CBM Report 2012-01

Nuclear matter physics at SIS-100The CBM Collaboration

24 February 2012

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Contents

1 Introduction 3

2 The CBM research programme at SIS-100 32.1 Exploring the properties of dense nuclear matter . . . . . . . . . . . . . . . . . . . . . . . 42.2 Probing the equation of state with flow measurements . . . . . . . . . . . . . . . . . . . . 52.3 Search for double hypernuclei . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62.4 Search for meta-stable multi-strange objects . . . . . . . . . . . . . . . . . . . . . . . . . . 72.5 The in-medium properties of vector mesons: search for the onset of chiral symmetry

restoration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.6 Measurement of the (semi-)photonic decays of π0, η and ω . . . . . . . . . . . . . . . . . . 92.7 Exploring the mechanisms of charm production and propagation in nuclear matter at

threshold energies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

3 Experimental setup 10

4 Feasibility studies 124.1 Acceptance for direct hadrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124.2 Hyperons, hypernuclei and strange di-baryons . . . . . . . . . . . . . . . . . . . . . . . . . 134.3 Di-electrons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.4 Open charm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144.5 Charmonium measurements with a staged muon detection system . . . . . . . . . . . . . . 17

5 Summary 17

CBM Report 2012-01The CBM Collaboration: Nuclear Matter Physics at SIS-100Darmstadt 2012Editors: P. Senger and V. Friese

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1 Introduction

The mission of high-energy nucleus-nucleus col-lision experiments worldwide is to investigate theproperties of strongly interacting matter underextreme conditions. At very high collision ener-gies as available at RHIC and LHC, the measure-ments concentrate on the study of the propertiesof deconfined QCD matter at very high tempera-tures and almost zero net baryon densities. Thegoal of the Compressed Baryonic Matter (CBM)experiment at the FAIR SIS100/300 acceleratorsystem is to explore the QCD phase diagram inthe region of very high baryon densities. In par-ticular, the experiments will focus on the searchfor the phase transition between hadronic andquark-gluon matter, the QCD critical endpoint,new forms of strange matter, in-medium modifica-tions of hadrons and the onset of chiral symmetryrestoration. A detailed discussion of the physics ofcompressed baryonic matter can be found in [1].

The research on compressed baryonic matterwill start already with beams from the SIS-100accelerator as an integral part of the physics pro-gramme of the FAIR modularized start versionas presented in the FAIR Green Paper [2]. Inthe SIS-100 energy range, the physics topics ofdilepton and strangeness production will be ad-dressed by two collaborations, HADES and CBM,sharing the same experimental area. The lay-outs and acceptances of both experiments are op-timized for different beam energy regimes butprovide sufficient overlap in beam energy to en-sure two independent evaluations of these chal-lenging measurements and to allow experimen-tal crosschecks for the lighter collision systems.The investigation of dilepton-, strangeness-, andcharm production over the full range of beam nu-clei and energies will be conducted with a ba-sis version of the CBM setup. This interna-tionally competitive research programme requireshigh-performance CBM subsystems such as high-rate detectors, free-streaming read-out electronics,high-speed data acquisition, and an ultra-fast on-line event selection performed on a computer farmconsisting of many-core CPUs accelerated withgraphics cards.

This document discusses the physics case forCBM at SIS-100 (section 2), defines the CBM ba-sis version (section 3) and its capability to measurethe relevant observables (section 4).

2 The CBM research programme atSIS-100

The SIS-100 accelerator will deliver beams ofheavy ions (Au) up to 11A GeV (

√sNN =

4.7 GeV), light ions (e.g. Ca) up to 14A GeV(√sNN = 5.3 GeV) and protons up to 29 GeV

(√sNN = 7.5 GeV). The research programme

based on these beams will address the followingfundamental questions:

• What is the equation of state of nuclear mat-ter at neutron star densities (up to 6 timessaturation density ρ0), and what are the rel-evant degrees of freedom at these densities?Are there new phases of QCD matter likequarkyonic matter?

• How far can we extend the chart of nuclei to-wards the third (strange) dimension by pro-ducing single and double hypernuclei? Doesstrange matter exist in the form of heavymulti-strange objects?

• To what extent are the properties of hadronsmodified in dense baryonic matter, and arethere signatures for chiral symmetry restora-tion?

• How is charm produced at threshold beam en-ergies, how does charm propagate in nuclearmatter, and what are the in-medium proper-ties of charmed particles?

Heavy-ion collisions in the SIS-100 energy rangewere pioneered at the AGS in Brookhaven withgold beams in the energy range from 2A −10.7A GeV. Using several experimental setups,global observables such as bulk hadronic spec-tra, kaon and Λ production, proton flow andparticle correlations were studied. The experi-ments at AGS concentrated on the measurementof hadrons, which undergo strong final state inter-actions and thus carry only limited information

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on the early and dense stage of the collision. Inparticular, no measurements of penetrating probessuch as lepton pairs were performed. The inves-tigation of strangeness production was restrictedto the measurements of kaons, Λ and φ, the latteronly at top AGS energy (11.7A GeV). No multi-strange hyperons were measured, with the excep-tion of 300 Ξ− in Au+Au collisions at 6A GeV.No particles carrying open or hidden charm weremeasured. Therefore, a second-generation exper-iment focused on the systematic measurement ofmulti-strange hyperons, lepton pairs and charm inproton-nucleus and nucleus-nucleus collisions hasa substantial discovery potential.

