Progress in Particle and Nuclear Physics 62 (2009) 375–380

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Progress in Particle and Nuclear Physics

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Review

Probing dense baryonic matterPeter Senger ∗GSI Helmholtzzentrum für Schwerionenforschung GmbH, Planckstr. 1, 64291 Darmstadt, Germany

a b s t r a c t

The Compressed Baryonic Matter (CBM) experiment will be one of the major scientificactivities at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt. Thegoal of the CBM research program is to explore the QCD phase diagram in the regionof high baryon densities using high-energy nucleus–nucleus collisions. This approach iscomplementary to the heavy-ion research program at RHIC and LHC where QCD matter atvery high temperatures is studied. The relevant observables and the layout of the proposedCBM experimental facility will be discussed.

© 2009 Elsevier B.V. All rights reserved.

1. Introduction

High-energy heavy-ion collision experiments worldwide are devoted to the investigation of strongly interacting matterunder extreme conditions. At very high collision energies as available at RHIC and LHC the research programs concentrateon the study of the properties of deconfined QCDmatter at very high temperatures and almost zero net baryon densities. Incontrast, moderate collisions energies are required to explore the QCD phase diagram at large baryon-chemical potentials,and to search for features like the critical endpoint, the predicted first order phase transition between hadronic and partonicmatter, and the chiral phase transition. The experimental discovery of these prominent landmarks of the QCD phase diagramwould be a major breakthrough in our understanding of the physics of strongly interacting matter. Equally important isquantitative experimental information on the in-medium properties of hadrons which are expected to be modified if chiralsymmetry is restored.Scientific progress is largely driven by new observations. Therefore, several experimental programs are planned to

explore the QCD phase diagram at large baryo-chemical potentials. The STAR and PHENIX collaborations at RHIC proposeto scan the beam energies, and to search for the QCD critical endpoint [1]. For the same reason, future measurements areenvisaged at CERN-SPS with the upgraded NA49 detector (NA61) using light and medium size beams [2]. At JINR in Dubna,a heavy-ion collider project (NICA) is discussed with the goal to search for the coexistence phase of nuclear matter [3].However, due to luminosity limitations these experiments are constrained to the investigation of bulk observables. Incontrast, the Compressed Baryonic Matter (CBM) experiment at FAIR in Darmstadt is designed for the detection of bulkand rare probes, and will benefit from the high-intensity heavy-ion beams provided by the FAIR accelerators [4].

2. Exploring the QCD phase diagram

Our knowledge on QCD matter as function of temperature T and baryon-chemical potential µB is illustrated in theleft panel of Fig. 1. Full and open circles represent the freeze-out points of heavy-ion collisions as derived from statistical(thermal) model fits to measured particle yields and ratios [5]. The dashed-dotted line – which roughly follows the freeze-out points – corresponds to a constant total baryon density of nb = 0.12 fm−3. Also included are calculations of freeze-outcurves for a hadron gas at constant energy density of (ε = 500 MeV/fm3).

∗ Tel.: +49 6159 712652; fax: +49 6159 712785.E-mail address: P.Senger@gsi.de.

0146-6410/$ – see front matter© 2009 Elsevier B.V. All rights reserved.doi:10.1016/j.ppnp.2008.12.007

376 P. Senger / Progress in Particle and Nuclear Physics 62 (2009) 375–380

Fig. 1. [Color online] Left panel: The phase diagram of hadronic and quark-gluon matter in the T–µb plane. The experimental values for the chemicalfreeze-out are shown together with results of lattice QCD calculations, the predicted critical point is marked by the open triangle [8]. Right panel: Theenergy dependence of freeze-out temperature and baryon chemical potential. The lines are parameterizations for T and µb . Figures taken from [5].

