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The Compressed Baryonic Matter Experiment at
FAIR in Darmstadt
Peter Senger
Gesellschaft f˜ur Schwerionenforschung m. b. H.
D­64291 Darmstadt, Planckstr. 1
I. THE FUTURE FACILITY FOR ANTIPROTON AND ION RESEARCH (FAIR)
The future Facility for Antiproton and Ion Research (FAIR) in Darmstadt will provide
unique research opportunities in the fields of nuclear, hadron, atomic and plasma physics [1].
The accelerators will deliver primary beams (protons up to 90 GeV, Uranium up to 35 AGeV,
nuclei with Z/A = 0.5 up to 45 AGeV) and secondary beams (rare isotopes and antiprotons)
with high intensity and quality. The facility comprises a double­ring synchrotron, rings for
accumulation, cooling and storage of primary and secondary beams, and dedicated detector
arrangements. The research program includes the study of nuclei far from stability, hadron
physics with antiproton beams, the study of compressed nuclear matter, the investigation
of plasmas induced by ion and laser beams and atomic physics.
The aim of the nucleus­nucleus collision experiments is to explore the QCD phase diagram
at high net baryon densities and moderate temperatures. This approach is complementary
to the studies of matter at high temperatures and low net baryon densities performed at
RHIC and LHC.
II. EXPLORING THE QCD PHASE DIAGRAM WITH HEAVY­ION COLLI­
SIONS
The exploration of the phase diagram of strongly interacting matter is a major field of
modern high­energy physics [2]. Of particular interest is the transition from hadrons to
partonic degrees of freedom which is expected to occur at high temperatures or high baryon
densities. These phases play an important role in the early universe and in the core of
neutron stars [3]. The discovery of this phase transition will shed light on fundamental
but still puzzling aspects of Quantum Chromo Dynamics (QCD): confinement and chiral
symmetry breaking. In particular, at high baryon densities one expects new phases of

strongly interacting matter [4]. The scientific progress in this exciting field, QCD at high
baryon densities, is driven by new experimental data.
In order to study the dynamics of strongly interacting matter far from its ground state,
laboratory experiments are performed with high­energy nucleus­nucleus collisions. The con­
ditions inside the transiently existing fireball are reflected in the abundances and the phase­
space distributions of the emitted hadrons. Important information on the early phase of the
collision is provided by the quark flavor of the observed hadrons. In particular, hadrons con­
taining strange or charm quarks are regarded as sensitive diagnostic probes of the collision
dynamics. The hadrons are either observed directly or ­ in the case of short­lived resonances
­ are identified via their hadronic or leptonic decay products.
Lattice QCD calculations at vanishing baryon chemical potential and finite temperature
predict the formation of a quark­gluon plasma above energy densities of about 1 GeV/fm 3
[5]. Such conditions may be created in central collisions between heavy nuclei already at
bombarding energies of about 10 AGeV [6, 7]. Recent lattice QCD calculations at finite
baryon chemical potential predict a critical endpoint of deconfinement phase transition at
µB # 400 MeV and T#160 MeV [8, 9].
Our current knowledge on the QCD phasediagram is illustrated in figure 1. The data
points correspond to chemical freeze­out and result from a statistical analysis of particle
ratios measured in Pb+Pb and Au+Au collisions at SIS, AGS, SPS and RHIC [10--12].
The solid curve along the freeze­out points represents a calculation with a constant baryon
density (baryons + antibaryons) of about # B = 0.75 # 0 [12]. The phase boundary between
quark­gluon matter and hadronic and the location of the critical endpoint as shown in
figure 1 is predicted by lattice QCD calculations [8, 9] which indicate that for values of µB
larger than about 400 MeV the phase transition is first order, whereas for µB smaller than
400 MeV there is a smooth cross over from the hadronic to the partonic phase (dotted line).
The search for the critical endpoint is a prime goal of the experiments.

baryonic chemical potential µ B [GeV]
temperature
T
[MeV]
n b =0.12 fm ­3
critical endpoint
Lattice QCD
dense
baryonic
medium
dilute
hadronic
medium
P. Braun­Munzinger et al. PLB 518 (2001)
F. Becattini et al. PRC 96 (2004)
R. Averbeck et al. nucl­ex/9803001
quark­gluon matter
0
25
50
75
100
125
150
175
200
0 0.2 0.4 0.6 0.8 1
FIG. 1: The phase diagram of strongly interacting matter plotted as a function of temperature
and baryon chemical potential. Full symbols: freeze­out points obtained with a statistical model
analysis from particle ratios measured in heavy collisions [10--12]. The critical endpoint at µB # 400
MeV is predicted by lattice QCD calculations [8, 9]. ''Dilute hadronic medium'': #B =0.038 fm -3
# 0.24 # 0 . ''Dense baryonic medium'': #B =1.0 fm -3 # 6.2 # 0 .
At ultra­relativistic beam energies provided by RHIC and the future LHC, partonic mat­
ter is expected to be produced at very high temperatures but at small baryon chemical
potentials. Similar conditions prevailed in the early universe. The complementary situation
exists in the core of a conventional neutron star: here the baryon density is very high and
the temperature very low. In the laboratory, fireballs at high baryon densities and moderate
temperatures are created at beam energies significantly below top SPS energies. This region
of the QCD phase diagram ­ marked by the hatched area in figure 1­ has not been studied
experimentally in detail.

