch final states were predicted by model calculations. New resonances were seen inKΛ and KΣ final states which were consistent with predictions obtained from isobarmodel calculations [Glander 2004].

The experiment at SAPHIR was also used to search for new sorts of resonances.A candidate of a new state was seen as peak in the nK+ invariant mass distributionat a mass value of 1540MeV and with a width corresponding to the experimentalresolution, which was produced in the reaction γp → nK0

sK+ [Barth 2003]. In quarkmodels such a peak is a candidate for a resonance with a minimal quark contentuudds i.e. with a pentaquark state. The existence of pentaquark states is possible inthe framework of the SM. The production of this pentaquark candidate called Θ+

was also reported from the CLAS Collaboration at JLAB (USA) [Stepanyan 2003]and several other experiments. However, neither the Θ+ nor any other pentaquark40 K. Heinloth, F. Klein, E. Klempt (HISKP), E. Paul, M. Ostrick, W. Schwille and R.Wedemeyer, technical staff and students.


Fig. 17. The SAPHIR detector at ELSA.

candidate was seen in following experiments. “Today there is little belief that theΘ+ is real, and it remains a mystery how so many experiments could have claimedstatistically significant evidence for the pentaquark” [Hicks 2012].

Since the early nineties, the Polarised Target group in Bonn41 built frozen spintargets and developed a holding coil for the installation of targets with longitudinal po-larisation in experimental set-ups [Dutz 1995]. Double-polarisation experiments withdifferent combinations of spin directions became possible.

The era of double polarisation experiments at ELSA began with an experimentto test the so-called Gerasimov-Drell-Hearm (GDH) sum rule. The rule relates totalhadronic cross sections for left- and right-handed circularly polarised photons on lon-gitudinal polarised nucleons to the anomalous magnetic moment and the mass of thenucleon.

The GDH experiment was carried out by an international collaboration42 at ELSAand, at lower energy, at the Mainz Microtron, MAMI. The circularly polarised photonswere obtained by bremsstrahlung of the electrons extracted from ELSA (and MAMI),after the originally transverse polarisation of the electrons had been rotated intolongitudinal polarisation. The polarised frozen spin targets were built and run byBonn. The GDH experiment confirmed the validity of the GDH sum rule at ELSAand at MAMI [Dutz 2005].

41 H. Dutz, S. Goertz and students. The group benefitted from a long experience withbuilding polarised targets in Bonn (see [Meyer 1988]).42 H. Dutz, D. Menze, B. Schoch and students from Bonn.


Tagging−System +Moeller−Polarimeter

Kryostat + pol. Target

Crystal Barrel

Midi−TAPS

TOF

Photon−kamera

γ −Veto

e−

Fig. 18. The CB-ELSA/TAPS experiment at ELSA.

In 1998, the SAPHIR detector was replaced by the CB-ELSA/TAPS experimen-tal set-up43. The current setup is shown in Fig. 18. The continuous electron beamfrom ELSA was used to produce an energy tagged polarised photon beam obtainedvia bremsstrahlung on a crystal. The photons were optionally linearly or circularlypolarised. The linear (transverse) polarisation was delivered for free by choosing thecorrect orientation of the crystal. The circular polarisation was obtained by rotatingthe transversely polarised electrons extracted from ELSA. The photon beams werescattered on polarised targets. The targets were used optionally with longitudinal ortransverse polarisation. The detector set-up was optimised to measure the produc-tion of neutral mesons decaying into photons within large angular acceptance. Themeasurements cover final states with one or more neutral mesons44.

A first experiment was carried out at a preliminary setup of CB-ELSA/TAPSexperiment and with unpolarised photon beam and targets. An interesting re-sult [Trnka 2005] was obtained for the photoproduction process of ω mesons. Fig. 19shows the comparison of the ω peak produced on hydrogen with that on a heaviernucleus (Niobium). The data suggest that the ω peak is lower when produced on nu-clear matter than when produced on hydrogen. This observation could be explainedas a dynamical consequence of a fundamental symmetry: strong interactions are, formassless particles, invariant against the change of the chirality of the quarks. In na-ture this symmetry is broken: quarks have a constituent mass, and vector mesonsare heavier than pseudoscalar mesons. QCD inspired models predict that the chiralsymmetry is recovered at high density and high temperature. The CB-ELSA/TAPS

43 F. Klein, E. Klempt (HISKP), H. Schmieden, B. Schoch, U. Thoma (HISKP), technicalstaff and students.44 Reactions measured are the production of neutral mesons in the reactions γp → pπ0, pηwith η → 3π0, pπ0π0 and pω with ω → π0γ and also γp → Σ+K0(π0).