The thermal model analysis of the data taken at the highest beam energies (SPS and RHIC) result in a limiting freeze-outtemperature of Tfo = 161± 4 MeV [5]. This value can be compared to lattice QCD predictions for the critical temperature Tcwhich, however, still are inconsistent (Tc = 192 ± 7 ± 4 MeV [6], and Tc = 151 ± 7 ± 4 MeV [7]). On the other hand, theL-QCD calculations agree on a crossover transition atµb = 0, and expect a critical endpoint and a first order phase transitionat finite values of µb. The open triangle in Fig. 1 represents the QCD critical endpoint predicted by Fodor and Katz [8].It is worthwhile to note that the thermal model fits to the experimental data extract freeze-out temperatures which

rise sharply up to√sNN = 7–8 GeV and then level off, while µb smoothly decreases all the way up to RHIC energies (see

right panel of Fig. 1). The observation of a limiting freeze-out temperature is nontrivial, and may indicate a change of thedegrees-of-freedom in the fireball which happens at FAIR (low SPS) beamenergies. At the same beamenergies (20–30AGeV)the kaon-to-pion ratio exhibits a sharp peak, and the inverse slope parameters of the kaon transverse mass spectra reach aplateau [9]. The coincidence of these observations at the same beam energy is very intriguing, and deserves further carefulinvestigations.

3. Towards the highest baryon densities

Very high baryon densities – which are comparable to those in the core of neutron stars – are predicted to be reached inheavy-ion collisions already at moderate beam energies. This is illustrated in Fig. 2 which depicts the density in the innervolume of central Au+ Au collisions at 20 AGeV as function of time calculated with various dynamical models: Three-FluidHydrodynamics (3-fluid), Parton–Hadron StringDynamics (PHSD), Ultrarelativistic QuantumMolecular Dynamics (UrQMD),the Quark-Gluon StringModel (QGSM), and the Giessen Boltzmann–Uehling–Uhlenbeckmodel (GiBUU) (for details see [10]and references therein). For each of these models, the dynamical evolution of head-on gold–gold collisions was calculatedand the density values at the center-ofmasswere extracted at consecutive points in time. Themaximumvalue of the densityvaries between 10 and 13 times saturation density depending on the model. A value of about 6 times saturation density isachieved for a substantial amount of time by all models. At these densities the nucleons most likely overlap, and partonicdegrees of freedommay prevail.Fig. 2 illustrates that heavy-ion collisions at FAIR energies are well suited to experimentally investigate the equation-of-

state of super-dense hadronic matter, and to search for the predicted first-order phase transition from hadronic to quark-gluon matter, the QCD critical endpoint, and the chiral phase transition. The general strategy for searching structures in theQCD phase diagram is to look for discontinuities in the excitation functions of bulk and rare observables.

4. Diagnostic probes of the high density fireball

The CBM experimentwill enter a new era of nuclearmatter research bymeasuring rare diagnostic probes never observedbefore at FAIR energies, and thus has a unique discovery potential. In order to obtain a complete picture, a comprehensiveset of observables will be measured in proton–proton, proton–nucleus, and nucleus–nucleus collisions over the full FAIRenergy range. The observables include:

P. Senger / Progress in Particle and Nuclear Physics 62 (2009) 375–380 377

Fig. 2. [Color online] The net baryon density ρ (t) at the center of a head-on Au+Au collision at 20 AGeV as obtained with the different dynamical models(see text). Taken from [10].

