Progress in Particle and

Nuclear Physics PERGAMON Progress in Particle and Nuclear Physics 50 (2003) 503-522

htrp://www.elscvier.comllocate/npe:

Highlights and Perspectives of the Mainz Microtron MAMI

T. WALCHER

Institutfir Kemphysik, Universitdt Main, M&z, Germany

An overview of the idea behind the physics of the MAMI laboratory and its real-

ization is given. The introduction attempts to show the importance of the physics

of hadrons in the general realm and emphasizes the low energy domain as the key

to study Quantum Chromo Dynamics (QCD). Next some highlights of results at

MAW are presented illustrating this idea. New significant experiments to proceed

with this approach to QCD are discussed. This is followed by a description of the

upgrade of the existing MAMI B with 0.885 GcV to MAW C with 1.5 GeV and of

the new experimental equipment making the new experiments possible.

1 Introduction

The presentation of some highlights and in particular of the pcrspcctives of the program at a facility like

MAMI needs a convincing justification. The study of strongly interacting systems is not unchallenged

today and the communitir,s in many countries have weakened the study of this field. Therefore, as an

introduction the deeper ideas behind the program at MAMI and its relation to the broader field of

strongly interacting particles are outlined. This outline was inspired by the following two references

[I, 21.

Quantum Chromo Dynamics (&CD) is today the accepted theory of strong interactions. It was

constructed on the basis of a series of experimental discoveries and their theoretical analyzes made in

the 1960’s and 70’s. I+om these emerged that hadrons arc made out of quarks with six flavors having

I/2 spin and carrying one of three “color” charges, the charges of strong interaction. Thcsc quarks

interact by means of eight colored gluons and are confined, i.e. are never observed freely. The observed

composed systems, hadrons, are always color singlets and their compositeness of quark and gluons is

hidden. The quark-gluon coupling is not a constant. At interaction momenta Q 2 1.5 (GeV/c) it is

“running”, i.e. a logarithmic function of the momentum and tends to zero at very large momenta; this

behavior is called “asymptotically free”.

All these observations could be molded into one relativistic quantum field theory, i.e. a field

theory based on the general principles of quantum mechanics, special relativity, and locality. The

quantum field theories which have turned out to bc the right ones for particle physics are gauge

0146~6410/03/$ - set front matter 0 2003 Elsevier Science BV. All rights reserved. PII: SOl46-6410(03)00045-0

504 T Walcher / Prog. Part. Nucl. Phys. 50 (2003) SOS-j22

invariant. For the quantum field theory for the electromagnetic fields, QED, the gauge transformation

is Abclian. For QCD the correct gauge group is non-Abelian, i.e. QCD is a Yang-Mills theory. It is

this feature which reproduces the asymptotic freedom of the coupling constant. Furthermore it turns

out that the ultraviolet divergcncics of QCD are fully renormalizablc to all orders.

All together QCD is our most complete and consistent physical theory [2]. Its validity is well

proven at high momenta in the asymptotic domain, where a perturbativc expansion in the small

coupling constant is possible. However, despite its beautiful and simple basic axioms it is very nonlinear

and cannot easily be solved in the non .pcrturbative regime.

Therefore, three most striking experimental facts cannot, yet be conclusively derived from &CD:

1. “mass gap”

There is a mass gap between the vacuum ground state and any QCD excitation. The excitation

spectrum of QCD must have a lower bound so that the excitations of the QCD vacuum have a

1 minimal energy. This is also required in order to explain the short range of the nuclear force.

2. “quark confinement”

Quarks and gluons arc ncvcr observed freely, they are confined.

3. “spontaneous chiral symmetry breaking”

The experimental fact, that there exists only one pseudo scalar pion and that (‘soft pion theorems”

are valid can be lead back to the spontaneous breaking of chiral symmetry.

This means that the supposed validity of QCD is not yet established in its most important

domain, the hadronic world, where this world comprises protons, neutrons, pions, kaons, . . . . deuterons,

nuclei, etc.. As is evident from the preceding sketch the problem is as well an expcrimcntal as a

theoretical one. This is the essential motivation for the physics at MAMI. But before some of the

past highlights and the future ideas for MAMI arc described the study of QCD is put into a broader

perspective.