III. THE FUTURE NUCLEUS­NUCLEUS COLLISION RESEARCH PROGRAM
The proposed heavy­ion collision experiment at FAIR in Darmstadt will simultaneously
measure observables which are sensitive to e#ects of high baryon densities and to phase
transitions. In particular, we focus on the investigation of:
. short­lived vector mesons. Their spectral properties, the mass and the width, can
be studied in the dense nuclear medium via their decay into lepton pairs. Since the
leptons are very little a#ected by the passage through the high­density matter, they
provide, as a penetrating probe, almost undistorted information on the conditions in
the interior of the collision zone.
. Charmonium production at beam energies close to the kinematical threshold. The J/#
mesons will be measured via their decay into electron­positron pairs. The multiplicity
of J/# mesons per participating nucleon as function of system size and beam energy
is expected to be sensitive to the deconfinement phase transition.
. open charm, e.g. D­mesons. The e#ective masses of D­mesons ­ a bound state of a
heavy charm quark and a light quark ­ are expected to be modified in dense matter
similarly to those of kaons. Such a change would be reflected in the relative abundance
of charmonium (c•c) and D­mesons.
. strange particles, in particular baryons (anti­baryons) which contain more than one
strange (anti­strange) quark.
. macro­dynamical e#ects, like collective flow of nuclear matter during the expansion
of the initially compressed system. Such probes provide constraints on the underly­
ing equation of state which is of great significance e.g. for astrophysical problems.
Moreover, the measurement of the flow of charmonium and multistrange hyperons
will provide new information on the behavior of these rare probes in dense baryonic
matter.
. characteristic event­by­event fluctuations near the critical point. The identification of
a critical point would provide direct evidence for the existence and the character of a
deconfinement phase transition in strongly interacting matter.

. search for exotica like pentaquarks, short­lived multi­strange objects, and precurser
e#ects of a color super­conducting phase at highest baryon densities [13].
IV. THE CBM DETECTOR
The experimental task is to identify both hadrons and leptons and to detect rare probes
in a heavy ion environment. The apparatus has to measure multiplicities and phase­space
distributions of hyperons, light vector mesons, charmonium and open charm (including
the identification of protons, pions and kaons) with a large acceptance. The challenge is
to filter out those rare probes in Au+Au (or U+U) collisions at reaction rates of up to
10 7 events per second. The charged particle multiplicity is about 1000 per central event.
Therefore, the experiment has to fulfill the following requirements: fast and radiation hard
detectors, large acceptance, electron and hadron identification, high­resolution secondary
vertex determination and a high speed trigger and data acquisition system. The CBM
experimental setup consists of the following detector components (see figure 2):
. Dipole magnet for momentum determination and #­ray deflection
. Radiation hard Silicon pixel/strip detectors for tracking and vertex determination
. Transition radiation detectors (TRD) for electron identification
. Ring imaging Cherenkov detector (RICH) for electron identification
. Resistive plate counters (RPC) for time of flight measurement
. Electromagnetic calorimeter for identification of electrons, photons and muons
. Diamond pixel detectors for TOF start signal

FIG. 2: Sketch of the planned Compressed Baryonic Matter (CBM) experiment. The beam enters
from the left hand side. The setup consists of a superconducting dipole magnet with a Silicon
tracker System inside, a Rich Imaging Cherenkov detector (RICH) for electron identification, the
three Transition Radiation Detectors (TRD), the Time­Of Flight (TOF) wall which is a Resistive
Plate Chamber (RPC) and the electromagnetic Calorimeter (ECAL) (from left to right). The total
length of the setup is approximately 12 m from target at the entrance of the dipole to the ECAL.
The proposed experimental setup o#ers the possibility to determine particle multiplicities
and phase­space distributions, the collision centrality and the reaction plane. The simul­
taneous measurement of electron­positron pairs and hadrons permits the study of cross
correlations. This synergy e#ect opens a new perspective for the experimental investigation
of nuclear matter under extreme conditions. Such systematic and detailed measurements
include the variation of experimental conditions like beam energy and atomic number of the
colliding nuclei and require a dedicated accelerator with high beam intensities, large duty
cycle, excellent beam quality and high availability.
The CBM Collaboration, which actually consists of 39 institutions from 14 countries, has
submitted a Technical Status report in January 2005 [14].

V. REFERENCES
[1] http://www.gsi.de/GSI­Future/cdr/
[2] Quark Matter 2004, J. Phys. G: Nucl. Part. Phys. 30 (2004)
[3] F. Weber, J. Phys. G: Nucl. Part. Phys. 27 (2001) 465
[4] F. Wilczek, Physics Today 53 (2000) 22 and hep­ph/0003183
[5] F. Karsch et al., Nucl. Phys. B 502, (2001) 321
[6] H. St˜ocker and W. Greiner, Phys. Rep. 137 (1986) 277
[7] V.D. Toneev et al., nucl­th/0309008 and Yu. Ivanov, private communication
[8] Z. Fodor and S.D. Katz, hep­lat/0402006, and JHEP 0404 (2004) 050
[9] S. Ejiri et al., hep­lat/0312006
[10] F. Becattini et al., Phys.Rev. C69 (2004) 024905
[11] J. Cleymans and K. Redlich, Phys. Rev. Lett. 81 (1998) 5284
[12] P. Braun­Munzinger, Nucl. Phys. A 681 (2001) 119c
[13] K. Rajagopal, Nucl. Phys. A661 (1999) 150c
[14] http://www­linux.gsi.de/~hoehne/report/cbmtsr_public.pdf