2.1 Exploring the properties of dense nu-clear matter

Nuclear collisions at SIS-100 energies producean initial state with net-baryon density severaltimes higher than the saturation density. Thisis illustrated in Figure 1, showing the net-baryondensity as function of time for various incidentbeam energies, obtained with the string-hadronictransport code HSD [3]. It was speculated thatat such extreme conditions a new phase of mat-ter might be created, in which quarks are stillconfined, but chiral symmetry is (partially) re-stored [4]. The bulk properties like e.g. energydensity are then dominated by quarks occupyingthe Fermi sea, whereas baryons represent the ex-cited states. This ”quarkyonic” phase is predictedto be located at large baryo-chemical potentialsand moderate temperatures (see Figure 2), and isthus in the regime of heavy-ion collisions at SIS-100 beam energies.

The concept of a chirally symmetric but stillconfined phase is still subject to theoretical de-bate (see e. g. [5]) and remains to be addressedexperimentally. Since a phase transition fromquarkyonic to hadronic matter would act as a ther-maliser, the experimental approach is to studythe hadronic freeze-out configuration in terms ofthe statistical model. Currently, thermal fits tohadron abundances below top AGS energy sufferfrom poor quality and a small number of measuredspecies. High-precision multiplicity measurements

Figure 1. Net-baryon density reached in cen-tral Au+Au collisions as function of elapsedtime calculated with the HSD transportcode [3]

Figure 2. Sketch of a possible QCD phasediagram with phase boundaries close to themeasured freeze-out curve (taken from [4])

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for a large variety of hadron species in the SIS-100energy domain can be expected to provide decisiveinsight into the thermal properties of the chemicalfreeze-out configuration and thus into a possiblephase transition at high net-baryon densities.

The yields and phase-space distributions ofmulti-strange hyperons (Ξ,Ω) are particularlypromising tools to study the properties and the de-grees of freedom of QCD matter at supra-nucleardensities and a possible transition to quarkyonicmatter. The threshold beam energies for reactionslike pp → Ξ−K+K+p or pp → Ω−K+K+K0pare 3.7 or 7.0 GeV, respectively. In nuclear col-lisions, however, Ξ− and Ω− can also be createdvia strangeness exchange reactions like ΛΛ →Ξ−p and ΛΞ− → Ω−n or ΛK− → Ξ−π0 andΞ−K− → Ω−π−, with the Λ and the K− pre-viously produced in independent reactions such aspp → K+Λp and pp → K+K−pp, which requireonly 1.6 and 2.5 GeV, respectively. Alternatively,three-body collisions involving Λ or kaons opennew production channels for Ξ and Ω with respectto p+p reactions. The production of multi-strangehyperons is thus expected to be enhanced at highdensities, and their yield to be sensitive to thebaryon density reached in the fireball. Moreover,the energy distributions of multi-strange hyper-ons provide information on the fireball tempera-ture and the radial flow at the time when they areemitted. Therefore, systematic measurements ofΞ− and Ω− production as function of beam energyand size of the colliding nuclei offer the possibilityto study the nuclear matter equation of state, orbaryon density fluctuations as they are expectedto occur when the system undergoes a first-orderphase transition. These fluctuations may also in-dicate the existence and the location of a QCDcritical endpoint.

Existing data on the production of multi-strange hyperons in nuclear collisions at SIS-100beam energies are very scarce as illustrated in Fig-ure 3, where the measured excitation function ofstrange particles is shown for central collision ofheavy nuclei (Au+Au, Pb+Pb) at beam energiesabove 2A GeV [6]. In particular, no data on Ω pro-duction below 40A GeV (

√sNN = 8.8 GeV) exist.

A systematic measurement of multi-strange hyper-

Figure 3. Yield of mesons, hyperons andanti-hyperons as function of collision energy,measured in central Au+Au or Pb+Pb colli-sions (taken from [6])

ons as diagnostic probes of dense nuclear matter atSIS-100 energies has thus a substantial discoverypotential.

2.2 Probing the equation of state withflow measurements

The collective motion of the final-state hadronsresulting from heavy-ion reactions contains impor-tant information on the collision dynamics. Theisotropic, radial flow allows to characterise thecollision system at kinetic freeze-out, i.e. whenelastic collisions of the produced particles cease.Anisotropic flow results from the conversion ofanisotropies in the density distribution into pres-sure gradients, and thus gives access to the equa-tion of state of dense nuclear matter. Moreover,the flow of strange particles and anti-baryons is de-termined by their in-medium potential and henceallows to address the restoration of chiral symme-try in the dense medium. Flow measurements at

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SIS-18 and AGS provided first insight into thesetopics, but left a number of features to be ad-dressed by a second-generation experiment.

At AGS, the experiment E895 measured theproton elliptic flow and reported a transitionfrom out-of-plane to in-plane emission at about6A GeV [7]. The data indicate an evolution froma stiff equation-of-state below 2A GeV to a softerone at higher beam energies. Such a softeningof the EOS was suggested to be indicative for aphase transition to a deconfined state [8]. Flowmeasurements of the FOPI collaboration at SIS-18, however, seem to exclude a stiff equation ofstate [9, 10], a conclusion which is in line with ear-lier results of KaoS and FOPI on kaon production(see e.g. [11, 12]).

Data on strange particle flow at SIS-100 ener-gies are scarce; the KaoS collaboration reporteddifferent emission patterns for K− and K+ inAu+Au collisions at 1.5A GeV, which could bereproduced by a microscopic transport model as-suming an attractive K−N in-medium poten-tial [13]. The sideward flow of K0 in Au+Au col-lisions at 6A GeV, measured by the E895 collabo-ration [14], shows a pattern which is quite distinctfrom that observed at lower energies and is not yetreproduced by model calculations.