Mπ γ [MeV/c2]

coun

ts /

[ 12

MeV

/c2 ]

NbLH2

| p→

ω |< 0.5 GeV/c

0

200

400

600

800

600 700 800 900Mπ γ [MeV/c2]

coun

ts /

[ 12

MeV

/c2 ]

Nb - LH2 dataMpeak = 722 MeV/c2

0

200

400

600 700 800 900

Fig. 19. Photoproduction of ω on hydrogen (left) and nyobium(right).

results suggest that the density of normal nuclei is already high enough for a reductionof the measured ω mass. This mass shift was so far not seen in other experiments.

The main physics goal of the CB-ELSA/TAPS experiment is to analyse the pro-duction of nucleon resonances in the mass range above the Δ(1232) resonance. Highstatistics event samples have been taken with various combinations of polarisationstates of photon beam and proton target.

A partial wave analysis group was set up in Bonn at the beginning of this centurywith the aim to include the increasing amount of data from ELSA and other labo-ratories45 in a coupled-channel analysis. Several new excitations of the proton werereported.

11 Merits of the experiments at the e±p collider HERA

In the early nineties, a new type of high-energy accelerator became available forexperiments: the electron-proton (ep) collider HERA at DESY in Hamburg. Electronsor positrons of 28 GeV were scattered at protons of 820 GeV (later 920 GeV) in head-to-head collisions. Two multi-purpose collider experiments, H1 and ZEUS, were setup at two of the four HERA interaction zones46. Bonn47 is member of the ZEUScollaboration.

The ZEUS detector of 2006 is shown in Fig. 20. The ep interaction region issurrounded, inside of a solenoid magnet, by tracking detectors comprising a microver-tex detector, a cylindrical central drift chamber and planar drift chambers inter-leaved with straw tube trackers48. The tracking detectors were encased by a large45 Similar experiments are in progress also at the accelerators MAMI in Mainz (photonenergies up to 1 GeV) and at CEBAF (photon energies up to 5 GeV) at JLAB (USA). Thedetectors of those experiments were tuned for the measurement of charged particles in thefinal states i.e. that they are somewhat complementary to CB-ELSA/TAPS.46 Two others were used for fixed-target experiments: HERMES using the electron beamfor DIS scattering on polarised nucleon targets, and HERA B using the proton beam for DISscattering on nuclear targets.47 I.C. Brock, J. Crittenden, B. Diekmann, K. Heinloth, E. Hilger, U. Katz, E. Paul andR. Wedemeyer, technical staff and students.48 The straw tube trackers replaced transition detectors, which were used for electron iden-tification up to the year 2000.


Micro Vertex Detector

Solenoid

Straw Tube Tracker +Planar Drift Chambers

Central Drift Chamber

Uranium CalorimeterMuon Chambers

p

e

Fig. 20. The ZEUS detector at the ep collider HERA.

Uranium-scintillator calorimeter, and further outside by a back-up calorimeter andmuon chambers. Bonn built, mainly with the manpower of students, planar driftchambers, transition radiation detectors, straw tube detectors and a pre-sampler infront of the Uranium-scintillator calorimeter. Students of Bonn also developed theread-out electronics and the basic software needed to analyse the data taken withthese components.

At the collider experiments at HERA, deep inelastic scattering was studied over awide kinematic range. The data were suitable for testing QCD. One of the quantitativeresults was the experimental verification that the strong coupling constant, αs, isrunning as a function of the inelasticity of the ep scattering process. A typical resultobtained in the ZEUS experiment is shown in Fig. 21. The coupling constant, αs,falls with increasing transverse energy of an inclusively produced hadronic jet, ingood agreement with the QCD prediction, and is consistent with the world averageat the Z0 mass.

The main physics goal of the experiments H1 and ZEUS was the measurementof proton structure functions and their analysis in terms of distribution functions ofthe partons contained in the proton. Fig. 22 shows parton distribution functions as afunction of the fraction of the proton momentum carried by the parton, x, which wereobtained from a fit to combined data of H1 and ZEUS experiments in 2006. Contribu-tions of valence quarks (u, d), gluons (g) and sea-quarks/antiquark pairs (S), which aretemporarily created by pair production from gluons, are separated from each other.Valence quarks dominate at large x, while gluons and sea quarks dominate at smallx. The shapes of the distributions were found to be consistent with those obtainedin pp collisions at the TEVATRON at FNAL (USA) and other experiments. Thissuggested that parton distribution functions are universal. Currently, there has beensubstantial progress in the development of tools for PDF fitting including all types ofdata in a global fit. Parton distribution functions cannot be predicted by perturbative


ZEUS

0.1

0.15

0.2

10 15 20 25 30 35 40 45

QCD(αs(MZ) = 0.1207 ± 0.0040)

ZEUS 82 pb-1

}th.} stat. } stat.+

syst.

