Narrow Results

In this paper, we study the effect of the dynamical screening of the four-quark interaction on the chiral phase transition and the baryon number fluctuations at finite temperature and magnetic field in an effective model inspired by QCD in the Coulomb gauge. The screening leads to a medium-dependent coupling that generates inverse magnetic catalysis at finite magnetic field. We observe the decreasing temperature of the chiral crossover, in agreement with lattice QCD predictions, and find a critical point for large magnetic fields. We also observe a strong enhancement of the baryon number fluctuations in the vicinity of the crossover.

At a critical finite chemical potential and low temperature QCD undergoes the chiral restoration phase transition. The folklore tradition is that simultaneously hadrons are deconfined and there appears the quark matter. We demonstrate that it is possible to have confined but chirally symmetric hadrons at a finite chemical potential and hence beyond the chiral restoration point at a finite chemical potential and low temperature there could exist a chirally symmetric matter consisting of chirally symmetric but confined hadrons. If it does happen in QCD, then the QCD phase diagram should be reconsidered with obvious implications for heavy ion programs and astrophysics.

We review recent results from the RHIC beam energy scan (BES) program, aimed to study the Quantum Chromodynamics (QCD) phase diagram. The main goals are to search for the possible phase boundary, softening of equation of state or first order phase transition, and possible critical point. Phase-I of the BES program has recently concluded with data collection for Au+Au collisions at center-of-mass energies of 7.7, 11.5, 19.6, 27 and 39 GeV. Several interesting results are observed for these lower energies where the net-baryon density is high at the mid-rapidity. These results indicate that the matter formed at lower energies (7.7 and 11.5 GeV) is hadron dominated and might not have undergone a phase transition. In addition, a centrality dependence of freeze-out parameters is observed for the first time at lower energies, slope of directed flow for (net)-protons measured versus rapidity shows an interesting behavior at lower energies, and higher moments of net-proton show deviation from Skellam expectations at lower energies. An outlook for the future BES Phase-II program is presented and efforts for the detailed study of QCD phase diagram are discussed.

The Roberge–Weiss (RW) phase transition of (2 + 1) flavor QCD at imaginary quark chemical potentials $({\mu }_u,{\mu }_d,{\mu }_s)=(i𝜃T,i𝜃T,i𝜃T)$ is investigated by employing the Polyakov loop extended Quark Meson model (PQM), where $T$ is temperature, and $𝜃$ is a dimensionless chemical potential. We calculate some thermodynamic quantities and draw the phase diagram. This work can be considered as a supplement of studying the RW transition by using the effective model.

In this paper, a search for power-law fluctuations with fractality and intermittency analysis to explore the QCD phase diagram and the critical point is summarized. Experimental data on self-similar correlations and fluctuations with respect to the size of phase–space volume in various high energy heavy-ion collisions are presented, with special emphasis on background subtraction and efficiency correction of the measurement. Phenomenological modeling and theoretical work on the subject are discussed. Finally, we highlight possible directions for future research.

We use the linear sigma model coupled to quarks, together with a plausible location of the critical end point (CEP), to study the chiral symmetry transition in the QCD phase diagram. We compute the effective potential at finite temperature and density up to the contribution of the ring diagrams, both in the low and high temperature limits, and use it to compute the pressure and the position of the CEP. In the high temperature regime, by comparing to results from extrapolated lattice data, we determine the model coupling constants. Demanding that the CEP remains in the same location when described in the high temperature limit, we determine again the couplings and the pressure for the low temperature regime. We show that this procedure gives an average description of the lattice QCD results for the pressure and that the change from the low to the high temperature domains in this quantity can be attributed to the change in the coupling constants which in turn we link to the change in the effective degrees of freedom.