In order to conclusively address the degree ofthermalisation, the equation of state, and thein-medium properties of strange particles, multi-differential flow measurements for a large varietyof hadron species, in particular strange hadronsand anti-baryons, are mandatory for a varietyof collisions systems and beam energies in theSIS-100 energy range. Such an experimentalprogramme requires a large-acceptance hadronspectrometer, good particle identification throughtime-of-flight measurements and decay topology,the determination of the reaction plane and col-lision centrality with good accuracy, and highenough statistics for systematic studies in termsof system size and beam energy.

2.3 Search for double hypernuclei

Hypernuclei, i.e. nuclei containing at least onehyperon in addition to nucleons, offer the fascinat-

ing perspective to explore the third, strange di-mension of the chart of nuclei. Their investigationprovides information on the hyperon-nucleon andeven on the hyperon-hyperon interaction, whichplay an important role in neutron star models.Most of the known hypernuclei were produced inexperiments with K− beams bombarding light nu-clei. In a strangeness exchange reaction, the squark is transferred from the K− to a nucleon,forming a Λ which is trapped in the nucleus. Inviolent collisions between the K− meson and thenucleon, a double-strange Ξ− hyperon may be pro-duced together with a K+. If the Ξ− is trapped inanother nucleus, the two strange quarks may betransferred to two nucleons forming two Λ, andthus a double hypernucleus is created. In mostof the experiments up to date, the decay of theΛΛ hypernucleus has been observed in emulsions.Only very few of such hypernuclei have been foundso far (see e.g. [15]).

In contrast to the method described above, wepropose to produce (double) hypernuclei in heavy-ion collisions at SIS-100 energies via coalescence ofΛ with nucleons or light nuclei in the final stateof the reaction. In such collisions, Λ hyperonsare produced abundantly. Although their maxi-mal yield (about 50 in central Pb+Pb collisions)is reached at beam energies above 40A GeV, stillabout four Λ per event are produced in centralAu+Au collisions at e. g. 4A GeV (see Figure 3).The yield of light nuclei like He, on the otherhand, increases rapidly with decreasing beam en-ergy, such that the coalescence probability has amaximum in the SIS-100 energy range. This isalso the prediction of the thermal model as il-lustrated in Figure 4, where the yield of (multi-strange) hypernuclei is shown as function of colli-sion energy [16]. The yield of 3He and 4He stronglyincreases with decreasing beam energy, and themaximum yield of 5

ΛΛH and 6ΛΛHe is predicted for

collision energies of√sNN = 4 − 5 GeV, corre-

sponding to 7A − 11A GeV beam energy for ex-periments with stationary targets. According tothis calculation, 2 · 10−2 3

ΛH, 4 · 10−6 5ΛΛH and

10−7 6ΛΛHe are expected per central collision.

Experimentally, the decay chain of the hyper-nuclei has to be reconstructed, for example

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Figure 4. Energy dependence of hypernu-clei yields at mid-rapidity for 106 centralAu+Au events as calculated with the statis-tical model [16]. The predicted yields of 3Heand 4He are included for comparison.

5ΛΛH → 5

ΛHe+π−, 5ΛHe → 4He+p+π−. One may

even search for the decay chain 6ΩH → 6

ΞHe + π−,6ΞHe → 6

ΛΛHe → 5ΛHe + p + π−, 5

ΛHe → 4He +p + π−.

2.4 Search for meta-stable multi-strangeobjects

Deeply bound objects with strangeness,e.g. strangelets, (strange) di-baryons, or kaonicclusters were proposed long ago as collapsedstates of matter, consisting of either baryonsor quarks [17, 18, 19]. The production yieldof double-K clusters in heavy-ion collisions wascalculated in terms of the statistical thermalmodel [20], predicting a pronounced maximumyield in the energy domain of SIS-100. Up to date,none of these objects have been observed, withthe exception of indications of a ppK− boundstate reported by the FINUDA experiment [21].Their existence or absence is an open issue in

high-energy physics and will give insight into themulti-quark dynamics of QCD since the one-gluonexchange is believed to play an essential role inthe production of di-baryons. Recently, the inter-est in the search for the H-dibaryon, a S = −2state with the valence quark structure uuddss, upto now with no avail [22], was renewed by latticeQCD calculations indicating a very small bindingenergy of this system [23, 24]. Generated bycoalescence of ΛΛ and Ξ−p pairs, the H-dibaryondecays weakly into Λpπ− with a decay lengthof about 5 cm [25]. A similar decay length isexpected for the (Ξ0Λ)b[26].

High-energy nuclear collisions, with kaons andΛ being abundantly produced in a single event,could provide a tool to create such compositeswith multiple units of strangeness. For SIS-100energies, however, no predictions for the multiplic-ities of bound multi-strange objects are available.At higher energies, calculations with a hybrid (mi-croscopic transport + hydrodynamics) model wereperformed [27], predicting e.g. a multiplicity ofabout 10−2 for the (Ξ0Λ)b in central collisions at30A GeV.

CBM as a high-rate experiment capable of iden-tifying hyperons is certainly suited to search fordecays of strange di-baryons like H → Λpπ−,(Ξ0p)b → p+Λ or (Ξ0Λ)b → ΛΛ in central Au+Aucollisions with unprecedented sensitivity.

2.5 The in-medium properties of vectormesons: search for the onset of chiralsymmetry restoration

One of the most important goals of heavy-ioncollision experiments is to search for signatures ofchiral symmetry restoration, which is expected tooccur at very high baryon densities and/or tem-peratures. An observable consequence would bea modification of hadron properties inside nuclei,or in hot and dense matter. Because of their pen-etrating nature, di-leptons provide direct accessto the properties of light vector mesons in denseand/or hot nuclear matter.