EjetT,B (GeV)

αs

Fig. 21. αs as a function of the transverse energy of an inclusively produced hadronic jetmeasured in the ZEUS experiment [Chekanov 2007].

QCD. Non-perturbative QCD calculations of parton distribution functions, based onLattice QCD can be expected in the future.

About 15% of the ep collisions in DIS are diffractive processes. For such events,the struck proton is isolated from other hadrons in the scattering process. It staysintact i.e. it does not exchange colour or other quantum numbers (except orbitalangular momentum) during the scattering process. Structure functions determinedfrom diffractive processes are not “distorted” by final-state interactions of the hadronsand therefore particularly interesting. First results were published. More data analysesare in progress.

Another interesting result obtained at HERA demonstrates the unification of elec-tromagnetic and weak interactions. Fig. 23 shows cross sections for charged cur-rent (CC) reactions, e±p → νe/νe + hadrons, and neutral current (NC) reactions,e±p → e± + hadrons, as a function of the four-momentum transfer squared, Q2, ob-tained from combined data of the experiments H1 and ZEUS. The Q2 dependenceof NC cross sections is dominated at low Q2 values by the photon propagator, andat large Q2 by the Z0 propagator, while the Q2 dependence of CC cross sections isdetermined by the W propagator throughout the whole Q2 range. Both cross sectionsbecome similar in size at large Q2 values i.e. around Q2 = M2

Z and M2W . All cross

sections are well described by SM predictions.


0.2

0.4

0.6

0.8

1

-410 -310 -210 -110 1

0.2

0.4

0.6

0.8

1

HERAPDF1.7 (prel.)

exp. uncert.

model uncert. parametrization uncert.

HERAPDF1.6 (prel.)

x

xf 2 = 10 GeV2Q

vxu

vxd

0.05)×xS (

0.05)×xg (

HE

RA

PD

F S

truc

ture

Fun

ctio

n W

orki

ng G

roup

June

201

1

HERA I+II inclusive, jets, charm PDF Fit

0.2

0.4

0.6

0.8

1

Fig. 22. Parton distribution functions of valence quarks, uv and dv, sea quarks and anti-quarks, S, and gluons, g, taken from picture archive of the ZEUS Collaboration. Address:www-zeus.desy.de.

12 Study of the composition of the nucleon spin at the CERN-SPS

A first experiment at CERN in which muons from the SPS were scattered on polarisedtargets was already carried out in the eigthies49. It was designed to measure the spindirection of valence quarks in the nucleon. It was already shown that the spin structureof nucleons is rather complex and that quite a few components beyond the valencequarks build up the nucleon spin.

Since the nineties, nucleon spin studies were continued in the HERMES experimentat DESY where polarised e± beams from HERA were scattered on polarised targets.A large amount of data was collected, and several of the components contributing tothe nucleon spin were successfully measured in this experiment.

Since the late 90’s, the nucleon spin structure has been studied in the COMPASSexperiment at CERN. A high intensity polarised muon beam and hadron beams fromthe SPS are scattered on polarised proton and deuteron targets. The experimentwas designed to improve the precision of of measurement of spin components and tomeasure so far missing components.

The COMPASS experiment at CERN is a two-level magnetic spectrometer of 60mlength (Fig. 24). A large angle spectrometer is located around a dipole magnet (SM1),preceded and followed by telescopes of trackers. The SM1 is followed by a RICHdetector (RICH1) and a hadron calorimeter (HCAL1). A second dipole magnet (SM2)