We construct a simple model for describing the hadron–quark crossover transition by using lattice QCD (LQCD) data in the $2+1$ flavor system, and draw the phase diagram in the $2+1$ and $2+1+1$ flavor systems through analyses of the equation of state (EoS) and the susceptibilities. In the present hadron–quark crossover (HQC) model, the entropy density $s$ is defined by $s=f_{\mathrm{\text{H}}}{s}_{\mathrm{\text{H}}}+(1-f_{\mathrm{\text{H}}})s_{\mathrm{\text{Q}}}$ with the hadron-production probability $f_{\mathrm{\text{H}}}$, where $s_{\mathrm{\text{H}}}$ is calculated by the hadron resonance gas model that is valid in low temperature $(T)$ and $s_{\mathrm{\text{Q}}}$ is evaluated by the independent quark model that explains LQCD data on the EoS in the region $400\lesssim T\le 500\mathrm{\text{MeV}}$ for the $2+1$ flavor system and $400\lesssim T\le 1000\mathrm{\text{MeV}}$ for the $2+1+1$ flavor system. The $f_{\mathrm{\text{H}}}$ is determined from LQCD data on $s$ and susceptibilities for the baryon-number $(B)$, the isospin $(I)$ and the hypercharge $(Y)$ in the $2+1$ flavor system. The HQC model is successful in reproducing LQCD data on the EoS and the flavor susceptibilities ${\chi }_{f{f}^{\prime }}^{(2)}$ for $f$, $f^{\prime }=\mathrm{\text{u}}$, $\mathrm{\text{d}}$, $\mathrm{\text{s}}$ in the $2+1+1$ flavor system, without changing the $f_{\mathrm{\text{H}}}$. We define the hadron–quark transition temperature with $f_{\mathrm{\text{H}}}=1/2$. For the $2+1$ flavor system, the transition line thus obtained is almost identical in ${\mu }_B\text{–}T$, ${\mu }_I\text{–}T$, ${\mu }_Y\text{–}T$ planes, when the chemical potentials ${\mu }_{\alpha }$$(\alpha =B,I,Y)$ are smaller than 250 MeV. This $BIY$ approximate equivalence is also seen in the $2+1+1$ flavor system. We plot the phase diagram also in ${\mu }_{\mathrm{\text{u}}}\text{–}T$, ${\mu }_{\mathrm{\text{d}}}\text{–}T$, ${\mu }_{\mathrm{\text{s}}}\text{–}T$, ${\mu }_{\mathrm{\text{c}}}\text{–}T$ planes in order to investigate flavor dependence of transition lines. In the $2+1+1$ flavor system, $\mathrm{\text{c}}$ quark does not affect the $2+1$ flavor subsystem composed of $\mathrm{\text{u}}$, $\mathrm{\text{d}}$, $\mathrm{\text{s}}$. Temperature dependence of the off-diagonal susceptibilities and the $f_{\mathrm{\text{H}}}$ show that the transition region at ${\mu }_{\alpha }=0$ is $170\lesssim T\lesssim 400\mathrm{\text{MeV}}$ for both the $2+1$ and $2+1+1$ flavor systems.

Classification of processes in heavy-ion collisions in the CBM experiment (FAIR/GSI, Darmstadt) using neural networks is investigated. Fully-connected neural networks and a deep convolutional neural network are built to identify quark–gluon plasma simulated within the Parton-Hadron-String Dynamics (PHSD) microscopic off-shell transport approach for central Au+Au collision at a fixed energy. The convolutional neural network outperforms fully-connected networks and reaches 93% accuracy on the validation set, while the remaining only 7% of collisions are incorrectly classified.

In high energy particle colliders, detectors record millions of points of data during collision events. Therefore, good data analysis depends on distinguishing collisions which produce particles of interest (signal) from those producing other particles (background). Machine learning algorithms in the current times have become popular and useful as the method of choice for such large scale data analysis. In this work, we propose and implement an artificial neural network architecture to achieve the task of identifying precisely the parent particles from all the candidates arising out of track reconstruction from collision data in the future Compressed Baryonic Matter (CBM) experiment. Our framework performs comparably to the existing computational algorithm for this task even with a simple network architecture.

Understanding the phase diagram of the QCD matter is one of the ultimate goals in high-energy nuclear physics. Event-by-event fluctuations of conserved charges are believed to be sensitive to the QCD phase structures. In this paper, we will review the measurements carried out in the Beam Energy Scan program at RHIC. Our focus will be on the technical details to overcome various difficulties of the measurements. We will also discuss about the current interpretations on the results and future prospects.