(A) Experimental opportunities

The properties of hadrons and nuclei have been investigated over the last 50 years and the precision

of its knowledge is frequently much better than the theoretical, QCD based, description. Therefore, a

further increase of the experimental precision will frequently not help in the future. Rather WC have to

perform qualitatively new experiments promising to bc meaningful to the mentioned three peculiarities

of &CD. The following three experimental tools appear to have this promise:

?? antiproton beams

Antiprotons produce in the annihilation with other nucleons gluon rich mesonic states. In par-

ticular the spectrum of glue balls would be highly relevant to the question of the mass gap and

the confinement. All quantum numbers of mesons are accessible and, therefore, we shall enlarge

our knowledge of the excitation spectrum which will in turn constrain the calculations.

7: Walcher / hog. I’art. Nucl. Phys. 50 (2003) 503-522

?? relativistic heavy ion beams

505

In relativistic heavy ion collisions hadrons, quarks and gluons experience high temperature and

density and, thcrcfore, the strong coupling constant will change, providing a direct test of the

foundations of QCD.

?? electromagnetic probe

The electromagnetic probe couples with sufiiciently good resolution, i.e. at high momenta, di-

rectly to the quarks allowing a complete mapping of the on- and off-shell momentum distributions

of the confined quarks. At low momenta the low energy theorems can be tested for light quarks

and in this way the mechanism of spontaneous chiral symmetry breaking be explored.

(B) Theoretical opportunities

Theoretical progress can be expected in the next, decade from the following developments:

?? lattice gauge QCD

An analytical solution of non perturbativc QCD appears to be impossible. However, promising

results have emerged for the discretixation of space and time on a four dimensional lattice and

a calculation of observables as a function of the lattice constant. Both the better analytical

understanding and formulation of this method as well as the ever increasing computer power

indicate that a decisive breakthrough is at, hand [3].

?? chiral dynamics

The observed hadrons do not show some of their internal degrees of freedom, as e.g. color and

explicit chiral symmetry. Therefore, it is obvious that in a good approximation to QCD the color

degrees of freedom can be intcgratcd out, and a theory be formulated in the effective hadron fields.

If one realizes the consequences of spontaneous chiral symmetry breaking the “chiral perturbation

theory” based on hadronic matter fields and almost massless Goldstone Bosons can be formulated

[4]. The limits of this theory have: to be explored further.

. QCD inspired models

As a quantum field theory QCD represents a many body problem. Therefore, effective models

based on quasi particles, e.g. “constituent quarks” : collective degrees of freedom, e.g. G,oldstone

Bosons, or empirical potentials for heavy quarks will reflect basic and important consequences of

QCD and give a deeper intuitive feeling of how QCD works. It will be very important to further

develop this approach.

The framework discussed shows that QCD is the model case of a relativistic quantum field theory.

Its study offers a deeper understanding of such theories and at the same time of the hadronic world. ’

so6 I: Walcher / Prog. Part. Nucl. Phys. 50 (2003) 503-522

2 Highlights at MAMI

‘2.1 Chiral Dynamics

As argued in the introduction one of the few possibilities to test non perturbativc QCD sibqificantly

is the study of chiral dynamics [4]. At MAMI several experiments testing chiral dynamics have been

performed:

polarixability of the pion [5]

polarizabilitics of the nucleon [7, 61

virtual polarizabilities [8]

photo threshold production of pions [9; IO]

electro threshold production of pions [ll, 121

The importance of pion loops has been most dramatically demonstrated in the already classical example

of photo production of neutral pions in the p(y, ~“)p reaction [13]. Fig. 1 shows the combined results

of three experiments at MAMI and Saskatoon (SAL).

ReE,-,+ 0.25

0.0

2 -z

. -0.75 m b -1.0 d - -1.25

-1.5

$ -1.75

p! -2.0

-2.25 MAM1 TAPS

-2.5 144 148 152 156 160

E, (MW

Figure 1: The real part of the electric s-wave amplitude compared to a calculation in the framework

of chiral perturbation theory [14] and a calculation using dispersion relations [15]. Note the prediction

of the low energy theorem LET.

The low energy theorem predicted a value of ReE o+ = -2.3 10-3/m, whereas the experimental

value is (-1.31* 0.05) 10-“/m,. The realization that this could be explained by one loop corrections

in the framework of chiral perturbation theory (ChPTh) was a real triumph and put, however, with

T. Walcher / Prog. Part. Nucl. Phys. 50 (2003) 503-522 507

some delay ChPTh into the focus of hadron physicists as is manifest from its application to lattice

gauge theory [3] and the shift of the program of some laboratories [16].