49 Spin-Muon Collaboration (SMC).

www-zeus.desy.de


]2 [

pb

/GeV

2/d

Qσd

-710

-510

-310

-110

10

]2 [GeV2Q310 410

p CC (prel.)+H1 e

p CC (prel.)-H1 e

p CC 06-07 +ZEUS e

p CC 04-06-ZEUS e

p CC (HERAPDF 1.0)+SM e

p CC (HERAPDF 1.0)-SM e

p NC (prel.)+H1 ep NC (prel.)-H1 e

p NC 06-07 (prel.)+ZEUS ep NC 05-06-ZEUS e

p NC (HERAPDF 1.0)+SM ep NC (HERAPDF 1.0)-SM e

y < 0.9 = 0eP

HERA

Fig. 23. Cross sections of the reactions e±p → e± + hadrons and e±p → ν/ν + hadronsin comparison to SM predictions, taken from the picture archive of the ZEUS collaboration.Address: www-zeus.desy.de.

with tracking detectors and calorimeter and muon detectors (Muon filters) completethe detector. A group from Bonn50 is a member of the COMPASS collaboration.Major contributions, made by Bonn, were building and running of polarised targetsand the development of trigger components. Data acquisition is in progress.

13 With the pp collider LHC to new physics

The Large Hadron Collider, LHC, at CERN is at present the world-wide largestparticle laboratory. Protons collide with protons, with energies up to 8 TeV per beam.The LHC opened a new energy range to search for the Higgs particle and physicsbeyond the SM.

Four experiments have been installed: the general-purpose detectors ATLAS andCMS, LHCb for heavy flavour physics and ALICE for heavy ion physics. The PhysicsInstitut Bonn is member of the ATLAS collaboration51.50 S. Goertz, F. Klein, M. Ostrick, J. Pretz and students.51 I.C. Brock, V. Buscher, K. Desch, J. Dingfelder, P. Fischer, M. Schumacher, E. vonThorne, N. Wermes, staff members, technicians and students.

www-zeus.desy.de.


Fig. 24. The COMPASS detector. The set-up is 60m long.

Fig. 25. The ATLAS detector (see text). It is 46 m long and 25 m wide and contains 100million sensors to measure particle production.

13.1 The ATLAS detector

A sketch of the ATLAS detector is shown in Fig. 25. It comprises the inner detector,electromagnetic and hadronic calorimeters and muon spectrometers and a magnetsystem. The inner detector combines high resolution detectors with continuous track-ing elements. It was designed to measure tracks of charged particles and secondaryvertices from particle decays in the environment of high track multiplicities and tighttime resolution.

Bonn contributed by developing, building and commissioning of a pixel detectorfor the Barrel which is part of the inner detector of ATLAS. A special laboratory was


Fig. 26. Modul of pixel detectors for the Barrel of ATLAS.

set up at the Physics Institut Bonn52 where the basic elements for the pixel detectors,semiconductor sensors and integrated electronic chips were developed. Fig. 26 showssuch a modul for the Barrel of ATLAS [Grosse-Knetter 2006]. It consists of sixteenfront-end chips with about 46 000 pixels of size 50 · 400 μm2, thirteen modules forma stave and 112 of the staves build up the Barrel.

13.2 New physics

Highest priority of experiments at the LHC was to search for the Higgs particle, inparticular, in the small mass window between 114 and 150 which was not excludedby previous Higgs particle searches (Section 8). Two experiments, ATLAS and CMS,discovered a resonance peak in the remaining mass range [ATLAS 2012,CMS 2012] inJuly 2012. The peak was seen at a mass of about 125GeV in several potential decaymodes. Measurements of branching ratios and consistency with the expected particlespin (0+) give evidence that the peak is the searched for Higgs particle. With thisdiscovery made at CERN the missing cornerstone of the SM has been found!

The LHC opens new possibilities to test QCD, in particular in processes whereheavy bosons W and Z are produced. Production cross sections for theses particlesare large at LHC energies. Moreover the LHC serves as top quark factory so thatdetailed studies of the t quark become possible. The high mass of the t opens newpossibilities to test QCD. Moreover, the t quark is predicted to have a large couplingto the Higgs particle.

The new energy range, which becomes available at the LHC, opens the door tosearch for new physics beyond the SM. A major challenge is the search for newparticles which are predicted by Supersymmetry (SUSY) models and other theories.

52 This laboratory was set up by the initiative of N. Wermes after he came on a physicschair in Bonn.


The production of dark matter, which is predicted in the framework of SUSY models,is also in the focus of the experiments.

14 Summary and outlook

Particle physics experiments began in Europe in the fifties with the 500 MeV electronsynchrotron at the Physikalisches Institut Bonn and the 28 GeV proton synchrotron(PS) at CERN. In the following fifty years, two more low energy electron acceleratorswere built in Bonn, while high-energy accelerators and storage rings for electrons,protons and their antiparticles were built at the large European laboratories CERNand DESY, with beam energies which were pushed from 28 GeV to 7 (8) TeV perbeam.

