Significant efforts have been made so far to explore the phase diagram of strongly interacting matter [1]. The region of high temperatures has been investigated ex-perimentally at RHIC (Relativistic Heavy Ion Collider) [2] and LHC (Large Hadron Collider) [3] experiments using heavy-ion collisions at very high energies [4–8]. In such collisions, the matter is produced at low net baryon densities, i.e. with al-most equal amount of baryons and antibaryons. Similar conditions are predicted to exist in the early universe several microseconds after the big bang. After hadroniza-tion, the system freezes out chemically at a temperature around 160 MeV [9]. This temperature coincides with the transition temperature predicted by Lattice QCD calculations for a chiral phase transition, which is found to be a smooth cross-over from partonic to hadronic matter [10].
Model calculations predict structures in the phase diagram at large baryon chem-ical potentials, such as a first order phase transition between hadronic and partonic matter, with a critical endpoint. Figure 1.1 illustrates the result of such a model [11].
In the hadronic phase, quarks and gluons cannot exist as free particles, they are con-fined (shown in yellow). At high temperatures and densities, a new state of matter, Quark-Gluon Plasma (QGP), can be created (shown in red). In such a state, quarks and gluons are deconfined and can move freely. A quarkyonic phase is predicted to exist in between, which has properties of both high density baryonic matter and deconfined and chirally symmetric quark matter. At very high baryon chemical po-tential and low temperature, one may anticipate that the ground state of strongly
interacting matter should form Cooper pairs leading to colour superconductivity (shown in blue).
Figure 1.1: Phase diagram of strongly-interacting matter [11].
Figure 1.2: Interaction rates of existing and planned experiments devoted to exploration of the phase diagram of strongly interacting matter at high net baryon densities [12]. The CBM experiment will run at unprecedent interaction rate: 2-3 orders of magnitude higher than other experiments.
Investigation of the properties, the equation of state and the degrees of freedom of dense baryonic matter is of fundamental interest, also for our understanding of astrophysical objects, such as neutron stars and neutron star mergers [11, 13].
According to model calculations, heavy-ion collisions at moderate beam energies are very suitable to produce and to investigate strongly interacting matter at very
high net baryon densities in laboratory experiments. There are several existing and planned experiments with focus on this collision energy: the beam energy scan at STAR1 at RHIC [15], NA61 at SPS2 with light and medium size ions [16], MPD3 at the NICA4facility at JINR5 [17], HADES6 at SIS18 [18], and the BM@N7experiment at JINR [19]. However, the expected yields of the observables for the experiments mentioned above are limited by low interaction rates or detector constraints [12].
The rate capabilities of existing and future heavy-ion experiments are shown in fig. 1.2. The Compressed Baryonic Matter (CBM) experiment at FAIR (Facility for Antiproton and Ion Research) [20] is designed to run at high interaction rates (up to 10 MHz) and is capable of measuring both bulk and rare probes with high precision [21]. The CBM experiment will run with gold beam energies from 2− 11 AGeV. According to the different models predictions, in central Au+Au collisions already at 5 AGeV, the nuclear fireball will be compressed to more than 6 times saturation density and will spend a relatively long time within the phase coexistence region or even beyond (see fig. 1.3).
Figure 1.3: Evolution of the central net baryon density as a function of elapsed time. Calculations were done by different transport models and 3-fluid hydrodynamics code for central Au+Au collision at5 AGeV (left panel) and 10 AGeV(right panel) [22].
A comprehensive study of the phase diagram at high net-baryon densities is the main focus of the CBM physics program. Operating at intermediate beam energies, where baryonic matter is expected to be compressed most, CBM will be able to address the following questions:
1Solenoidal Tracker at RHIC [14]
2Super Proton Synchrotron, CERN, Switzerland
3Multi-Purpose Detector
4Nuclotron-based Ion Collider fAcility
5Joint Institute for Nuclear Research, Russia
6High Acceptance Di-Electron Spectrometer
7Baryonic Matter at Nuclotron
• Does the phase diagram of nuclear matter exhibit structures like a first order phade transition and a critical point at high densities? The following observ-ables yield information:
– The existence of a plateau in the caloric curve (the fireball temperature vs. the collision energy) would indicate a first-order phase transition [23].
The slope of the invariant mass distribution of dilepton pairs can serve as a temperature measurement [24]. The region 1−2.5 GeV/c2 refers to the thermal radiation of dilepton pairs.
– The directed flow of hadrons is sensitive to the details of the phase tran-sition [25].
– Yields of strange hadrons consistent with the thermal model are indica-tors of the phase transition (in particular Ω baryons) [26]. The equili-bration of strange baryons could not be understood in terms of hadronic two-body relaxation processes in the limited life time of the fireball. If system undergoes a transition from a partonic phase to the hadronic fi-nal state, the equilibration is driven by multi-body collisions in the high particle density regime near the phase boundary.
– Lattice QCD calculations show that high-order event-by-event fluctua-tions of conserved quantities (electrical charge, baryon number, strange-ness) are expected to be sensitive to the proximity of the critical point [27, 28].
– The existence of QGP can be confirmed by charmonium suppression [29].
This effect is expected due to colour screening of heavy quarks in the deconfined phase.
• Is there a restoration of the chiral symmetry at high densities?
– An important consequence of this effect is the in-medium modifications of hadrons. Particularly, the spectral function of vector mesons (for exam-ple, ρ-meson) will be modified [30]. The invariant mass distribution will be investigated via lepton pair measurements for different collision sys-tems. The thermal radiation includes a broadened in-medium ρ-meson, radiation from the QGP, and dileptons from multi-pion annihilation. The latter reflects ρ−a1 chiral mixing and, therefore, provides a direct link to chiral symmetry restoration.
• What is the equation of state of the nuclear matter at high net baryon density?
– This question can be answered by measurements of collective flow of hadrons, which is generated by the density gradient of the early fire-ball [31,32].
– Directed flow v1 is sensitive to the softening of the equation of state [25].
– Splitting in the elliptic flow v2 for different particle types is determined by the baryon chemical potential [33].
– Other promising observables are multi-strange hyperons, which are pro-duced in sequential collisions of Λ hyperons and kaons, and, therefore, they are sensitive to the fireball density [34]. This sensitivity is largest at lower beam energies close to or even below the production threshold in elementary collisions.