However, from the beginning the convergence radius of ChPTh was questioned. It turned out

that higher loops corrections to the s-wave amplitudes are almost as important as the the one loop

corrections. But it could be shown that the p-wave amplitudes bffered a much better convergence (171

which was confirmed experimentally [9]. Most of these impressive tests of the validity of ChPTh were,

however, performed at the photon point, i.e. at, Q2 = 0 (Geb’/c~)~. 0 II tl le other hand, the pion electro

production experiments wcrc until recently all performed at relatively large momentum transfers of

Q’ > 0.1 (GeV/c)’ and the agreement was frequently assumed to be less meaningful since the external

momenta were already larger than the pion mass [ll, 81. Therefore, at MAMI an experiment at half

the squared momentum transfer for the ~(~,e’~)~~ reaction was performed [12]. The results for the

transverse s-wave amplitude &+(-(1’) and the longitudinal Co+(-q2) are depicted in fig. 2.

005

-q2 [(GeW$]

h

000 0.05

q2 [(GN/c)~]

Figure 2: The Q2 = -q2 dependence of the s-wave amplitudes Eo+ and Lo+

The open circle rcprcscnts the old result of ref. [ll] which took the pwave amplitude from the

ChPTh calculation. The filled circle were derived by assuming that the pwave amplitudes P1,2,3 0: <.i

where $is the three momentum of the virtual photon and k the momentum of the pion in the cm frame.

It is clear that neither the ChPTh calculations nor the MAID phase shift program can reproduce the

data. This deviation hints possibly to a deeper problem. It appears that ChPTh has difficulties to

describe the amplitudes at finite Q*. The early agreement could have been fortuitous and enforced bi

508 T. Walcher/ Prog. Part. Nucl. Phys. 50 (2003) 503-522

including theoretical biases either through the assumptions about the p-wave amplitudes or the fitting

of some 0(p”) counter term. This discrepancy clearly asks for a further study. It may be a unique

possibility to get some deeper insight, into ChPTh.

2.2 Gerassimov-Drell-Hearn (GDH) sum rule

Sum rules reprcscnt a possibility to test, some aspects of the internal structure of composite systems

without having to resolve all the exclusive channels. Tho Gerassimov-Drell-Hearn sum rule connects

the anomalous magnetic moment of the nucleons, i.e. a structure constant intimately related to their

internal structure and consequently their constituency, to the difference of the absorption of polarized

real photons by longitudinally polarized nucleons as given by cq. (1).

p” _ _ P I M ~l/dW) - ‘T3/2bJ)dW _ 7re2 Ic2

0 w 2m2 (1)

where alI2 and 0312 and are the cross sections for that spin of the nucleon antiparallel and parallel to the

photon spin respectively, w the excitation cncrgy, m the nucleon mass, and K the anomalous magnetic

moment. This sum rule is based on very general ingredients like analyticity and causality. Therefore,

it would be very difficult indeed to explain a deviation from the theoretical value of the right hand

side of eq. (1) IzD1’ = -204 ph . However, it is not easy to measure the cross sections over the full

w range required by eq. (1). In recent years a combined effort has been made by a collaboration, the

“GDH collaboration”, at MAMI and ELSA to study it for the first time.

At MAMI the non magnetic detector DAPHNE from Saclay has been used to measure the hclicity

dependent cxclusivc cross sections from 200 MeV to 800 McV with the Glasgow tagged photon facility

of the A2@MAMI collaboration. DAPHNE was complemented by a Cherenkov detector from Gent

University and a forward plastic wall from Tiibingen University. The decisive polarized target was

provided by the ELSA group and represented a heroic effort given the narrow and long horizontal

access to the center of DAPHNE. However, once all components worked much more than just the sum

rule could be dctcrmincd. Fig. 3 shows the results for the 7r”p, x1-n, and ~7rN channels[l8, 19, 20, 211.