At thermal equilibrium, different chemical freeze-out conditions have been proposed so far. They have an ultimate aim of proposing a universal description for the chemical freeze-out parameters ($T_{ch}$ and ${\mu }_b$), which are to be extracted from the statistical fitting of different particle ratios measured at various collision energies with calculations from thermal models. A systematic comparison between these conditions is presented. The physical meaning of each of them and their sensitivity to the hadron mass cuts are discussed. Based on availability, some of them are compared with recent lattice calculations. We found that most of these conditions are thermodynamically equivalent, especially at small baryon chemical potential. We propose that further crucial consistency tests should be performed at low energies. The fireball thermodynamics is another way of guessing conditions describing the chemical freeze-out parameters extracted from high-energy experiments. We endorse the possibility that the various chemical freeze-out conditions should be interpreted as different aspects of one universal condition.

The in-medium modifications of hadron properties such as masses and decay widths have been a major focus of the scientific work of Gerry Brown and the insights gained by him and his collaborators made them major drivers of this field for several decades. Their prediction of experimental signals in di-lepton pair production in relativistic heavy-ion collisions was instrumental in initiating large experimental campaigns which continue till today. In this chapter, we review recent results which elucidate the relation of hadronic spectral properties at finite temperature and density to the restoration of spontaneously broken chiral symmetry.

We investigate the role of suspected resonance states, that are yet to be confirmed experimentally, on different thermodynamic quantities as well as the higher-order fluctuations and the correlation between conserved charges using ideal hadron resonance gas (HRG) model. We have discussed the temperature dependence of the various thermodynamic quantities and compared them with the lattice QCD result. We observe that the values of the bulk thermodynamic variables such as pressure, energy density, entropy density and second-order susceptibilities are increased by the inclusion of the additional resonances. Further, we find that the hadronic phase of lattice QCD result of ${\chi }_S^2,{\chi }_{BS}^{11}$ and ${\chi }_{QS}^{11}$ can be described well in ideal HRG model with the additional resonances. We have also studied the $\sqrt{s_{NN}}$ dependence of the fluctuation observables of net-proton, net-kaon and net-charge. Proper experimental acceptance cuts have been used in the model to compare with the data from experimental measurements. It has been observed that the experimental data of lower-order fluctuation observable can be described well by the ideal HRG model. However, higher-order fluctuation observables cannot be described by ideal HRG model for all $\sqrt{s_{NN}}$ studied here, which indicates the possibility of presence of critical or nonequilibrium physics for those energies. The effect of additional resonances on fluctuation observables of net-charge at different $\sqrt{s_{NN}}$ has also been studied. Finally, the chemical freeze-out parameters have been extracted from the experimental data of ${\sigma }^2/M$ of net-proton and net-charge using two different sets of hadronic spectra. We find that for both the sets, the extracted temperature is slightly lower than those obtained from the hadronic yields. Moreover, it is observed that the extracted temperature of the system gets further reduced with addition of more resonances.

The Compressed Baryonic Matter (CBM) experiment will investigate high-energy heavy-ion collisions at the international Facility for Antiproton and Ion Research (FAIR), which is under construction in Darmstadt, Germany. The CBM research program is focused on the exploration of QCD matter at neutron star core densities, such as study of the equation-of-state and the search for phase transitions. Key experimental observables include (multi-) strange (anti-) particles, electron-positron pairs and dimuons, particle correlations and fluctuations, and hyper-nuclei. In order to measure these diagnostic probes multi-differentially with unprecedented precision, the CBM detector and data acquisition systems are designed to run at reaction rates up to 10 MHz. This requires the development of fast and radiation hard detectors and readout electronics for track reconstruction, electron and muon identification, time-of-flight (TOF) determination and event characterization. The data are read-out by ultra-fast, radiation-tolerant, and free-streaming front-end electronics, and then transferred via radiation-hard data aggregation units and high-speed optical connections to a high-performance computing center. A fast and highly parallelized software will perform online track reconstruction, particle identification and event analysis. The components of the CBM experimental setup will be discussed and results of physics performance studies will be presented.