In heavy-ion collisions at the SPS, the CERESand NA60 collaborations found a significantly en-hanced yield of lepton pairs in the invariant mass

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range between 200 and 700 MeV/c2 [28, 29]. Fig-ure 5 depicts the di-muon excess yield measuredby NA60 in In+In collisions at 158A GeV, definedrelative to the di-muon yield from known hadronicdecays including the ω and the φ meson. Micro-scopic calculations suggest that the excess yieldis dominated by ππ annihilation, which proceedsthrough the ρ vector meson according to the vectordominance model. The shape and the magnitudeof the observed excess can be explained assum-ing that the ρ mass distribution is substantiallybroadened. Model calculations indicate that thecoupling of ρ to baryons and anti-baryons plays acrucial role [30].

Figure 5. Excess mass spectrum of muonpairs measured by NA60 in In+In collisionsat 158A GeV, compared to model calcula-tions [29]

The HADES collaboration has performed pre-cision measurements of the invariant-mass spec-tra of electron-positron pairs in nuclear collisionsat beam energies of 1A − 2A GeV. The excess ofdi-lepton yield in C+C collisions in the invariant-mass range 0.2 − 0.7 GeV/c2 as reported by theDLS experiment was confirmed [31]. However, itwas shown that the spectrum measured in C+Ccorresponds to a superposition of lepton pairs from

p+p and p+n collisions [32]. In contrast, heaviersystems like Ar+KCl show a di-lepton excess yieldrelative to the nucleon-nucleon reference data [33].This effect is illustrated in Figure 6 depicting thedi-electron invariant-mass spectra for Ar+KCl col-lisions (symbols), and for a superposition of p+pand p+n collisions (shaded area), both normalizedto the measured pion yields.

Figure 6. Invariant-mass spectrum of e+e−

pairs measured by HADES in Ar+KCl colli-sions. The shaded area shows the super-position of p + p and p + n data as a refer-ence [33].

At SIS-100, the HADES di-electron programmewill be continued towards larger collisions systemsand to higher energies, where the net-baryon den-sities are substantially higher and the in-mediumeffects can thus be expected to be more pro-nounced. In particular, the precise and systematicmeasurement of the in-medium mass distributionof the short-lived ρ meson is expected to provideinformation on the conditions and degrees of free-dom inside the hot fireball produced in heavy-ioncollisions. While HADES at SIS-100, being lim-ited by detector granularity, will investigate col-lision systems up to Ni+Ni, the measurement ofdi-electron spectra in heavier systems is a centralpart of the CBM physics programme at SIS-100.

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2.6 Measurement of the (semi-)photonicdecays of π0, η and ω

The particular challenge of di-electron measure-ments in heavy-ion collisions is the large physicalbackground of the Dalitz decays of π0, which arethe dominant contribution at invariant masses be-low 200 MeV, and of η and ω, dominating at in-variant masses from 200 - 500 MeV. A quantitativeinterpretation of the di-electron spectra requires aprecise knowledge of the contribution from thesechannels. An independent measurement of yieldsand phase-space distributions of π0, η and ω isthus indispensable. Such a measurement will beperformed with a photon detector (calorimeter) inthe (semi-)photonic decays π0 → γγ, η → γγ andω → π0γ.

2.7 Exploring the mechanisms of charmproduction and propagation in nu-clear matter at threshold energies

Hadrons containing charm quarks are verypromising diagnostic probes of hot and dense nu-clear matter. The cc pairs are created in hard col-lisions in the initial stage of the nucleus-nucleusreaction and then propagate through the densemedium. If this medium is partonic, the forma-tion of charmonium states will be suppressed byDebye screening, and the charm quarks will finallycoalesce with light quarks to hadrons with opencharm. Indeed, an anomalous suppression of theJ/ψ yield relative to muon pairs from Drell-Yanprocesses was observed by the NA50 collabora-tion in central Pb+Pb collisions at 158A GeV [34].This observation was based on reference data mea-sured in proton-nucleus collision at 400 GeV. Withnew measurements of charmonium in p+A colli-sions at 158 GeV, where the absorption cross sec-tion in cold nuclear matter turned out to be twiceas large as at 400 GeV, the NA60 experiment re-ported the relative charmonium yield in In+Into be compatible with absorption in cold nu-clear matter; an anomalous suppression of about25 % remains visible in the most central Pb+Pbcollisions [35]. The experience at SPS showedthat in order to disentangle charmonium absorp-

tion in cold nuclear matter and shadowing ef-fects from charmonium dissociation due to Debyescreening in partonic matter, high-precision multi-differential data on charmonium and open charmproduction in nucleus-nucleus and proton-nucleuscollisions are needed.

Model calculations, including the cold nuclearmatter effects like initial state parton shadowingand final state dissociation of the cc-bar pairs dueto the interaction with the target nucleons, werecarried out in order to estimate the charmoniumproduction in p+A collisions in the FAIR energydomain [36]. The results indicate that at theselower energies, the amount of normal nuclear sup-pressions for two different physical scenarios ofcc hadronization (namely color singlet model andcolor octet model) are distinguishably different, afeature unseen at SPS energies. In addition, thedifferential J/ψ production cross section dσ/dy asa function of the cms rapidity y, for a particularp+A collision system, is also found to be differentfor two different physical scenarios of J/ψ forma-tion. Thus in addition to serve as a reference base-line for calibration of conventional nuclear effectsin case of A+A collisions, a precision measure-ment of the charmonium production cross sectionin proton-induced collisions with different nucleartargets might also help to shed light on the muchdebated issue of color neutralization.