The agreement of the experimental results for the channels with one pion with the calculations of

the phase shift code MAID is very good for the first resonance region up to 500 MeV [24]. For

the higher resonances the agreement is less good [23]. From the data of the difference of the total

cross sections the integral of eq. (1) can be calculated from 200 MeV to 800 MeV with the result

Icu” = (-226 f 5slat. f 12,,,,,)@. This has to be corrected for the missing regions frorn 0 MeV to P

200 MeV and above 800 MeV [22]. With these theoretical corrections the value of the left hand side of

cq. (1) yields IFDH = (-202 f 5stal. f 12,,,1. f 10lh,,.)llb. The agreement within the error bars is very

satisfactory. The replacement of the theoretical corrections by the cross sections rneasured at ELSA is

in progress [23]. However, it appears that the separated channels carry more significant information

T. Wulcher/Pro~.Part.Nucl.Phys.50(2OO3)503-522 509

GDH

i ----MAID(d)

-200 1

100 150 200 250 300 350 400 450 500 550 600 650 700 750 600 650

photon energy (MeV)

Figure 3: The difference of the helicity dependent photo production cross sections for the channels rap

(open squares), ,+n (filled triangles), and x7rN (filled diamonds) as a function of the photon energy.

The filled circles show the total cross section difference as determined with a special analysis method

[19]. The curves are calculations with the phase shift code MAID for the respective channels [22].

than the total cross sections and their integrals. Separated channels couple to individual resonances

differently and allow the determination of small resonance amplitudes. Experimental results from

MAMI showing this will be published in a forthcoming paper [25].

Another intcrcsting issue is the GDH sum rule for the neutron as measured with a polarized

deuteron target. Data of limited statistical accuracy have been taken at ELSA and MAMI but have

not yet been analyzed. A new experiment is in progress at MAMI and aims to produce data of the

same accuracy as for the proton.

2.3 Parity violation in p(Z, e’)p

The nucleon carries no open strangeness. However, several experimental results and their theoretical

interpretation suggest that there may bc an hidden strangeness content in the quark sea or its non

perturbative equivalents. In deep inelastic scattering of neutrinos from the proton a 2% contribution

to the momentum fraction is estimated. Some theoretical explanations of the x-nucleon sigma term

derive a relative contribution of the strange quark mass to the nucleon mass of ‘y = 2(N]sS]N)/(N]ufi+

510 T. Walcher/ Prog. Part. Nucl. Phys. SO (2003) 503-522

&IN) = 0.2 f 0.2. A third hint to a contribution of a strange sea to the nucleon structure comes

from deep inelastic scattering of polarized leptons from polarized proton targets. The spin dependent

structure functions derived from these expcrimcnts suggest a contribution of -12% of the strange sea

to the nucleon spin.

All this evidence is highly model dependent and it is of great interest to pursue experiments

which are more direct.ly sensitive to the strange content of the nucleon. Such a possibility is offered

by the parity violating scattering of high intensity polarized electrons from a high density unpolarized

proton target in the p(c,e’)p reaction. The physics background of this reaction and a review of the

results obtained so far arc presented in a contribution to this conference [26].

At MAMI major experimental effort (271 was made to build a competitive and adequate set up

of which the salient features are summarized in the following:

polarized electron source

At MAMI a polarized electron source with 80% polarixation and up to 30pA is routinely used.

machine stability

It is essential to have a spatially and energetically very stable beam. The energy stabilization

of MAMI has reached 6E/E = 10e6 and the local displacements at the target are smaller than

10 /Nl.

polarimctry

A Moellcr polarimetcr is available to allow repeated measurements of the polarization with an

absolute accuracy of about 2%. An on-line-Compton-laser-back-scattering polarimeter is being

built.

high power hydrogen target

A high power hydrogen target taking up to 50 pA of beam current has been built and is running

reliably. It will also allow for the planned measurements on the deuteron.

monitor system

An extensive monitor system surveys all possible helicity correlated changes: beam current, beam

energy, beam position, beam angle, luminosity.

PbFz calorimeter

The central part of the set up is the calorimeter consisting of 1022 PbFz crystals of which 511

have been used for the first round of measurements. The geometrical arrangement on a circular

sector around the beam axis allows to measure the scattered electrons at an average forward

angle of 35’ emphasizing the electric part of the cross section. In turning the calorimeter by

180’ one can access backward anglrs around 145’ and emphasize the magnetic parts. The four

T. Walcher / Pros. Part. Nucl. Phys. 50 (2003) 503-522 511

momentum squared rangrs from about 0.1 (GeV/c)* up to 1 (GeV/c)’ once the energy upgrade

of MAMI is completed (see section 4).