• Hadron yields and phase-space distributions. The excitation function (from 2 to 45 AGeV) of hadron yields and phasespace distributions (including multi-strange hyperons) will provide information about the fireball dynamics and thenuclear matter equation of state over a wide range of baryon densities. A non-monotonic behavior of the inverse slopeas function of beam energy would signal a change in the nuclear matter properties at a certain baryon density. Theinverse-slope distribution as a function of particle mass is related to the particle freeze-out time, and, hence, may helpto disentangle the early from the late collision stages.• Collective flow. The strength of the elliptic flow v2 measured as a function of transverse momentum for various particlespecies reflects the initial pressure of the system. The scaling of v2 with the quark content of the particles may serve asindication for flow generation in a partonic phase. The onset (or the disappearance) of the scaling behavior at a certainbeam energy would signal a change in the degrees-of-freedom of the matter. The vanishing of directed flow at a certainbeam energy would indicate a strong softening of the equation-of-state.• Open and hidden charm. The transport properties of open charm mesons in dense matter – which depend on theinteraction with the medium and, hence, on the structure of the medium – can be studied via the yield, the elliptic flowand the momentum distributions of charmed particles. In a baryon-dominated medium these observables are expectedto differ forD and D̄mesons. A globalmT -scaling of allmesons – in particular for strange and charmed particles – indicatesin-medium modifications which may be related to the onset of chiral symmetry restoration. The crossing of the phasetransition may be indicated by sudden changes of charm particle ratios such as the ψ ′/(J/ψ) ratio and the (J/ψ)/Dratio when measured as function of beam energy. The elliptic flow of charmonium is a measure of the initial pressure ofpartonic nature.• Dileptons. A precise measurement of the dilepton invariant mass spectrum up to about 1 GeV provides information onthe in-medium properties of the vector meson spectral function as a signal of the chiral symmetry restoration in the hotand dense matter. At higher invariant masses the spectrum contains a substantial contribution from thermal dileptonsfrom the early partonic phase. The different origin of the dileptons is also reflected in the inverse slope of their transversemomentum spectra. The experimental determination of dileptons emitted from the high-density phase of the collisionrequires the measurement (and subtraction) of contributions from very early nucleon–nucleon collision, from the dilutecorona, and from freeze-out.• Fluctuations and correlations. The presence of a phase coexistence region is expected to cause strong fluctuations fromevent to event in the charged particle number, baryon number, strangeness-to-pion ratio, average transversemomentumetc. Similar effects are predicted to occur in the vicinity of the QCD critical endpoint.

5. Selected predictions

The experimental challenge is to identify signatures for the partonic phase which survive hadronization. In the followingwe discuss two examples for observables which are generated in the early phase of the collision, and are only little affectedby final state interactions during the evolution of the fireball.

5.1. Collective flow

The measurement of the collective flow of particles emitted in heavy-ion collisions provides unique information on thespace–time evolution of the fireball. For example, the strength of the elliptic flow v2 and its dependence on the particle

378 P. Senger / Progress in Particle and Nuclear Physics 62 (2009) 375–380

Fig. 3. [Color online] Elliptic flow v2 for charged hadrons normalized to the constituent quark number versus transverse momentum per constituentquark as calculated with the AMPT transport model with string melting [12], for Au+Au collisions at a center of mass energy

√sNN = 7 GeV for an impact

parameter b = 7 fm (at midrapidity |y| < 1).

transverse momentum sheds light on the degrees of freedom which prevail in the early stage of the collision. In particular,the scaling of v2 with the number of constituent quarks observed at RHIC is interpreted as a direct signature for partoniccollectivity (see QM2008 proceedings [11]).Fig. 3 depicts the elliptic flow v2 for charged hadrons normalized to the constituent quark number as function of

transverse momentum per constituent quark calculated at midrapidity (|y| < 1) with the AMPT transport code with stringmelting [12]. The calculations were performed for mid-central Au + Au collisions at

√sNN = 7 GeV (corresponding to

25 AGeV fixed target beam energy). The full string-melting option of the AMPT code assumes that the initially producedmatter is 100% partonic. The hot and dense partonic medium generates a pressure which drives the quark flow. The v2 flowcomponent of hadrons is obtained from the v2 of the corresponding quarks using the coalescence model.The calculations presented in Fig. 3 demonstrate that approximate constituent quark number scaling is expected in the

case of an early partonic phase. In order to locate the phase transition from hadronic to partonic matter, future experimentswill have to scan carefully the beam energies measuring the elliptic flow of many particles (including multi-strange andcharmed particles), and search for the onset of constituent quark number scaling of the elliptic flow.

