Matter In Heavy-ion Collisions Research Articles

The present and future of QCD

This White Paper presents an overview of the current status and future perspective of QCD research, based on the community inputs and scientific conclusions from the 2022 Hot and Cold QCD Town Meeting. We present the progress made in the last decade toward a deep understanding of both the fundamental structure of the sub-atomic matter of nucleon and nucleus in cold QCD, and the hot QCD matter in heavy ion collisions. We identify key questions of QCD research and plausible paths to obtaining answers to those questions in the near future, hence defining priorities of our research over the coming decades.

Measurements of global polarization of QCD matter in heavy-ion collisions

The experimental data of the global polarization of Λ hyperon, ϕ and K<sup>*0</sup> vector mesons in high-energy heavy ion collision confirm the new phenomenon of global polarization of hot-dense QCD matter, which has attracted extensive attention from researchers and has become a new hot research direction in the frontier of high-energy nuclear physics. This paper reviews the recent global polarization measurements. We focus on the global polarization measurements of Λ hyperon and ϕ, K<sup>*0</sup> mesons, carried out by the solenoidal tracker detector (STAR) collaboration group at the Relativistic Heavy Ion Collider (RHIC) at its Phase I of Beam Energy Scan program, and extend to the global polarization measurements containing multiple strange quark particles, such as Ξ, Ω and the local polarization studies of Λ along the beam direction. In the paper, we also briefly comment on the measurements at higher energy from the large hadron collider (LHC) and at very low energy in HADES experiment. In the end of the paper, the physical information given by these experimental results is also briefly discussed.

Specific heat and its high-order moments in relativistic heavy-ion collisions from a multiphase transport model

Energy dependence of specific heat extracted from temperature fluctuation of Au + Au collisions at $\sqrt{{s}_{NN}}$ = 7.7 to 200 GeV was investigated by using a multiphase transport (AMPT) model. The results were compared with those from other models and some differences at low $\sqrt{{s}_{NN}}$ were found. To explain the above differences and describe the properties of the hot dense matter at low $\sqrt{{s}_{NN}}$, a new quantity ${C}_{v}^{*}$ was derived for describing specific heat in heavy-ion collisions. It was found that, by using ${C}_{v}^{*}$ together with its high-order moments (skewness and kurtosis), thermal properties of the hot dense matter can be described and different thermal properties with or without parton process can be clearly distinguished. The proposed observable provides a way to learn the property of QCD matter in heavy-ion collisions.

Photon production rate calculation from quark-gluon plasma using MEQM in heavy-ion collision

It is widely accepted that a huge and intense magnetic field is created along with the exotic state of matter in heavy-ion collision. The photon yield from quark-gluon plasma created in a magnetic environment is studied using a thermal quasi-particle mass. The quarks acquire this mass depending on the temperature and nonzero value of quark chemical potential which gets modified as magnetized effective quark mass (MEQM) in the presence of magnetic field. The total photon rate is shown for one loop process incorporating MEQM. It is found that the emission rate to lowest order increases strongly with increasing of the quark chemical potential and temperature. The enhanced yield in photon is also compared with the recently reported theoretical results.

Formation of large chunk of nuclear matter in heavy-ion collisions

In terrestrial laboratory with existing accelerators, the maximum multinucleon system produced so far is limited to that with total nucleon number being less than the sum of two colliding nuclei. Such situation may hinder our studies on the properties of nuclear many-body system. Here a way of producing large multinucleon system via multi-nucleus collision device Collider Plus in terrestrial laboratory is proposed. It is shown that large chunk of nucleonic matter can be produced through nuclear ternary fusion reactions at lower beam energies and also large block of dense matter can be formed via ternary collisions of heavy nuclei at intermediate energies. These investigations may shed lights on the studies of the synthesis of superheavy elements and the properties of bulk superdense nuclear matter.

Study of the Spectator Matter in Heavy Ion Collisions at the BM@N Experiment

Study of the Spectator Matter in Heavy Ion Collisions at the BM@N Experiment

Thermalization of nuclear matter in heavy-ion collisions at Fermi energies

We analyze the time evolution of the kinetic properties of nuclear matter produced in heavy-ion collisions at Fermi energies. The collision system is simulated using Constrained Molecular Dynamics (CoMD) transport calculations whose output is the isospin, position, and momentum of the nucleons. Focusing on central 35A⋅MeV 40Ca+40Ca collisions we utilize this information to extract localized momentum distributions in volume elements of 8fm3 and time steps of 5fm/c. We then parameterize the single-particle momentum distributions with thermally motivated fit functions in the local rest frame of each cell. While the transverse-momentum distributions are well reproduced by thermal ones, the longitudinal ones carry a marked imprint of the initial nuclear motion which we capture by introducing a centroid motion into our fit functions. In particular, we find that Fermi distributions yield significantly better fits than Boltzmann ones, a consequence of the Pauli blocking implemented in CoMD. From the fits we extract the time dependence of the thermodynamic and collective properties of the excited nuclear medium. We find that the transverse temperature gradually rises to about 6MeV, which is accompanied by a dissipation of the initial centroid motion of the incoming nuclei which vanishes at about 100fm/c after initial impact. We are therefore able to track the transition of beam energy into random kinetic energy for nucleons, suggesting a three-dimensional equilibration of energy in the late stages of the collision.

