Large Directed Flow of Open Charm Mesons Probes the Three-Dimensional Distribution of Matter in Heavy-Ion Collisions.

We point out that metastable knots of deconfined hot quark matter may exist in the quark-gluon plasma due to local parity violating effects. These knots are stabilized by strong magnetic fields which induce electric currents via the chiral magnetic effect. The magnetic field in the knot resembles the spheromak plasma state which appears in the nuclear fusion devices. The size of the knot is inversely proportional to the chiral conductivity of the plasma, and the parity (P) and charge-parity (CP) symmetries are broken inside the knot. We argue that these knots may be created in noncentral heavy-ion collisions.

In non-central relativistic heavy ion collisions, the colliding nuclear system possesses a huge global orbital angular momentum in the direction opposite to the normal of the reaction plane. Due to the spin-orbit coupling in strong interaction, such a huge orbital angular momentum leads to a global spin polarization of the quark matter system produced in the collision process. The global polarization effect in high energy heavy ion collisions was first predicted theoretically and confirmed by STAR experiments at the Relativistic Heavy Ion Collider in Brookhaven National Laboratory. The discovery has attracted much attention to the study of spin effects in heavy ion collision and leads to a new direction in high energy heavy ion physics—Spin Physics in Heavy Ion Collisions. In this paper, we briefly review the original ideas, the calculation methods, the main results and recent theoretical developments in last years. First, we present a short discussion of the spin-orbit coupling which is an intrinsic property for a relativistic fermionic quantum system. Then we review how the global orbital angular momentum can be generated in non-central heavy ion collisions and how the global orbital angular momentum can be transferred to the local orbital angular momentum distribution in two limit model---Landan fireball model and Bjorken scaling model. After that, we review how we can describe the scattering process with initial local orbital angular momentum in the formalism of scattering cross section in impact parameter space and how we calculate the polarization of the quarks and antiquarks in quark gluon plasma produced in non-central heavy ion collisions after single or multiple scattering. We also give a brief review on how the global polarization can be predicted from the formalism of relativistic hydrodynamics with the generalized Cooper-Frye formula with spin. Finally, we discuss how the quark's polarization can be transferred to the final hadron's polarization. We focus on the hyperon's polarization and vector meson's spin alignment produced 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.

In non-central collisions between ultra-relativistic heavy ions, the\nfreeze-out distribution is anisotropic, and its major longitudinal axis may be\ntilted away from the beam direction. The shape and orientation of this\ndistribution are particularly interesting, as they provide a snapshot of the\nevolving source and reflect the space-time aspect of anisotropic flow.\nExperimentally, this information is extracted by measuring pion HBT radii as a\nfunction of angle with respect to the reaction plane. Existing formulae\nrelating the oscillations of the radii and the freezeout anisotropy are in\nprinciple only valid for Gaussian sources with no collective flow. With a\nrealistic transport model of the collision, which generates flow and\nnon-Gaussian sources, we find that these formulae approximately reflect the\nanisotropy of the freezeout distribution.\n

Global polarization of Λ hyperons as a probe for vortical effects in A+A collisions at HADES

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.

Heavy quarks, i.e. charm and beauty, are produced at the initial stages of heavy-ion collisions, on a time scale shorter than the medium formation time, and are sensitive to the initial angular momentum of the system and the magnetic field produced perpendicular to the reaction plane (defined by the impact parameter direction and beam direction) in non-central heavy-ion collisions. In the presence of a large angular momentum and initial magnetic field, the charm quark can be polarized. Experimentally, the heavy-flavor polarization can be probed by measuring the spin density matrix element (ρ00) of spin-1 mesons such as D*+ mesons. Any deviation of the parameter from 1/3 can be attributed to the spin alignment of the D∗+ meson. In these proceedings, we present the first measurement of the ρ00 parameter of prompt D*+ mesons in Pb–Pb collisions at √sNN = 5.02 TeV, exploiting the data sample collected by the ALICE Collaboration during LHC Run 2. A comparison with the J/ψ polarization measurement is also reported to investigate the effect of the magnetic field. Moreover, the ρ00 parameter of D*+ mesons measured in pp collisions is also presented.

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

We report the first results of elliptic (v_{2}), triangular (v_{3}), and quadrangular (v_{4}) flow of charged particles in Pb-Pb collisions at a center-of-mass energy per nucleon pair of sqrt[s_{NN}]=5.02 TeV with the ALICE detector at the CERN Large Hadron Collider. The measurements are performed in the central pseudorapidity region |η|<0.8 and for the transverse momentum range 0.2<p_{T}<5 GeV/c. The anisotropic flow is measured using two-particle correlations with a pseudorapidity gap greater than one unit and with the multiparticle cumulant method. Compared to results from Pb-Pb collisions at sqrt[s_{NN}]=2.76 TeV, the anisotropic flow coefficients v_{2}, v_{3}, and v_{4} are found to increase by (3.0±0.6)%, (4.3±1.4)%, and (10.2±3.8)%, respectively, in the centrality range 0%-50%. This increase can be attributed mostly to an increase of the average transverse momentum between the two energies. The measurements are found to be compatible with hydrodynamic model calculations. This comparison provides a unique opportunity to test the validity of the hydrodynamic picture and the power to further discriminate between various possibilities for the temperature dependence of shear viscosity to entropy density ratio of the produced matter in heavy-ion collisions at the highest energies.