?? electronics and data acquisition

The real challenge of this set up is the idea to count the individual scattering events. The typical

elastic rates are 10 MHz and the inelastic 90 MHz. In order to achieve the required statistical

accuracy of 10m7 about lOI events have to be collected. Therefore, the two different event types

have to be identified and counted on line. This is accomplished by an integrated electronic

capable of evaluating the signals from neighboring crystals and storing the events within 20 ns.

The intrinsic asymmetry given by the weak form factors without strangeness contribution is at the

given kinematical parameters A0 = -5.7 lO-‘j. The frst physical asymmetry, i.e. including Ao,

obtained after 600 hours is:

A phys. = (-7.7 f 0&tat. f 0.7sy.d. f 0.5pd) . 1o-6

From this value the contribution due to strange quarks is:

A phya. - Ao = (-2.0 f o.b,t. f 0.7nmiin. f 0.5,0~) . W6

The measurements and their analysis is in progress and first final results with an accuracy of better

than 10m6 will be available soon. Together with the results from the Bates laboratory and the Thomas

Jefferson laboratory they will allow a first decomposition of the weak form factors [26].

3 New significant experiments

The new MAMI C with its almost doubled energy of 1.5 GeV will provide the opportunity to pursue a

serious of experiments to probe further into the structure of strongly interacting systems, i.e. hadrons

and nuclei. The following experiment are considered to be particularly significant in the spirit of the

introduction:

0 form factors

Recent, exciting results at MAMI on the electric form factor of the neutron [28] suggest a series

of precision experiments on all four nucleon form factors the electric and magnetic of the proton

and neutron at small and moderate of four momentum transfers of 0 < Q2 < 2 (G~V/C)~.

?? effective field theories and chiral dynamics

This active field at MAMI as been already mentioned. It will be extended to the strangeness

sector where little is known about the validity of effective field theories. But also the threshold

production of one or several of the 7r,q,p, and w rncsons will give interesting insight into the

mesonic structure of the nucleon.

512

0 nuclei

T. Walcher/ Prog I’arf. Nucl. Phys. 50 (2003) 503-522

Several experiments of one and two nucleon knock out on light, nuclei are planned for which the

new MAW C will offer a broader kinematical range than was accessible so far. Due to the higher

energy several production thresholds arc passed and mesons can be produced in nuclei allowing

the study of their medium modifications.

The possibility to investigate hypcr nuclei will be studied. Such studies will greatly profit from

the large solid angle spectrometers at forward angles KAOS which will become available at MAMI

(see section 4.3). The existing’ spectrometer B can be already positioned at ‘?‘.

0 resonances in nucleons

The study of resonances in nuclcons are evidently a significant access to the internal structure

of nucleons. Of particular interest are small amplitudes either of weak resonances or as small

admixtures to dominating transitions. Fig. 4 shows the range of resonances accessible by photo

excitation at MAMI C. The decisive problem is that that at these masses mesons can be produced.

Therefore all resonances arc broad and lie in the t-channel continuum of the nucleon. This

means that the overlapping resonances have to be separated from the non resonant continuum

and one from the other. Even if one can separate the individual resonancra this problem will be

become fundamental above an invariant mass of about 2 GeV/c?. Here the phase advances of

the very broad resonances and the non resonant background with increasing energy may become

comparable and consequently undistinguishable. Therefore, techniques like speed plots will fail.

The only solution will be to fit models for resonances and background directly to the data, an

attempt which failed, however: for the giant resonances, i.e. the excitation continuum, of nuclei.

Below 2 GeV/c? one can use the different couplings of the mesons to the different resonances as

shown in fig. 4(a) and fig. 4(b) f or a selective study of e.g. the ,911 resonances. An even more

powerful possibility to select one resonance and separate it from the background is the use of

polarization variables. At MAMI polarized electrons as well as linearly and circularly polarized

photons are available. They can be used with polarized proton, deutcron and 3x targets.

Further a proton recoil polarimeter exists with the spectrometer A of the Al collaboration as

well as a neutron polarimetcr.