Smallest QCD droplet and multiparticle correlations in p−p collisions

The collective evolution of produced matter in heavy-ion collisions is effectively described by hydrodynamics from time scales greater than the inverse of the temperature, $\ensuremath{\tau}\ensuremath{\gtrsim}1/T$. In the context of the Gubser solution, I show that the hydrodynamization condition $\ensuremath{\tau}\phantom{\rule{0.16em}{0ex}}T\ensuremath{\gtrsim}1$ is translated into an allowed domain in the spatial system size and the final multiplicity for hydrodynamics applicability. It turns out that the flow measurements in $p\text{\ensuremath{-}}p$ collisions are inside the domain of validity. I predict that by approaching the boundaries of the allowed domain the hydrodynamic response to the initial ellipticity changes its sign. I follow a rather model-independent approach for the initial state where, instead of modeling the initial energy density of individual events, the initial system size and ellipticity event-by-event fluctuation are modeled. The model, initial state $\mathrm{fluctuation}+\mathrm{Gubser}$ solution$+$Cooper-Frye freeze-out, describes the multiplicity and transverse momentum dependence of two-point and four-point correlation functions (${c}_{2}{2}$ and ${c}_{2}{4}$) in an accurate agreement with $p\text{\ensuremath{-}}p$ collision experimental measurements. In particular, the sign of the four-point correlation function is the same as the observation, which failed to be described correctly in previous studies. I also predict a signal for the sign change in the hydrodynamic response that can be inspected in future experimental measurements of two-point and four-point correlation functions at lower multiplicities.

Impact of momentum resolution on factorial moments due to power–law correlations between particles

The effect of momentum resolution on factorial moments due to the power–law correlation function is studied. The study is motivated by searching for the critical point of the strongly interacting matter in heavy-ion collisions using the intermittency method. We observe that factorial moments are significantly affected by the finite momentum resolution. The effect is superficially significant compared to intuitive expectations. The results depend on the power of the correlation function and the number of uncorrelated particles.

Study of Signals of Hot and Dense Nuclear Matter in Heavy-Ion Collisions at NICA Energies Using Micro- and Macroscopic Models

Study of Signals of Hot and Dense Nuclear Matter in Heavy-Ion Collisions at NICA Energies Using Micro- and Macroscopic Models

Isentropic evolution of the matter in heavy-ion collisions and the search for the critical endpoint

We study the isentropic evolution of the matter produced in relativistic heavy-ion collisions for various values of the entropy-per-baryon ratio of interest for the ongoing and future experimental searches for the critical endpoint (CEP) in the QCD phase diagram: these includes the current beam-energy-scan (BES) program at RHIC and the fixed-target collisions foreseen for the near future at various facilities. We describe the hot-dense matter through two different effective Lagrangians: the PNJL (Polyakov–Nambu–Jona–Lasinio) and the PQM (Polyakov-quark-meson) models. We focus on quantities expected to have a direct experimental relevance: the speed of sound, responsible for the collective acceleration of the fireball, and the generalized susceptibilities, connected to the cumulants of the distributions of conserved charges. In principle they should affect the momentum spectra and the event-by-event fluctuations of the yields of identified particles. Taking realistic values for the initial temperature and the entropy-per-baryon ratio we study the temporal evolution of the above quantities looking for differences along isentropic trajectories covering different regions of the QCD phase diagram, passing far or close to the CEP or even intersecting the first-order critical line.

Examination of an isospin-dependent single-nucleon momentum distribution for isospin-asymmetric nuclear matter in heavy-ion collisions

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.

Reconstruction and simulation of the performance of the Electromagnetic Calorimeter on MPD

The main goal of MPD at NICA is to investigate the hot and dense baryonic matter in heavy-ion collisions over a wide range of atomic masses, from Au+Au collisions at a center-of-mass energy of √sNN = 11 GeV (for Au79+) to proton-proton collisions with √spp= 20 GeV. A shashlyk type Electromagnetic Calorimeter is an important detector of the MPD to identify electrons and photons, and measure their energy with high precision. The performance of the ECal in detecting photons and reconstruct π0 were simulated and analyzed. π0 signal can be reconstructed from two photons and it is a very important probe to study the chiral symmetry restoration and flow signal. The time capability of the ECal was also explored for particle identification. The performance of ECal can be got in the simulartion, energy and spatial resolution are about 6% and 4.8 mm for 1-GeV photons. Reconstructing efficiency for photon and π0 can reach to 80% and 50%. π can be separated from K for P<1.9 GeV/c and K can be separated from π and proton for P<1.1 GeV/c with an efficiency higher than 90%.