The mission of the Compressed Baryonic Matter (CBM) experiment at the future Facility for Antiproton and Ion Research (FAIR) in Darmstadt is to explore the QCD phase diagram at high net baryon densities likely to exist in the core of neutron stars. The CBM detector system is designed to perform multi-differential measurements of hadrons and leptons in central gold-gold collisions at beam energies between 2 and 11 A GeV with unprecedented precision and statistics. In order to reduce the systematic errors of the lepton measurements, which generally suffer from a large combinatorial background, both electrons and muons will be measured with the same acceptance. Up to now, no di-muon measurements have been performed in heavy-ion collisions at beam energies below 158 A GeV. The main device for electron identification, a Ring Imaging Cherenkov (RICH) detector, can be replaced by a setup comprising hadron absorbers and tracking detectors for muon measurements. In order to obtain a complete picture of the reaction, it is important to measure simultaneously leptons and hadrons. This requirement is fulfilled for the RICH, which has a low material budget, and only little affects the trajectories of hadrons on their way to the Time-of-Flight (TOF) detector. In contrast, the simultaneous measurement of muons and hadrons within the same experimental acceptance poses a substantial challenge. This article reviews the simulated performance of the CBM experiment for muon identification, together with the possibility of simultaneous hadron measurements.

The future gold beams with energies of up to 3.8AGeV from the Nuclotron at JINR in Dubna are well suited to study the equation of state of dense baryonic matter, and to explore the microscopic degrees-of-freedom emerging at neutron star densities. The relevant observables in heavy-ion collisions at these energies include yield and multidifferential distributions of (multi-) strange particles, collective flow of identified particles, fluctuation of conserved quantities and hypernuclei. In order to measure these observables in Au $+$ Au collisions with rates of up to 50kHz, the Baryonic Matter@Nuclotron setup will be upgraded with a highly granulated and fast hybrid tracking system, and a forward calorimeter for event plane determination. The physics program, the detector upgrades and physics performance studies will be presented.

We review recent theoretical developments relevant to heavy-ion experiments carried out within the Beam Energy Scan program at the Relativistic Heavy Ion Collider. Our main focus is on the description of the dynamics of systems created in heavy-ion collisions and establishing the necessary connection between the experimental observables and the QCD phase diagram.

The chiral phase transition temperature $T_c^0$ is a fundamental quantity of QCD. To determine this quantity we have performed simulations of (2 + 1)-flavor QCD using the Highly Improved Staggered Quarks (HISQ/tree) action on N_τ=6, 8 and 12 lattices with aspect ratios N_σ/N_τ ranging from 4 to 8. In our simulations we fix the strange quark mass to its physical value $m_s^{\text{phy}}$, and vary the values of two degenerate light quark masses m_l from $m_s^{\text{phy}}/20$ to $m_s^{\text{phy}}/160$ which correspond to a Goldstone pion mass m_π ranging from 160 MeV to 55 MeV in the continuum limit. We employ two estimators T₆₀ and T_δ to extract the chiral phase transition temperature $T_c^0$, after taking the chiral limit, thermodynamic limit and continuum limit, we present our current estimate for $T_c^0={132}_{-6}^{+3}\text{MeV}$.

We review recent theoretical developments relevant to heavy-ion experiments carried out within the Beam Energy Scan program at the Relativistic Heavy Ion Collider. Our main focus is on the description of the dynamics of systems created in heavy-ion collisions and establishing the necessary connection between the experimental observables and the QCD phase diagram.

The in-medium modifications of hadron properties such as masses and decay widths have been a major focus of the scientific work of Gerry Brown and the insights gained by him and his collaborators made them major drivers of this field for several decades. Their prediction of experimental signals in di-lepton pair production in relativistic heavy-ion collisions was instrumental in initiating large experimental campaigns which continue till today. In this chapter, we review recent results which elucidate the relation of hadronic spectral properties at finite temperature and density to the restoration of spontaneously broken chiral symmetry.