5.2. Charm

The measurement of hadrons containing charm quarks as a function of beam energy may provide direct evidence for adeconfinement phase transition. This is demonstrated in Fig. 4which depicts the ratio of J/ψ over the sumofD and D̄mesonsas a function of available energy in the nucleon–nucleon system as predicted by the HSD hadronic transport model [13] (forcentral Au+ Au collisions) and by the statistical hadronization model SHM [14] (for the corresponding values of T and µB).The SHM assumes complete dissociation of charmonium in the quark-gluon plasma, followed by statistical production ofJ/ψ mesons, Dmesons andΛC during hadronization.For a typical FAIR beamenergy of

√sNN = 7GeV the hadronic transportmodel (HSD) predicts a J/ψ overD+D̄ ratiowhich

is about 5 times larger than the result of the statistical hadronizationmodel (see Fig. 4).Within theHSD transportmodel boththe J/ψ meson and the D(D̄) meson production excitation functions are calculated using independent parameterizationswhich were fitted to experimental data. For example, the lowest threshold for charm production in hadronic collisions isdefined by the process p+p→ D̄+Λc+p resulting in a value of

√sthr = 5.07 GeV. In SHM, the lowest threshold is defined

by the process p+ p→ p+ p+ cc̄ resulting in a value of√sthr = 4.5 GeV. Due to the different threshold definitions in the

cross-section parameterizations, the abundance of cc̄ pairs in the SHM is about 7 times higher than the abundance of DD̄and D̄Λc pairs in HSD at a beam energy of

√sNN = 7 GeV. However, in SHM the J/ψ over D+ D̄ ratio is independent of the

total abundance of charm and anticharm quarks in the fireball (at least at FAIR energies where this number is small), anddepends only on the temperature and the baryon chemical potential.In conclusion, the J/ψ overD+ D̄ ratio as shown in Fig. 4 is sensitive to the conditions inside the reaction volume, and the

twomodels describe to extreme scenarios: a purely partonic fireball (SHM) versus a hadronic fireball (HSD). However, if onlypart of the fireball volume undergoes a deconfinement phase transition, or if the primordially produced J/ψ mesons are notfully suppressed by the plasma, the difference between the hadronic and partonic scenario will be reduced. Nevertheless,when measuring carefully the excitation function of J/ψ and D (D̄) meson production in heavy-ion collisions, their ratioshould exhibit a discontinuity when entering the deconfined phase.

P. Senger / Progress in Particle and Nuclear Physics 62 (2009) 375–380 379

Fig. 4. [Color online] Ratio of J/ψ over D + D̄ mesons as a function of available energy in the nucleon–nucleon system predicted for central Au + Aucollisions by the HSD hadronic transport model [13] and by the statistical hadronization model SHM [14] which assumes a QGP initial state.

6. The Compressed Baryonic Matter (CBM) experiment

The future Facility for Antiproton and Ion Research (FAIR) in Darmstadt/Germany will provide nuclear beams withenergies up to 45 AGeV [4]. Therefore, nucleus–nucleus collisions in the FAIR energy range arewell suited to explore the QCDphase diagram at high baryon densities. The goal of the CBM experiment is to study rare and bulk particles including theirphase-space distributions, correlations and fluctuations with unprecedented precision and statistics. These measurementswill be performed in nucleus–nucleus, proton–nucleus, and proton–proton collisions at different beam energies. Most ofthe rare probes like lepton pairs, multi-strange hyperons and charm will be measured for the first time in the FAIR energyrange.The experimental task is to identify both hadrons and leptons in a heavy-ion environment. A particular experimental

difficulty is the identification of D-mesons which is based on the selection of secondary vertices with high accuracy. Theexperimental challenge is to select rare events in nucleus–nucleus collisions with charged particle multiplicities of about1000 per central event at reaction rates of up to 10 MHz. Such measurements require fast and radiation hard detectors, fastand self-triggered read-out electronics, a high-speed data acquisition system, and online event selection based on full trackreconstruction.A schematic view of the proposed CBM experimental facility is shown in Fig. 5. Inside a large aperture dipole magnet