At SIS-100, both hidden and open charm mea-surements can be performed in proton-induced re-actions using beams with energies up to 29 GeV.Open charm measurements address the under-standing of charm production near threshold, theproperties of charmed particles at saturation den-sity and their propagation in cold nuclear matter.They are complementary to the PANDA researchprogramme on charm in nuclear matter. More-over, they serve as a reference for nucleus-nucleuscollisions to be measured with the full CBM setupat SIS-300. The total charm production cross sec-tion can be determined by measuring separatelyD+, D−, D0 and D0. Up to now, no data on opencharm production in proton-induced reactions atSIS-100 energies exist (Figure 7).

A particularly challenging and interesting ex-periment is to study charm production in Au+Au

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Figure 7. Existing measurements of thecharm production cross section in p+A col-lisions as function of collision energy (takenfrom [37])

collisions at sub-threshold beam energies, wherethe yield is expected to be extremely small (Fig-ure 8). The threshold beam kinetic energies forcharm production in nucleon-nucleon collisionswith stationary targets are 11.16 GeV for p+ p→J/ψ+p+p, 11.95 GeV for p+n→ Λc+D−+p and14.92 GeV for p+p→ D++D−+p+p. At SIS-100,they can be approached with heavy ions (beamsup to 10A GeV) and surpassed with medium-sizednuclei (up to 14AGeV for Z/A=0.5). Since nodata on charm production in nuclear collisions ex-ist below SPS energies, the discovery potential ofsuch measurements is large.

3 Experimental setup

The measurement of bulk hadrons, multi-strange hyperons, hypernuclei, lepton pairs andcharmed particles in nuclear collisions at SIS-100energies requires a large-acceptance, high-rate de-tector system. The Compressed Baryonic Matter(CBM) experimental setup is being designed tofulfill these requirements. It comprises:

• A micro-vertex detector (MVD) for the high-precision measurement of the decay verticesof charmed hadrons.

• A spectrometer with a silicon tracking sys-

Figure 8. Predictions of the HSD model formeson multiplicities in central Au+Au col-lisions as function of incident beam en-ergy [37]

tem (STS) inside a superconducting magnetwith large aperture (polar angle acceptance2.5 − 25 for all azimuth angles). The STSwill measure the trajectories of the producedparticles in the magnetic dipole field, deter-mine their momenta and reconstruct hyper-ons by their decay topology.

• A RICH detector for the identification ofelectron-positron pairs from the decay of low-mass vector mesons.

• A muon detection system (MUCH) for themeasurement of charmonium via its decayinto muon pairs. This MUCH start version,consisting of two detector triplets, will be up-graded to the full system with seven tripletsfor deployment at SIS-300. Both the MUCHand the RICH detectors will be movable in or-der to be used alternatively for muon or elec-tron measurements, respectively.

• An intermediate tracking detector with threeto four TRD layers allowing to match tracksreconstructed in the STS to the TOF mea-surement. This TRD constitutes the startversion of the full TRD to be used at SIS-300,which will consist of about ten detector lay-ers for the identification of high-momentumelectrons.

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Figure 9. Three flavours of the CBM basis version with the HADES detector in front. Upper panel:CBM setup for the measurements of bulk hadrons, multi-strange hyperons and open charm consist-ing of the magnet, MVD (open charm only), STS, one TRD station as tracker, TOF, ECAL, and PSD.Centre panel: CBM setup for the measurements of di-electrons with an additional RICH detector.Bottom panel: CBM setup for measurements of charmonium with a MUCH start version replacingthe RICH.

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Table 1. Observables and detector combinations at SIS-100

Observables Collisions systems Detectors

Hadrons, hyperonse+e− from low-mass vector mesons

up to Ni+Ni at 8A GeV HADES

Hadrons, hyperons, hypernuclei,photonic decays of low-mass vec-tor mesons

up to Au+Au at 11A GeV Magnet, STS, TRD, TOF,ECAL, PSD, DAQ/FLES

e+e− from low-mass vector mesons up to Au+Au at 11A GeV Magnet, MVD, STS, RICH,TRD, TOF, PSD, DAQ/FLES

D mesons p+A up to 30 GeVCa+Ca up at 14A GeV

Magnet, MVD, STS, TRD,TOF, PSD, DAQ/FLES

Charmonium p+A up to 30 GeVCa+Ca up to 14A GeVAu+Au up to 11A GeV

Magnet, MVD, STS, MUCH,TRD, TOF, PSD, DAQ/FLES

Photons Au+Au up to 11A GeV Magnet, STS, ECAL, PSD,DAQ/FLES

• A large-area time-of-flight detector (TOF)large-area detector consisting of multi-gap re-sistive plate chambers for the identification ofpions, kaons and protons.

• An electromagnetic calorimeter (ECAL) forthe measurement of photons from light vectormeson decays.

• A forward calorimeter (PSD) for the determi-nation of the collision centrality and the re-action plane by the measurement of projectilespectators.

The detector combinations of the CBM startsetup are shown in Figure 9 together with theHADES detector (left side), which will be trans-ferred from its current location and be used forthe measurements of hadrons and electron pairsin collisions of nuclei with masses up to Ni. Thebeam enters from the left and can be focused al-ternatively on the HADES or on the CBM target.The top panel shows the CBM start setup for themeasurement of bulk hadrons, hyperons and opencharm. The configuration for di-electron measure-ments, with an additional RICH detector, is shownin the centre panel, while the bottom panel showsthe setup for the measurement of charmonium, us-

ing the start version of the muon detector system.Table 1 summarises the observables and the cor-responding CBM detector combinations.