Tab. 1 lists the 16 polarization observables in principle accessible with polarized photons if

combined with different. polarized targets and recoil measurements. The the cross section u,

and the photon beam asymmetry 2: have been investigated intensively in recent years at MAMI

[9, lo] for tests of chiral dynamics. The observable E is nothing but the GDII observable already

discussed in subsection 2.2. For illustration of the power of polarization observablrs one further

example of a planned experiment, for MAMI C will be presented in more detail in the following.

T Walcher/ Prog. Part. Nucl. Phys. 50 (2003) 503-522 513

70

60

SO

s40 3

b 30

20

10

0 200 400 600 800 loo0 1200 140(

E, WV)

(a) piorl production y+N --f N’ + N+n (b) eta production yfN+N’+N+q

1.4

1.2

1.0

2 3 0.8

b 0.6

2ca 400 600 800 IIN0 12lxl 1400

E, WV)

1

E-, WV)

(c) Kahn production y+N -+ N* -P Y+K+

Figure 4: The range of photo excitation of nucleon resonances N’ covered by MAMI C

The differential cross section for the production of charged and neutral pions with linearly po-

larized phot.ons and longitudinally polarized protons in the T + fl-+ N’ + 7P-O reaction is given

by

$@‘, 4) = $$I){1 - p,Ccos(2@) + p,p,Gsin(2qS)} (2)

where 4 is the angle of t,hc polarization direction with respect, to the production plane and 6’

the azimuthal angle of the pions with respect to the photon direction. The double polarization

observable G depends on two amplitudes:

G cc ImM,.. - R.eMI+ (3.)

514 71 Walcher / Prog. Part. Nucl. Phys. 50 (2003) 503-522

polarization

photon none target recoil target&recoil

x’ yj zj x’ x’ z’ z’ direction

X Y z X Z X Z

unpolarized u 0 T 0 0 P 0 T,l -L,, T,l LZI

linearly polarized -C H (-P) -G 0,~ -‘1’ OZ! (-&I) (T,O (--&I) (-T’)

circularly polarized 0 P’ 0 -E -c,, 0 -c,j 0 0 0 0

Table 1: The 16 polarization observablcs in photo production. The observablcs in brackets can be

accessed by other experimentally easier combinations of polarizations.

where Ml+ is the well known A(1232) resonance amplitude and MI- is the amplitude of the

so called Roper P,,(1440) resonance which has a particular interest since it would be a radial

excitation of the nucleon and, therefore, provide a meauingful access to the compressibility of the

nucleon. The sensitivity to the excitation of this amplitude is shown in fig. 5. The crystal ball

0.4

0.2

0.0

-0.2

-0.4

- P,,(14w E7 = 550 McV

. . . . . . .:. . .

. . . . . . .-..-._._

,..I . . ..____._.. . ..I . .

-0.6 I 0 30 60 90 120 150 180

&r (de@

0.4 8 I 0 I 1 ’ - P,*(14w e, = 9o”

0.2

-0.6 n a ’ a . a . 240 360 480 600 720

Er WV)

Figure 5: The double polarization observable G as a function of the azimuthal angle 0, and the photon

energy E7. The dotted curve shows G without the contribution of the R.oper resonance P,,(1440), the

full curve with it.

CBQMAMI (see subsection 4.2) will provide the large acceptance needed for the neutral mesons

and will be the basis for many experiments with very good precision.

T Walcher / Prog. Purr. Nucl. Phys. SO (2003) 503-522

4 Perspectives of MAMI

515

4.1 Energy upgrade of MAMI: MAMI C

The Mainz Microtron MAMI B [29] started its physics program about 10 years ago after the continuous

wave (CW) race track microtron technique (RTM) had been developed and realized in several steps:

I979 MAMI-test with 15 McV (van de Graff injector plus one RTM), 1983 MAMI A with 180 MeV

(van dc Graff injector plus two RTM), and 1990 MAMI B with 885 MeV (RF injector plus three

cascaded R,TM). The microtron principle excels by its reliability and an excellent cw beam quality,

i.e, very small phase space, excellent energy spread and stability, and practically no halo (see beam

parameters given in tab. 2) [30].

In order to extend the program at MkMI to the physics outlined in the introduction and to be

able to perform the experiments described in the previous section 3, an energy upgrade became very

desirable. The existing equipment with the 3-spectrometer set up (Al collaboration), the Glasgow

tagging facility with its many specialized detectors as DAPHNE from Saclay and TAPS (A2 collabo-

ration), and the calorimeter for the parity violating electron scattering (A4 collaboration) represented

already a major asset to justify an energy increase. All setups could be used at cncrgics up to 2 GeV.