Experiments at accelerators in Bonn have also been carried out for the past fiftyyears. The current experiment is dedicated to study nucleon resonances and the pro-duction of hadrons in double-polarisation experiments. Circularly and linearly po-larised photons are scattered on polarised nucleons of frozen spin targets. Bonn ben-efits from a long tradition in building polarised targets. Moreover, Bonn was and isa place for work in research and development of new detector components for bothfuture accelerators and particle scattering experiments.

Also since fifty years ago, Bonn participated in external experiments carried at highenergy accelerators at CERN and DESY. Current experiments are the COMPASSexperiment at the CERN-SPS for quantifying the spin-structure of the nucleon, andthe ATLAS experiment at the large hadron collider (LHC) for testing the validity ofthe SM and predictions beyond the SM in a new range of high energy.

Acknowledgements. I am grateful to my colleagues in Bonn for their support. In particularI would like to thank Eberhard Klempt for careful reading of the manuscript, H. Dutz forchecking the sections about polarised targets and S. Mergelmeyer for helping me in technicalproblems.

References

The ALEPH Collaboration et al. 2006. Precision Electroweak Measurement on the ZResonance. Phys. Rep. 427: 257

Althoff, K.-H. 1969. Das 2.5 GeV-Electronen-Synchrotron der Universitat Bonn. Sonderdruckdes Landesamts fur Forschung, JAHRBUCH 1969, Westdeutscher Verlag, Koln undOpladen

Althoff, K.-H. et al. 1977. Photoproduction of pions on polarised protons and neutrons inthe second resonance region. Nucl. Phys. B 131: 1

Anderson, H.L. et al. 1952. Total Cross Sections of Positive Pions in Hydrogen. Phys. Rev.85: 936

Apsimon, R.J. et al. 1989. Inclusive photoproduction of single charged particles at high P(T).Z. Phys. C 43: 63

ATLAS Collaboration. 2012. Observation of a New Particle in the Search for the StandardModel Higgs Boson with the ATLAS detector at the LHC. Phys. Lett. B 716: 1

Barnes, V. et al. 1964. Observation of a Hyperon with Strangeness Minus Three. Phys. Rev.Lett. 12: 204

Barth, J. et al. 2003. Evidence for the positive-strangeness pentaquark Θ+ in photoproduc-tion with the SAPHIR detector at ELSA. Phys. Lett. B 572: 127

Baum G. and U. Koch 1969. A Source of Polarized Electrons. Nucl. Instr. and Meth. 71:189


Boden, B. et al. 1991. Elastic electron deuteron scattering on a tensor polarised solid ND3

target. Z. Physik C 49: 175Brandt, S. 1963. Wechselwirkung von negativen π-Mesonen eines Impulses von 10 GeV mit

Protonen unter besonderer Beruecksichtigung der Erzeugung seltsamer Teilchen und derMethoden zur Auswertung von Blasenkammerbildern. Ph.D thesis, Bonn

Breidenbach, M. el al. 1969. Observed Behavior of Highly Inelastic Electron-ProtonScattering, Phys. Rev. Lett. 23: 935

Brianti, G. 2004. CERN’s contributions to accelerators and beams. Eur. Phys. J. C 34: 15Burmester, R. 2010. Die vier Leben einer Machine. Deutsches Museum. Abhandlungen und

Berichte. Neue Folge. Band 26. Wallstein VerlagChekanov, S. et al. 2007. Jet-radius dependence of inclusive-jet cross sections in deep inelastic

scattering at HERA. Phys. Lett. B 649: 12CMS Collaboration. 2012. Observation of a new boson at a mass of 125 GeV with the CMS

detector at the LHC. Phys. Lett. B 716: 36Courant, E.D., S. Livingston, H.S. Snyder 1952. The Strong-Focusing Synchrotron - a New

High Energy Accelerator. Phys. Rev. 88: 11190Drees, J. and W. Paul 1964. Beschleunigung von Elektronen in einem Plasmabetatron. Z.

Phys. 180: 340Dutz, H. et al. 1995. An internal superconducting holding coil for frozen spin targets. Nucl.

Instr. and Meth. A 256: 111Dutz, H. et al. 2004. Experimental Check of the Gerasimov-Drell-Hearn Sum Rule on H.

Phys. Rev. Lett. 93: 032003Edwards, R. et al. 2011. Nuclear Physics from Lattice QCD. arXiv:1104.5152Englert, F. and R. Brout 1964. Broken symmetry and the Mass of Gauge Vector Bosons.