4 Feasibility studies

The proposed detector setup was simulatedwith respect to the physics observables describedin section 2. The simulations were performedwith the CBM software package cbmroot, usingGEANT3 as transport engine and the UrQMDmodel for the generation of background events.Realistic detector geometries, including passivematerials, and realistic detector response func-tions with charge sharing and single-channel in-efficiencies were employed. All results were ob-tained after full event reconstruction, using thereconstruction algorithms developed for fast on-line data processing.

4.1 Acceptance for direct hadrons

Figure 10 shows the phase space coverage of theCBM setup for pions, kaons and protons from theprimary vertex, simulated for central Au+Au col-lisions at 6A GeV. Both the kaons and the protonsare identified by their time of flight, measured in

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Figure 10. Phase-space coverage of CBM for charged pions, kaons and protons in the y − pt planefor central Au+Au collisions at 6A GeV. The hadrons are identified by a time-of-flight wall 10 m down-stream of the target with a time resolution of 80 ps. Mid-rapidity is at ycm = 1.28.

the TOF detector located at 10 m downstreamof the target with an assumed time resolution of80 ps. The identification limits the momentumrange for kaons to p < 4 GeV. The distributionsfor other SIS-100 energies look similar; over the en-tire energy range, the forward rapidity hemisphereis largely covered by the acceptance.

4.2 Hyperons, hypernuclei and strange di-baryons

Hyperons and neutral kaons are measured bytopological reconstruction of their weak decay ver-tex in the STS. Figure 11 shows the simulatedinvariant-mass spectra for K0 → π+π−, Λ→ pπ−

and Ξ− → Λπ− for central Au+Au collisions at4A GeV beam momentum. The analysis doesnot employ the identification of the secondariesby time of flight. For all three channels, excel-lent signal-to-background ratios are obtained af-ter proper cuts on the decay topology. The back-ground can still be reduced substantially by iden-tifying the protons and pions with a TOF wallbased on MRPC technology which will be locatedat about 10 m distance downstream the target,providing a resolution of 80 ps or better. This willbe of particular importance for the measurementof Ω− → ΛK− near or even below threshold.

Table 2 lists the multiplicities of hyperons atSIS-100 energies as predicted by the hadron gasmodel [38]. The detection efficiency, including

Figure 11. Reconstructed Λ (upper panel) andΞ− (lower panel) for central Au+Au collisionsat 4A GeV using the UrQMD event generator,the CBM model in the GEANT transport code,and the event reconstruction algorithms inthe cbmroot software package. The spectracorrespond to 104 collisions for Λ and to 108

collisions for Ξ−.

13

geometrical acceptance and branching ratios, is15% for Λ and 3% for the multi-strange hyper-ons. In the absence of a trigger signature, the hy-peron measurements will be performed at the dataarchival rate of 2 · 104/s. Assuming the multiplic-ity in minimum bias events to be 25% of the onein central events results in the estimate of mea-sured hyperon yield per week of run time given intable 3. Remarkably, even at the deep-thresholdenergy 4A GeV, several hundreds of Ω

+will be

measured per week.

No simulations for the detection of hypernucleihave been performed so far. Using the multiplici-ties as predicted by the statistical model (see Fig-ure 4), conservatively assuming a detection effi-ciency of 1% and applying the same scaling of mul-tiplicities from central to minimum bias events asfor hyperons, we estimate the number of detectedhypernuclei in one week at 2 · 104 interactions persecond to be 1.2 · 106 for 3

ΛH, 480 for 5ΛΛH and 12

for 6ΛΛHe.

4.3 Di-electrons

Electron-positron pairs from the decay of low-mass vector mesons produced in heavy collisionsystems will be measured with one of the CBMbasis configurations consisting of the magnet, theMVD, the STS, the RICH and the TOF detector.Electrons are identified using the information fromboth RICH and TOF, which suppresses chargedpions by more than three orders of magnitude. Acareful strategy for the suppression of backgroundelectrons from π0 Dalitz decays and γ conversionin the target both on the single-track and on thepair level is applied in order reduce the combina-torial background. The result of a simulation ofcentral Au+Au collisions at 10A GeV based onthis setup is presented in Figure 12. Leptonic de-cays of light vector mesons were embedded intobackground UrQMD events with multiplicities aspredicted by the HSD model. The spectrum rep-resents 80,000 events which can be measured in 3- 4 seconds, and contains 4.2 ω mesons and 0.15 φmesons. Table 4 summarises the expected yieldsof low-mass vector mesons at this beam energy forone week of data taking at 2 · 104 interactions per

Figure 12. Simulation of the invariant mass ofelectron-positron pairs as measured in cen-tral Au+Au collisions at 10A GeV with theCBM electron basis version (MVD, STS, RICH,TOF).

second.

4.4 Open charm

For the measurement of hadrons containingopen charm, with typical decay length of the orderof 100 µm, the MVD will reconstruct the decayvertex with a resolution of 50 − 80 µm depend-ing on the decay channel. As this device has aread-out time which is large compared to e.g. theSTS, a certain amount of event pile-up (20 - 30at an interaction rate of 1 MHz) must be toler-ated. Moreover, being located close to the eventvertex, it suffers from delta electrons produced bythe beam in the target. These features were prop-erly taken into account in the simulations.