there is a Silicon Tracking and Vertexing System which consists of two parts: a Micro-Vertex Detector (MVD, 2 silicon pixellayers) and the Silicon Tracking System (STS, up to eight layers of silicon micro-strip detectors). The Silicon detector arrayhas to provide the capabilities for track reconstruction, determination of primary and secondary vertices, and momentumdetermination. Particle identificationwill be performedby time-of-flightmeasuredwith a large area Resistive Plate Chamber(RPC) wall. The left panel of Fig. 5 depicts the setup with the Ring Imaging Cherenkov (RICH) detector for the identificationof electrons from low-mass vector-meson decays. The Transition Radiation Detector (TRD) will provide charged particletracking and the identification of high energy electrons and positrons. The Electromagnetic Calorimeter (ECAL) will be usedfor the identification of electrons and photons. The muon detection/hadron absorber system is shown in the right panelof Fig. 5. It consists of 5 double or triple stations of highly granulated gaseous micro-pattern chambers (for example Gas-Electron Multiplier (GEM) detectors) sandwiched by iron plates with a total thickness equivalent to 13 absorption lengths.The status of detector R&D and recent results of detailed simulations are documented in [15].The measurement of rare probes such as open charm or vector mesons decaying into lepton pairs requires efficient

background suppression and very high interaction rates. In order to select events containing rare observables, the tracksof each collision have to be reconstructed and filtered online with respect to physical signatures. This concept represents aparadigm shift for data taking in high-energy physics experiments: CBM will run without hierarchical trigger system. Self-triggered read-out electronics, a high-speed data processing and acquisition system, fast algorithms, and, last but not least,extremely radiation hard detectors are indispensable prerequisites for a successful operation of the experiment. The tracksproduced in the Silicon tracking system by a central Au + Au collision at 25 AGeV are shown in the left panel of Fig. 6,whereas the right panel depicts the corresponding track reconstruction efficiency obtained with fast algorithms (for detailssee [15]).A first version of read-out electronics for the STS and GEM detectors has been developed and built. This read-out chain

consists of a self-triggered front-end chip mounted on a PCB, a readout controller, and a data acquisition system. The firsttest of Silicon Strip and GEM detectors equipped with the full read-out chain was successfully performed at GSI using aproton beam with an energy of 2.3 GeV in September 2008.

380 P. Senger / Progress in Particle and Nuclear Physics 62 (2009) 375–380

Fig. 5. [Color online] The CBM experimental facility with electron detectors RICH and TRD (left panel) and with the muon detection system (right panel).

Fig. 6. Left: Illustration of a central Au+ Au collision at 25 AGeV generated with the UrQMD model, and tracked through the CBM Silicon Detector usingthe GEANT3 transport code. Right: track reconstruction efficiency for central Au+ Au collisions at 25 AGeV using fast algorithms [15].

In summary, the goal of the CBM experiment at FAIR is to explore the phase diagram of strongly interacting matter inthe region of the highest baryon densities. The CBM research program addresses fundamental aspects of nonperturbativeQCD including the equation-of-state of highly compressed baryonic matter, the predicted first-order deconfinement phasetransition and its critical endpoint, the restoration of chiral symmetry at highbaryondensities, and the in-mediumpropertiesof hadrons. The corresponding key observables comprise low-mass vector mesons decaying into lepton pairs which serveas penetrating probes, hidden and open charm produced at threshold beam energies, (multi-) strange particles, and globalfeatures like collective flow and event-by event fluctuations. The CBM experiment will be realized until 2015 when the firstbeams from the FAIR accelerators will be available. The CBM Collaboration actually consists of more than 400 persons from54 institutions and 15 countries.

References

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