We study the single electron spectra from $D-$ and $B-$meson semileptonic decays in Au+Au collisions at $\sqrt{s_{\rm NN}}=$200, 62.4, and 19.2 GeV by employing the parton-hadron-string dynamics (PHSD) transport approach that has been shown to reasonably describe the charm dynamics at RHIC and LHC energies on a microscopic level. In this approach the initial heavy quarks are produced by using the PYTHIA which is tuned to reproduce the FONLL calculations. The produced heavy quarks interact with off-shell massive partons in QGP with scattering cross sections which are calculated in the dynamical quasi-particle model (DQPM). At energy densities close to the critical energy density the heavy quarks are hadronized into heavy mesons through either coalescence or fragmentation. After hadronization the heavy mesons interact with the light hadrons by employing the scattering cross sections from an effective Lagrangian. The final heavy mesons then produce single electrons through semileptonic decay. We find that the PHSD approach well describes the nuclear modification factor $R_{\rm AA}$ and elliptic flow $v_2$ of single electrons in d+Au and Au+Au collisions at $\sqrt{s_{\rm NN}}=$ 200 GeV and the elliptic flow in Au+Au reactions at $\sqrt{s_{\rm NN}}=$ 62.4 GeV from the PHENIX collaboration, however, the large $R_{\rm AA}$ at $\sqrt{s_{\rm NN}}=$ 62.4 GeV is not described at all. Furthermore, we make predictions for the $R_{\rm AA}$ of $D-$mesons and of single electrons at the lower energy of $\sqrt{s_{\rm NN}}=$ 19.2 GeV. Additionally, the medium modification of the azimuthal angle $\phi$ between a heavy quark and a heavy antiquark is studied. We find that the transverse flow enhances the azimuthal angular distributions close to $\phi=$ 0 because the heavy flavors strongly interact with nuclear medium in relativistic heavy-ion collisions and almost flow with the bulk matter.

Kinetic and chemical equilibrations play important roles in the formation of the quark-gluon plasma (QGP) in relativistic heavy-ion collisions (HICs). These processes further influence the production of hard and electromagnetic probes in HICs, in particular, the thermalization of heavy quarks, which are produced at an extremely early time before the formation of the QGP. I calculate the drag and diffusion coefficients of heavy quarks in the prehydrodynamic quantum chromodynamic (QCD) plasma with the state-of-the-art QCD effective kinetic theory (EKT) solver. The time, momentum, and angular dependencies of these coefficients for gluon and quark contributions are presented separately, showing the effects of isotropization and chemical equilibration from the QCD plasma. I also provide a simple formula to estimate the heavy quark drag and diffusion coefficients, as well as its energy loss, within the prehydrodynamic plasma at different coupling strengths based on the attractor theory. Finally, I discuss the validity of these estimations with leading-order calculations and leading-logarithmic rescaling factors. Published by the American Physical Society 2024

Heavy quarks, i.e. charm and beauty, are produced in the initial stage of heavy-ion collisions, on a time scale shorter than the medium formation time, and are sensitive to the large initial magnetic field produced perpendicular to the reaction plane, defined by the impact parameter direction and beam direction, in non-central heavy-ion collisions. In the presence of a large initial magnetic field, the charm quark can be polarised. The quark polarisation is then expected to be transferred to the hadron in the hadronisation process. We will present the first measurement of D*+ polarisation in Pb–Pb collisions at √SNN = 5.02 TeV, exploiting the large data sample collected by the ALICE Collaboration during the LHC Run 2 and we will compare these new results with the existing J/ψ polarisation measurements.

Within a transport model using nucleon momentum profiles as the input from a parameterized isospin-dependent single-nucleon momentum distribution, with a high momentum tail induced by short-range correlations, we employ \(^{197}\)Au + \(^{197}\)Au collisions at 400 MeV/nucleon to examine the effects of the short-range correlations on the pion and flow observables in probing the nuclear symmetry energy. We investigate how reliable this isospin-dependent single-nucleon momentum distribution is and determine the corresponding parameter settings. Apart from the significant effects of the short-range correlations on the pion and flow observables that are observed, we also find that the theoretical simulations of the \(^{197}\)Au + \(^{197}\)Au collisions with this momentum distribution using two sets of parameters, extracted from the experimental analysis and the self-consistent Green’s function prediction, can reproduce the neutron elliptic flows of the FOPI-LAND experiment and the \(\pi ^-/\pi ^+\) ratios of the FOPI experiment, respectively, under the symmetry energy setting in a particular range. Therefore, we conclude that this parameterized isospin-dependent single-nucleon momentum distribution is reliable for isospin-asymmetric nuclear matter. Correspondingly, two sets of parameters extracted from both the experimental analysis and the self-consistent Green’s function prediction cannot be excluded according to the available experimental information at present.

It has been observed that the yield of neutral pions at high transverse momentum (pT $>$ 5 GeV/c) region is strongly suppressed in central Au+Au collisions at Relativistic Heavy Ion Collider (RHIC), compared to the one expected in p+p collisions. This suppression may be due to an energy loss of hard scattered partons in the medium (jet quenching), that results in a decrease of the yield at a given pT. The magnitude of the suppression would depend on the path length of scattering partons in the medium, and therefore is associated with azimuthal angle from reaction plane in non-central collisions. Studying the path length dependence of energy loss would give additional information on understanding the energy loss mechanism. We discuss the parton energy loss mechanism using the nuclear modification factor ($R_{AA}$) of neutral pion with respect to reaction plane. A new reaction plane detector was installed in the PHENIX detector in RHIC Year-7 run, and improved the reaction plane resolution. More precise measurement of the hadron suppression with respect to path length is expected using the detector. I will report about analysis status of neutral pion production in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV.