Quarkonia and Deconfined Quark–Gluon Matter in Heavy-Ion Collisions

Transition to hot quark matter in relativistic heavy-ion collision

Results obtained from the search for the critical point of strongly interacting matter in relativistic heavy-ion collisions at the CERN-SPS (experiments NA49 and NA61/SHINE) and the RHIC beam energy scan program (experiments STAR and PHENIX) are discussed. Although some intriguing signals were found, establishing firm evidence for the critical point remains a challenging enterprise.

The NICA (Nuclotron-based Ion Collider fAcility) project is under realization at the Joint Institute for Nuclear Research (JINR, Dubna). The main goal of the project is an experimental study of hot and dense strongly interacting matter in heavy ion (up to Au) collisions at center-of-mass energies up to 11 GeV per nucleon. The physics program will be performed at two experiments, BM@N (Baryonic Matter at Nuclotron) at beams extracted from the Nuclotron, and MPD (Multi-Purpose Detector) at the NICA collider. The aim of the BM@N experiment is to study interactions of relativistic heavy ion beams with fixed targets. The scientific program comprises studies of nuclear matter in the intermediate energy range between experiments at the SIS and NICA/FAIR facilities. The BM@N experiment has recorded first experimental data in the carbon, argon and krypton beams of kinetic energy per nucleon ranging from 2.3 to 4.5 GeV per nucleon. The first measurement of short range correlations of nucleons in carbon nucleus was performed in inverse kinematics with the carbon beam and liquid hydrogen target. The MPD detector is under construction to study hot and baryon rich QCD matter in heavy ion collisions at the NICA collider in the energy range $\sqrt{s_{NN}} = 4 \div 11$ GeV. Physics program includes the study of collective phenomena, $\Lambda$ polarization, dilepton, hyperon and hypernuclei production under extreme conditions of highest baryonic density.

Studies of extremely dense matter in heavy-ion collisions at J-PARC

Thermalized matter created in noncentral relativistic heavy-ion collisions is expected to be tilted in the reaction plane with respect to the beam axis. The most notable consequence of this forward-backward symmetry breaking is the observation of rapidity-odd directed flow for charged particles. On the other hand, the production points for heavy quarks are forward-backward symmetric and shifted in the transverse plane with respect to the fireball. The drag on heavy quarks from the asymmetrically distributed thermalized matter generates substantial directed flow for heavy flavor mesons. We predict a very large rapidity-odd directed flow of D mesons in noncentral Au-Au collisions at sqrt[s_{NN}]=200 GeV, several times larger than for charged particles. A possible experimental observation of a large directed flow for heavy flavor mesons would represent an almost direct probe of the three-dimensional distribution of matter in heavy-ion collisions.

The deconfined state of QCD matter in heavy-ion collisions has been a topic of paramount interest for many years. Quarkonium suppression in heavy-ion collisions at the Relativistic Heavy-Ion Collider (RHIC) and Large Hadron Collider (LHC) experiment indicates the formation of quark-gluon plasma (QGP) in heavy-ion collision experiments. Experiments at LHC have given indications of deconfined QCD matter effect in asymmetric p−Pb nuclear collisions. Here, we use a theoretical model to investigate the bottomonium suppression in Pb−Pb, 2.76 TeV, 5.02 TeV and in p−Pb at 5.02 TeV center-of-mass energies under a QGP formation scenario. Our current formulation is based on a unified model consisting of suppression due to color screening, gluonic dissociation along with collisional damping. Regeneration due to correlated quark and antiquark pairs has also been taken into account in the current work. We obtain here the net bottomonium suppression in terms of survival probability under the combined effect of suppression plus regeneration in the deconfined QGP medium. We concentrate here on the centrality, number of participants and transverse momentum, and \(p_{T}\) dependence of bottomonium 1S and 2S state suppression in Pb−Pb and p−Pb collisions at mid-rapidity. We compare our model predictions for bottomonium 1S and 2S suppression to the corresponding experimental data obtained from the LHC. We find that the experimental observations on transverse momentum \(p_{T}\)- and centrality \(N_{PART}\)-dependent suppression agree reasonably well with our model predictions.

Detecting Strangeness -4 Dibaryon States

Photons from thermalizing matter in heavy ion collisions

Using the test-particle method to solve the transport equation derived from the Nambu-Jona-Lasino (NJL) model, we study how phase separation occurs in expanding quark matter like that in a heavy ion collision. To test our method, we first investigate the growth rates of unstable modes of quark matter in a static cubic box and find them to agree with the analytical results that were previously obtained using the linear response theory. In this case, we also study the higher-order scaled density moments in the quark matter, which have values of 1 for a uniform density distribution or a distribution where the nonzero density regions all have same value, and they are found to increase with time and saturate at values significantly larger than 1 after the phase separation. The skewness of the quark number event-by-event distribution in a small subvolume of the system is also found to increase, but this feature disappears if the subvolume is large. For the expanding quark matter, two cases are considered: one using a blast-wave model for the initial conditions and the other using initial conditions from a multiphase transport (AMPT) model. In both cases, we find that the expansion of the quark matter is slowed down by the presence of a first-order phase transition. Also, density clumps appear in the system and the momentum distribution of partons becomes anisotropic, which can be characterized by large scale density moments and nonvanishing anisotropic elliptic and quadrupolar flows, respectively. The large density fluctuations further lead to an enhancement in the dilepton yield. In the case with the AMPT initial conditions, the presence of a first-order phase transition also results in a narrower distribution of partons in rapidity. These effects of density fluctuations can be regarded as possible signals for a first-order phase transition that occurs in the baryon-rich quark matter formed in relativistic heavy ion collisions.

Strangelets in heavy ion physics

The suppression of single inclusive hadron spectra in heavy-ion collision as compared to the scaled expectation from proton-proton collisions has long been regarded as an interesting tool to study properties of the bulk matter in heavy-ion collisions. However, the limitations of this class of observables has become increasingly obvious, and both experimental and theoretical efforts are now made to go beyond single hadrons to fully reconstructed jets. Monte-Carlo simulations of in-medium parton showers are currently considered the most promising tool to theoretically access jet physics in heavy-ion collisions. In this paper, I review some of the first results obtained with the MC code YaJEM (Yet another Jet Energy-loss Model) for both single hadron and jet observables.

The relativistic heavy-ion collider (RHIC) was constructed to achieve an asymptotic state of nuclear matter in heavy-ion collisions, a near-ideal gas of deconfined quarks and gluons denoted quark–gluon plasma or QGP. RHIC collisions are indeed very different from the hadronic processes observed at the Bevalac and AGS, but high-energy elementary-collision mechanisms are also non-hadronic. The two-component model (TCM) combines measured properties of elementary collisions with the Glauber eikonal model to provide an alternative asymptotic limit for A–A collisions. RHIC data have been interpreted to indicate formation of a strongly-coupled QGP (sQGP) or "perfect liquid". In this review, I consider the experimental evidence that seems to support such conclusions and alternative evidence that may conflict with those conclusions and suggest different interpretations.

QCD sum rules predict that the change of the strange quark condensate 〈¯ss〉 in hadron matter at finite baryon density causes a shift of the peak position of the di-electron spectra from φ-meson decays. Due to the expansion of hadron matter in heavy-ion collisions, the φ peak suffers a smearing governed by the interval of density in the expanding fireball, which appears as an effective broadening of the di-electron spectrum in the φ region. The emerging broadening is sensitive to the in-medium change of 〈¯ss〉. This allows to probe directly in-medium modifications of 〈¯ss〉 via di-electron spectra in heavy-ion collisions at SIS energies with HADES.

Deconfined QCD matter in heavy-ion collisions has been a topic of paramount interest for many years. Quarkonia suppression in heavy-ion collisions at the relativistic Heavy Ion Collider (RHIC) and Large Hadron Collider (LHC) experiments indicate the quark-gluon plasma (QGP) formation in such collisions. Recent experiments at LHC have given indications of hot matter effect in asymmetric p–Pb nuclear collisions. Here, we employ a theoretical model to investigate the bottomonium suppression in Pb–Pb at sqrt{s_{NN}}=2.76, 5.02 TeV, and in p–Pb at sqrt{s_{NN}}=5.02 TeV center-of-mass energies under a QGP formation scenario. Our present formulation is based on an unified model consisting of suppression due to color screening, gluonic dissociation along with the collisional damping. Regeneration due to correlated Qbar{Q} pairs has also been taken into account in the current work. We obtain here the net bottomonium suppression in terms of survival probability under the combined effect of suppression plus regeneration in the deconfined QGP medium. We mainly concentrate here on the centrality, N_text {part} and transverse momentum, p_{T} dependence of Upsilon (1S) and Upsilon (2S) states suppression in Pb–Pb and p–Pb collisions at mid-rapidity. We compare our model predictions for Upsilon (1S) and Upsilon (2S) suppression with the corresponding experimental data obtained at the LHC energies. We find that the experimental observations on p_t and N_text {part} dependent suppression agree reasonably well with our model predictions.

Anisotropic flow phenomena are a key probe of the existence of Quark-Gluon Plasma. Several new observables associated with correlations between anisotropic flow harmonics are developed, which are expected to be sensitive to the initial fluctuations and transport properties of the created matter in heavy-ion collisions. I review recent developments of correlations of anisotropic flow harmonics. The experimental measurements, together with the comparisons to theoretical model calculations, open up new opportunities of exploring novel QCD dynamics in heavy-ion collisions.