For the reactions envisaged the maximal nominal momentum of 735 McV (spectrometer A), 870 MeV

(spectrometer B), and 551 McV (spectrometer C) sufficrs since the higher energy goes in most cases in

recoil energy and mass production. Only in forward direction, particularly important for strangeness

production, a higher momentum spectrometer is required and will bc made available by GSI (see sec-

tion 4.3). The Glasgow tagger can bc upgraded for a moderate invcstmcnt and the A4 calorimeter will

not need any modification.

The ideal cncrgy increase to 2 GeV could not be realized since the existing experimental halls

available for housing the extension are not large enough. The building of new accelerator halls was

excluded due to the financial limits of the project. Another mandatory constraint was the wish to

continue the operation of MAMI B during the construction of MAMI C, a necessity for a university

laboratory. Therefore, the energy upgrade had to bc limited to 1.5 GeV, which is, however, well

above some important thresholds. Additionally, the emphasis of the future program is on threshold

production of mesons and the study of the well defined, i.e. separable, nucleon resonances. Fig. 6

depicts the floor plan of the existing MAMI B together with the new MAMI C under construction.

Some important parameters arc listed in tab. 2. Again financial considerations on investments as

well as on running costs rnadc it clear that the rnicrotron principle with a warrn rf structure is the

most economical solution. A superconducting option though possible did also not offer any decisive

advantages for the physics program. However, the proven RTM design cannot be scaled to 1.5 GeV

since the reversing 180’ bending magnets would have the extremely high weight of 4000 tons. Al&

516 7: Wakher/ Frog. Part. Nucl. Phys. SO (2003) 503-522

Figure 6: The floor plan of the MAW facility with the existing MAMI B consisting of the race track

microtrons RTMl, RTM2, and RTM3 and the experimental facilities comprising the S-spectrometer

setup (Al collaboration), the tagger facility (A2 collaboration), the calorimeter and Compton po-

larimetcr halls (A4 collaboration), and the hall for applied radiation physics (Xl collaboration). For

MAMI C a fourth stage, the harmonic double sided microtron IIDSM, is under construction.

would the many turns needed degrade the beam quality due to the quantum fluctuations of the

synchrotron radiation. Therefore, the new double sided microtron (DSM) design shown in fig. 7 was

chosen. This scheme is known since a long time but was never realized successfully It has two

essential difficulties. Firstly, one has to design achromatic bends for a broad momentum range with

excellent focusing properties. The lay out shown in fig. 7 accomplishes this by using a specially

adjusted gradient field in the dipole magnets compensating for vertical edge defocusing without any

additional quadrupoles for the separated beams in the dispersive region between the bends. The only

transverse focusing needed is done by four quadrupoles doublets on the axes of the accelerating rf

Z Walcher/Prog Purt. Nucl. Phys. 50 (2003) 503-522 517

Figure 7: The lay out of the harmonic double sided microtron HDSM, the fourth stage of MAMI under

construction. The essential parameters are listed in tab. 2

sections. Such designs bccamc feasible through modern computer codes capable of calculating reliably

inhomogcncous field configurations. The second problematic aspect of DSMs is its over focusing in

the longitudinal direction. Since the magnet field is not constant in the bending magnets some phase

slip of the electron bunches towards steeper regions of the rf wave occurs during acceleration. The

resulting strong longitudinal focusing leads to a risk of instabilities. However, it was realized that in

one of the two linacs both originally designed for the first harmonic frequency of MAMI B, only every

second bucket is occupied. Therefore, the MAMI B frequency could be used for this linac resulting in

a very satisfactory phase stability during acceleration. The DSM with the basic and its first harmonic

frequency is addressed as the harmonic double sided microtron (HDSM) [31, 321. All magnets are by

end of 2002 in house and the first one measured performs to specifications. The vacuum system will be

build in 2003. The rf acceleration sections and the 4.9 GHz klystron are being produced and expected

for delivery by beginning of 2004. The upgrading of the beam handling systems has been finished.