Results of the simulated measurement of Dmesons in p+C collisions at 30 GeV beam momen-tum are shown in Figure 13 for the four-particledecays of neutral D mesons (left) and the three-particle decays of charged D mesons (right). Thesimulations suggest that clean signals can be ob-tained. No deterioration is observed for an eventpile-up of up to 50. The performance of CBM forthe detection of D mesons is shown in Table 5.

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Table 2. Hyperon multiplicities in central Au+Au collisions for different beam kinetic energies ascalculated with the hadron gas model [38]

Beam energy Ξ− Ω−Λ Ξ

+Ω

+

4.0A GeV 1.0 · 10−1 2.0 · 10−3 8.0 · 10−6 5.7 · 10−5 1.0 · 10−5

6.0A GeV 2.8 · 10−1 5.6 · 10−3 5.5 · 10−4 1.5 · 10−4 3.0 · 10−5

8.0A GeV 4.5 · 10−1 1.6 · 10−2 8.0 · 10−3 2.4 · 10−3 6.7 · 10−4

10.7A GeV 6.0 · 10−1 2.5 · 10−2 1.5 · 10−2 4.6 · 10−3 1.3 · 10−3

Table 3. Expected hyperon yields measured per week at a minimum-bias interaction rate of 2 · 104/s.The minimum bias multiplicity is assumed to be 25% of the one in central collisions.

Beam energy Ξ− Ω−Λ Ξ

+Ω

+

4.0A GeV 9.0 · 106 1.8 · 105 3.6 · 103 5.3 · 103 9.0 · 102

6.0A GeV 2.6 · 107 5.0 · 105 2.4 · 105 1.4 · 104 2.8 · 103

8.0A GeV 4.0 · 107 1.4 · 106 3.6 · 106 2.0 · 105 6.0 · 104

10.7A GeV 5.4 · 108 2.2 · 106 6.8 · 106 3.8 · 105 1.2 · 104

Table 4. CBM performance for low-mass vector mesons measured via their decay into e+e− in Au+Aucollisions at 10A GeV and an interaction rate of 2 ·104/s. Shown are the multiplicity in central eventsas predicted by the HSD model, the scaled multiplicity for minimum bias events, the branching ratiointo e+e−, the detection efficiency and the expected yield per week.

ρ ω φ

Multiplicity central 9 19 0.12Multiplicity min. bias 2.25 4.74 0.03

branching ratio 4.7 · 10−5 7.1 · 10−5 3.1 · 10−4

efficiency 4% 4% 5%yield per week 5.1 · 104 1.6 · 105 5.6 · 103

Table 5. CBM performance for D mesons measured via their decay into charged hadons in p+C col-lisions at 30 GeV. The multiplicities for central events were taken from the HSD model; they werescaled down by a factor of three for minimum bias events as suggested by HSD. The total detec-tion efficiency, including geometrical acceptance and branching ratio, is given. The yield per weekassumes a minimum-bias interaction rate of 106/s.

D+ D− D0 D0

Decay channel K+π+π− K−π−π+ K−π−π+π+ K+π+π−π−

Multiplicity central 2.7 · 10−8 5.5 · 10−8 2.9 · 10−8 8.8 · 10−8

Multiplicity min. bias 9.0 · 10−9 1.8 · 10−8 9.7 · 10−9 2.9 · 10−8

branching ratio 9.5% 9.5% 8.1% 8.1%efficiency 13% 13% 1.7% 1.7%

yield per week 67 134 8 24

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Figure 13. Performance of the CBM setup for the detection of D mesons on p+C collisions at 30 GeVbeam momentum. Multiplicities were taken from the HSD model predictions. The top panel showsthe reconstruction of the four-particle decay D0 → Kπππ, the bottom panel the three-particle decayD± → Kππ.

Figure 14. Staged setup of the muon system. Left: Setup with two detector triplets for the measure-ment of charmonium in p+A collisions (top) and simulated invariant mass spectrum of muon pairsfor p+Au collisions at 25 GeV (bottom). Centre: Setup for the measurement of charmonium in A+Acollisions at SIS-100 (top) and simulated invariant-mass spectrum of muon pairs for Au+Au collisionsat 10A GeV (bottom). Right: Setup for the measurement of low-mass vector mesons in A+A colli-sions at SIS-100 (top) and simulated invariant-mass spectrum of muon pairs for Au+Au collisions at8A GeV (bottom). The darker histogram shows the result obtained with an additional time-of-flightmeasurement in the absorber system.

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4.5 Charmonium measurements with astaged muon detection system

The muon detector system can be set up inthree stages (see Figure 14). For the detectionof charmonium in p+A collisions at SIS-100, twodetector triplets are needed. The first station willbe constructed from GEM detectors; for the lastone, the TRD can be used. Simulations of p+Aucollisions, with J/ψ decays inserted according tothe multiplicity as predicted by the HSD model,show that a clean signal can be obtained with thissetup.

In the high track density environment ofAu+Au collisions, an additional detector triplet,possibly realised with straw tubes, is needed inorder to correctly match the signals after theabsorber with tracks reconstructed in the STS.Simulations of the system Au+Au at 10A GeVdemonstrate that even at this sub-threshold en-ergy, the J/ψ is visible above the combinatorialbackground.

The right side of Figure 14 shows a muon setupwith four detector triplets and 90 cm of iron ab-sorber. This system will be capable to measurelow-mass vector mesons through their decay inmuon pairs as demonstrated in the lower rightpanel for Au+Au collisions at 8A GeV. It con-stitutes a subset of the full detector system to beoperated at SIS-300 for the measurement of char-monium and low-mass vector mesons in Au+Aucollisions up to 35A GeV.