Brief History of the Search for Critical Structures in Heavy-ion Collisions

The paper briefly presents history, status, and plans of the search for the critical structures - the onset of fireball, the onset of deconfinement, and the deconfinement critical point - in high energy nucleus-nucleus collisions. First, the basic ideas are introduced, the history of the observation of strongly interacting matter in heavy ion collisions is reviewed, and the path towards the quark-gluon plasma discovery is sketched. Then the status of the search for critical structures is discussed - the discovery of the onset of deconfinement, indications for the onset of fireball, and still inconclusive results concerning the deconfinement critical point. Finally, an attempt to formulate priorities for future measurements - charm quarks vs the onset of deconfinement and detailed study of the onset of fireball - closes the paper.

Development of shashlik electromagnetic calorimeter for the NICA/MPD

The main goal of the NICA heavy-ion program at JINR is an experimental investigation into the properties of nuclear matter within the energy region of the maximum baryonic density. The Multi Purpose Detector (MPD) is one of the detectors at NICA collider, which is optimized for the study on properties of hot and dense matter in heavy-ion collisions and, in particular, the search for a manifestation of possible deconfinement and/or chiral symmetry restoration phase transitions, critical end-point and mixed quark-hadron phase. Electromagnetic calorimeter (ECal) is an important part of MPD . The particular goal of MPD-ECal is to measure position and energy of photons and electrons. Taking all factors (high energy resolution, high segmentation, large enough distance to the vertex, small Moliere radius, high magnetic field and high time resolution) into consideration, a shashlik-type electromagnetic calorimeter is selected. Therefore the tower consists of 220 layers of lead plates and scintillator plates whose thicknesses are namely 0.3mm/1.5mm. The tower cell is 4×4 cm2 in size. To reduce the dead zones effect, all towers should be cut from four sides at small angles, which will significantly improve the position resolution and the performance homogeneity of the ECal. Currently, energy deposition, energy resolution and position reconstruction are studied through Monte Carlo simulations. A prototype of shashlik Ecal has been built and the cosmic test results show that the number of photoelectrons (Npe) is around 522. By comparing with the previous ECal experimental results and simulation results, it is indirectly considered that the energy resolution of the prototype has reached 5%/√E (GeV) . Key technologies to increase light yield are being studied and will be used in the development of new prototypes.

Centrality and transverse momentum dependent suppression of Upsilon (1S) and Upsilon (2S) in p\u2013Pb and Pb\u2013Pb collisions at the CERN Large Hadron Collider

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.

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

We aim to study extremely dense matter in heavy-ion collisions at 1 −19 AGeV/c at a future project of J-PARC (J-PARC-HI). We will search for the first order phase boundary and its critical end point in the QCD phase diagram. We also aim at studying the properties of dense matter related to neutron stars and neutron star mergers, in particular the equation of state (EOS). We expect to produce the world's highest rate of 1011Hz of heavy-ion beams, with ion species from p to U. We design spectrometers based on a large dipole magnet to measure hadrons, dimuons, and hypernuclei. We evaluate some of key performance of the spectrometers based on detailed simulations.

Prospects for the study of event-by-event fluctuations at MPD/NICA project

One of the main physics goals of the Multi Purpose Detector (MPD) is to investigate hot and dense baryonic matter in heavy ion collisions at NICA energies to search for the possible critical end point (CEP). Since the location of CEP is not clear the entire accessible region of the QCD phase diagram needs to be explored by scanning the full range of available beam energies. In case of CEP existence it can be observed by abnormal fluctuations of various quantities such as net-proton multiplicity. This task requires excellent particle identification (PID) capability over as large as possible phase space volume. The identification of charged hadrons is achieved at the momenta of 0:1 – 3 GeV/c. The results of hadron identification and preliminary possibility estimation of the study of event-by-event fluctuations at MPD will be presented.

Photons from thermalizing matter in heavy ion collisions

We investigate the production of direct photons in heavy ion collisions within the modified bottom–up thermalization scenario, which we show to be related to the thermalizing Glasma framework. The dynamics of the parton system up to thermalization/equilibration can be described by two momentum scales, by means of which we express the photon invariant yield excluding the prompt pQCD contribution. We derive an analytic formula, which provides an estimate of photon production from thermalizing matter for a wide range of collision systems and energies. We compare the yield with that measured in the PHENIX Au+Au run04 and the combined run07+run10 data sets at sNN=200GeV. We also make theory-data comparisons for Pb+Pb at 2760 GeV and d+Au at 200 GeV collision energies. Finally, we make predictions for the direct photon invariant yield from collisions of U+U, Cu+Au, He3+Au at 200 GeV and Pb+Pb at 5020 GeV.

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

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.