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7: Walcher / Pros. Part. Nucl. Phys. SO (2003) 503-522

The completion and commissioning of MAMI C is scheduled for end of 2004

519

4.2 Crystal Ball (CBQMAMI)

One of the greatest concerns with the upgrade to MAMI C was the desire to match the new op

portunities offered by appropriate experiments. The limited resources of the laboratory facing the

building of a new accelerator, pursuing an ambitious and competitive physics program at MAMI B,

and developing new dctcctors, reprcscntcd some sever limits and threatened to impose some unac-

ceptable compromises. A new accelerator without, matching detectors is not worth much and a shut

down with no physics and consequently no students would have put the laboratory in a very difficult

situation. Fortunately, two well proven detectors became available just at the right time. The Crystal

Ball conceived for SPEAR at the Stanford Linear Accelerator Center (SLAC) and used at DESY and

in recent years at the Brookhavn National Laboratory (BNL) was given on a long term loan from the

US Department of Energy and forms the basis of a new international collaboration, the CB@MAMI

collaboration. Fig. 8 shows a sketch of this legendary assembly of 672 NaJ crystals. It will be furnished

with new electronics and a new data acquisition system. Two cylindrical wire chambers similar to the

ones already in use for DAPHNE and a layer of thin plastic detectors surrounding the target will

be used for charged particle trcking. A central addition will be a new polarized target explained in

Figure 8: The Crystal Ball developed and built at, SLAC and used for many years at SLAC, DESY,

and BNL together with the TAPS detector as a forward detector wall as it will be installed at MAMI.

section 3 which will be realized in a collaboration with the universities at Bonn and Bochum. The’

520 T. Walcher / Prog. Part. Nucl. Phys. 50 (2003) 503~;22

TAPS detector will scrvc as a forward wall giving an almost 100% covcragc of the solid angle. Due to

the fast response of the .UaJ of CB and BaF of TAPS the high flux of polarized photons provided by

the Glasgow tagging facility can be used.

4.3 KAoSQMAMI

The KAOS spcctromcter has been .used to investigate subthreshold kaon production in heavy ion

collisions [33]. It, is therefore designed to look for forward produced kaons in a large momentum bitt

and solid angle with good resolution, just the requirements for the electro production experiments of

strangcncss at MAMI C. The KAOS spectrometer can be well integrated in the 3-spectrometer set

up and will rcprcsent, a unique 4-spectromctcr set up with unparalleled figure of merit for coincidence

experiments. The spectrometer hall of the Al collaboration provides sufficient space. The maximal

momentum of the KAOS spectrometer is 1.6 GcV with a solid angle of 20 msr at a resolution of 1. 10m4 for

kaons and 2 lo-” for electrons and a momentum bitt of up to f20 %. Its short length of 6 m provides a good

survival probability for kaons. A possible arrangement of the spectromctcr at MAMI is depicted in fig. 9. A

ta

Figure 9: One possible arrangement of the GSI-KAOS spectrometer optimized for forward detection of

kaons.

new focal plane detector system based on scintillating fibers is being developed. It promises a good spatial

resolution of 150 pm, good timing and conscqucntly the handling of a high count rate. It will be realized by

using 0.83 mm diameter fibers and a multi-anode phototube read out with 32 channels/tube. The whole focal

plane detector will consist of 4000 charmels with 4 fibers/channel.

7: Walcher/Prog. I’art. Nucl. Phys. SO (2003)503-522 521

5 Conclusions

This overview tried to go back and justify why the study of QCD is more than a study of hadrons and other

strongly interacting systems. The exarnplc of MAMI shows that with moderate investment and personnel

significant physics like chiral dynamics, small amplitudes of nucleon resonances aud the form factors of the

nucleon can be pursued. F’undarnental aspects of QCD like spontaneous symmetry breaking and non per-

turbative methods can be studied with reasonable expenses in the frame work of a university. The upgrade

was unanimously agreed by the Senate of the University of Mainz comprising a large majority of theologists,

lawyers, physicians, arts scholars, and a small minority of scientists. When the Noble Laureate Jack Stein-

hcrger visited Maim in 2000 hc wrote afterwards a letter to the author of this contribution saying “I am

ashamed to have been so ignorant about this work”. Considering the difficulties the field of ha&on and QCD

physics cxpcricnces today this suggests that either the physics is not fromulatcd clearly enough or that the

community is closed to much.

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