5 Summary

Heavy-ion collisions in the SIS-100 energy rangeare an ideal tool for the production of hadronicmatter at neutron star core densities, and, hence,offer the unique opportunity to investigate funda-mental properties of strongly interacting systemsand its constituents: the nuclear matter equationof state, exotic new phases such as quarkyonicmatter, in-medium modifications of hadrons as asignature for chiral symmetry restoration, hyper-nuclei and multistrange objects, charm productionat threshold beam energies, and charm propaga-tion in nuclear matter. Pioneering experiments

at the AGS were limited to the measurement ofthe most abundant hadrons like pions, protons,kaons and lambdas which freeze out in the lateand dilute stage of the collision. New insight willcome from the investigation of observables whichare sensitive to the early and dense phase of thefireball evolution. The basis version of the CBMdetector system at SIS-100 is designed to performcomprehensive and precise measurements of diag-nostic probes of dense matter like multi-strangeparticles, lepton pairs, charmed hadrons, and theircorrelations with the bulk particles.

In conclusion, the combination of the HADESdetector with the basis version of the CBM set-upis very well suited to start an internationally com-petitive nuclear-matter research programme witha substantial discovery potential using beams fromSIS-100. A world-wide unique experimental searchfor quark-gluon matter at ultra-high baryon den-sities will become possible with the full version ofthe CBM detector system using high-energy andhigh-intensity beams from SIS-300.

References

[1] B. Friman et al. (Eds.), The CBM PhysicsBook, Lect. Notes Phys. 814, Springer-VerlagBerlin Heidelberg 2011

[2] FAIR Green Paper - The Modularized StartVersion, Darmstadt 2009, http://www.gsi.de/documents/DOC-2009-Nov-124-1.pdf

[3] W. Ehehalt and W. Cassing, Nucl. Phys. A602 (1996) 449

[4] A. Andronic et al., Nucl. Phys. A 837 (2010)65

[5] K. Fukushima, Phys. Lett. B 695 (2011) 387

[6] C. Blume, J. Phys. G 31 (2005) S57

[7] C. Pinkenburg et al. (E895 Collaboration),Phys. Rev. Lett. 83 (1999) 1295

[8] P. Danielewicz et al., Phys. Rev. Lett. 81(1998) 2438

17

[9] G. Stoicea et al. (FOPI collaboration),Phys. Rev. Lett. 92 (2004) 072303

[10] W. Reisdorf et al. (FOPI collaboration),Nucl. Phys. A 876 (2012) 1

[11] C. Sturm et al. (KaoS collaboration),Phys. Rev. Lett. 86 (2001) 39

[12] C. Fuchs et al., Phys. Rev. Lett. 86 (2001)1974

[13] F. Uhlig et al. (KaoS collaboration),Phys. Rev. Lett. 95 (2005) 012301

[14] P. Chung et al. (E895 Collaboration),Phys. Rev. Lett. 85 (2000) 940

[15] H. Takahashi et al., Phys. Rev. Lett. 87(2001) 212502

[16] A. Andronic et al., Phys. Lett. B 697 (2011)203

[17] A. R. Bodmer, Phys. Rev. D 4 (1971) 1601

[18] R. L. Jaffe, Phys. Rev. Lett. 38 (1977) 195,erratum ibid. 38 (1977) 617

[19] Y. Akaishi and T. Yamazaki,Phys. Rev. C. 65 (2002) 044005

[20] A. Andronic, P. Braun-Munzinger andK. Redlich, Nucl. Phys. A 765 (2006) 211

[21] M. Agnello et al. (FINUDA collaboration),Phys. Rev. Lett. 94 (2005) 212303

[22] T. Sakai, K. Shimizu and K. Yazaki,Progr. Theor. Phys. Suppl. 137 (2000) 121

[23] S. R. Beane et al. (NPLQCD collaboration),Phys. Rev. Lett. 106 (2011) 162001

[24] T. Inoue et al. (HAL QCD collaboration),Phys. Rev. Lett. 106 (2011) 162002

[25] J. F. Donoghue, E. Golowich and B. E. Hol-stein, Phys. Rev. D 34 (1986) 3434

[26] J. Schaffner-Bielich, R. Mattiello andH. Sorge, Phys. Rev. Lett. 84 (2000) 4305

[27] H. Stocker et al., Nucl. Phys. A 827 (2009)624c

[28] G. Agakichiev et al. (CERES collaboration),Eur. Phys. J. C 41 (2005) 475

[29] S. Damjanovic et al. (NA60 collaboration),Nucl. Phys. A 774 (2006) 715

[30] H. van Hees and R. Rapp, Phys. Rev. Lett. 97(2006) 102301

[31] G. Agakishiev et al. (HADES collaboration),Phys. Lett. B 663 (2008) 43

[32] G. Agakishiev et al. (HADES collaboration),Phys. Lett. B 690 (2010) 118

[33] G. Agakishiev et al. (HADES collaboration),Phys. Rev. C 84 (2011) 014902

[34] B. Alessandro et al. (NA50 collaboration),Eur. Phys. J. C 39 (2005) 335

[35] R. Arnaldi et al. (NA60 collaboration),Nucl. Phys. A 830 (2009) 345c

[36] P. P. Bhaduri, A. K. Chauduri and S. Chat-topadhyay, Phys. Rev. C 84 (2011) 054914

[37] W. Cassing, E. L. Bratkovskaya andA. Sibirtsev, Nucl. Phys. A 691 (2001) 753

[38] A. Andronic, P. Braun-Munzinger andJ. Stachel, Phys. Lett. B 673 (2009) 142; er-ratum ibid. B 678 (2009) 516

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