Phys. Rev. Lett. 13: 321Freytag, D. et al. 1965. Measurement of the Cross Sections for the Photoproduction of

Positive Pions on Hydrogen in the Region of the first Resonance. Z. Phys. 186: 1Gattringer, C. and C.B. Lang 2010. Quantumchromodynamics on the Lattice, SpringerGell-Mann, M. and Y. Ne’eman 1964. The Eightfold Way. Frontiers in Physics. Publisher

Benjamin, New YorkGell-Mann, M. 1964. A Schematic Model of Baryons and Mesons. Phys. Lett. 8: 214Glander, K.-H. et al. 2004. Measurement of γp → K+Λ and γp → K+Σ0 at photon energies

up to 2.6 GeV. Eur. Phys. J. A 19: 251Grosse-Knetter, J. 2008. Concept, realization and characterization of serially powered pixel

modules. Nucl. Instr. and Meth. A 568: 252Hicks, K.H. 2012. On the conundrum of the pentaquark. Eur. Phys. J. H 37: 1Higgs, P. 1964. Broken Symmetries and the masses of the Gauge Bosons. Phys. Rev. Lett.

13: 508Hillert, W. 2006. The Bonn Electron Stretcher Accelerator ELSA: Past and Future. Eur.

Phys. J. A 28: 139Husmann, D. and W.J. Schwille 1988. ELSA - die neue Bonner Elektron-Stretcher-Anlage.

Physikalische Blaetter 44, Nr.2: 40Kolanoski, H. 1984. Two-Photon Physics at e+e− Storage Rings. Springer Tracts of Modern

Physics 105Leutwyler, H. 1994. On the foundations of chiral perturbation theory. Annals of Physics

235: 165Lohrmann, E. and P. Soeding 2009. Von schnellen Teilchen und hellem Licht, 50 Jahre

Deutsches Elektronensynchrotron. Wiley-VCH Verlag, WeinheimLoring, U. et al. 2001. The light baryon spectrum in a relativistic quark model with instanton

induced quark forces: The non-strange baryon spectrum and ground states, Eur. Phys.J. A 10: 395

Maharana, K. et al. 1979. Baryonium masses in a quark model. Phys. Rev. D 20: 2920Meyer, W. 1985. Present status of polarised targets for high intensity photon and electron

beams. Nucl. Phys. B 446: 1Meyer, W. 1988. Polarised target physics at the Bonn electron accelerators. Physikalisches

Institut Bonn. BN-IR-88-60


Nakamura, K. et al. 2010. Review of Particle Properties. J. Phys. G 37: 075021Paul, E. 1976. Interference experiments with neutral kaons. Springer Tracts of Modern

Physics 79: 53Paul, S. 2009. The puzzle of neutron lifetime. Nucl. Instr. and Meth. A 611: 157Paul, W. et al. 1989. Measurement of the neutron lifetime in a magnetic storage ring. Z.

Phys. C 45: 25Perl, M.L. et al. 1975. Evidence for Anomalous Lepton Production in e+e− annihilation.

Phys. Rev. Lett. 35: 635Plass, G. 2011. The CERN proton synchrotron: 50 years of reliable operation and continued

development. Eur. Phys. J. H 36: 439Schildknecht, D. 2006. Vector Meson Dominance. Acta Phys. Pol. B 37: 595Schwela, D. et al. 1967. Evolution of Multipoles for Photo- and Electroproduction of Pions.

Z. Phys. 202: 452Soding, P. 2010. On the discovery of the gluon. Eur. Phys. J. H 35: 1Stahl, A. 1999. Physics with Tau Leptons. Springer Tracts of Modern Physics 160Stepanyan, S. et al. 2003. Observation of an Exotic S = 1 Baryon in Exclusive Photo-

production from the Deuteron. Phys. Rev. Lett. 91: 252001Trnka, D. et al. 2005. Observation of In-Medium Modifications of the ω Mesons. Phys. Rev.

Lett. 94: 192303Tsemelis, E. 2011. Jack Steinberger: Memories of the PS and of LEP. Eur. Phys. J. H 36:

455Zweig, G. 1964. An SU(3) Model for Strong Interactions and its Breaking. CERN Report

Nr. TH 401, unpublished; Zweig, G. 1964. An SU(3) Model for Strong Interactions andits Breaking: II. CERN Report Nr. TH 412, unpublished