High Baryon Density Region Research Articles

Antiflow of mesons in the high baryon density region

Antiflow of mesons in the high baryon density region

The compressed baryonic matter (CBM) experiment at FAIR—physics, status and prospects

The Compressed Baryonic Matter (CBM) experiment is one of the major scientific pillars of the Facility for Antiproton and Ion Research (FAIR). The physics programme of CBM is centred around the exploration of the QCD phase diagram and nuclear matter equation-of-state in the region of high baryon densities. The respective rare probes will be accessed by using nucleus-nucleus collisions in the energy range at peak interaction rates of up to 10 MHz and a trigger-less data acquisition scheme. This article reviews CBM physics goals with the experimental observables. The progress and current status (as of January 2022 before sanctions imposed on Russia and suspension of collaboration with Russian institutes) of the comprising detector sub‐systems, including their performance in FAIR Phase‐0 experiments will also be presented.

Recent Hypernuclei Measurements in the High Baryon Density Region with the STAR Experiment at RHIC

Recent Hypernuclei Measurements in the High Baryon Density Region with the STAR Experiment at RHIC

Light Nuclei and Hyper Nuclei Collectivity Measurements at High Baryon Density Region

High energy heavy-ion collisions produce large amounts of light nuclei and hype nuclei, especially at high baryon density around collision energy of several GeV. These light nuclei and hyper nuclei carry the information of nucleon-nucleon and hyperon-nucleon interactions and affect the chemical composition and properties of the collision system. This proceeding is a brief review for the recent measurements on light nuclei collective flow, including directed flow v1, elliptic flow v2, and high order flow coefficients, at finite and high baryon densities from different experiments. The light nuclei production mechanism is discussed based on comparisons of the measurements and model calculations. The first measurement for hyper nuclei v1 at 3 GeV by STAR is also reported, which may imply the hyper nuclei are formed via the coalescence of nucleons and hyperon.

Effects of a phase transition on two-pion interferometry in heavy ion collisions at $$\sqrt {{s_{{\rm{NN}}}}} = 2.4 - 7.7\,\,{\rm{GeV}}$$

Hanbury-Brown-Twiss (HBT) correlations for charged pions in central Au+Au collisions at $\sqrt{s_\mathrm{NN}}=2.4 - 7.7~\text{GeV}$ (corresponding to beam kinetic energies in the fixed target frame from $E_{\rm{lab}}=1.23~\text{to}~30~\text{GeV/nucleon}$) are calculated using the UrQMD model with different equations of state. The effects of a phase transition at high baryon densities are clearly observed in the HBT parameters that are explored. It is found that the available data on the HBT radii, $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$, in the investigated energy region favors a relatively stiff equation of state at low beam energies which then turns into a soft equation of state at high collision energies consistent with astrophysical constraints on the high density equation of state of QCD. The specific effects of two different phase transition scenarios on the $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$ are investigated. It is found that a phase transition with a significant softening of the equation of state below 4 times nuclear saturation density can be excluded using HBT data. Our results highlight that the pion's $R_{O}/R_{S}$ and $R^{2}_{O}-R^{2}_{S}$ are sensitive to the stiffness of the equation of state, and can be used to constrain and understand the QCD equation of state in the high baryon density region.

Model Study of the Energy Dependence of Anisotropic Flow in Heavy-Ion Collisions at sNN = 2–4.5 GeV

The anisotropic flow is one of the important observables sensitive to the equation of state (EOS) and transport properties of the strongly interacting matter created in relativistic heavy-ion collisions. In this work, we report a detailed multi-differential study of the directed (v1), elliptic (v2), triangular (v3), and quadrangular (v4) flow coefficients of protons in relativistic heavy-ion collisions at sNN = 2.2–4.5 GeV using several hadronic transport models. Recent published results for Au + Au collisions at sNN = 2.4 GeV from HADES experiment and at sNN = 3.0 GeV from the STAR experiment have been used for comparison. The study motivates further experimental investigations of the anisotropic collective flow of protons and neutrons in a high baryon density region.

The Topmetal-CEE prototype, a direct charge sensor for the beam monitor of the CSR external-target experiment

The Cooler-Storage-Ring External-target Experiment (CEE) which is being constructed since 2020 is a spectrometer to study the properties of nuclear matter at high baryon density region. We present the design and simulation results of a sensor named Topmetal-CEE which is being developed for an online beam monitor of the CEE. A prototype of the Topmetal-CEE has been designed in a standard CMOS 130 nm process and is being fabricated. The Topmetal-CEE prototype has 180 channels with a pitch of 100 μm. In each channel, electrons are collected by a charge collection electrode which is a top-most metal exposed to the surrounding media, amplified by a charge sensitive amplifier, and then fed into a discriminator. The output of the discriminator is split into two paths which record Time of Arrival (ToA) and Time over Threshold (ToT) of the collected signal, respectively. In order to reduce the dead time, 180 channels are split into two separate parts and then the information in each part is read out by a data-driving priority readout scheme independently. Each channel has its own address encoded by an address encoder. The ToA, ToT and address of each channel are framed and encoded, and then transferred off chip with a serial speed of 4.4 Gbps.

Mean-field update in the JAM microscopic transport model: Mean-field effects on collective flow in high-energy heavy-ion collisions atsNN=2–20 GeV energies

The beam energy dependence of the directed flow is a sensitive probe for the properties of strongly interacting matter. Hybrid models that simulate dense (core) and dilute (corona) parts of the system by combining the fluid dynamics and hadronic cascade model describe well the bulk observables, strangeness productions, and radial and elliptic flows in high-energy heavy-ion collisions in the high baryon density region. However, the beam energy dependence of the directed flow cannot be described in existing hybrid models. We focus on improving the corona part, i.e., the nonequilibrium evolution part of the system, by introducing the mean-field potentials into a hadronic cascade model. For this purpose, we consider different implementations of momentum-dependent hadronic mean fields in the relativistic quantum molecular dynamics (RQMD) framework. First, Lorentz scalar implementation of a Skyrme type potential is examined. Then, full implementation of the Skyrme type potential as a Lorentz vector in the RQMD approach is proposed. We find that scalar implementation of the Skyrme force is too weak to generate repulsion explaining observed data of sideward flows at $\sqrt{{s}_{NN}}<10$ GeV, while vector implementation gives collective flows compatible with the data for a wide range of beam energies $2.7<\sqrt{{s}_{NN}}<20$ GeV. We show that our approach reproduces the negative proton directed flow at $\sqrt{{s}_{NN}}>10$ GeV discovered by experiments. We discuss the dynamical generation mechanisms of the directed flow within a conventional hadronic mean field. A positive slope of proton directed flow is generated predominantly during compression stages of heavy-ion collisions by the strong repulsive interaction due to high baryon densities. In contrast, at the expansion stages of the collision, the negative directed flow is generated more strongly than the positive one by the tilted expansion and shadowing by the spectator matter. At lower collision energies $\sqrt{{s}_{NN}}<10$ GeV, the positive flow wins against the negative flow because of a long compression time. On the other hand, at higher energies $\sqrt{{s}_{NN}}>10$ GeV, negative flow wins because of shorter compression time and longer expansion time. A transition beam energy from positive to negative flow is highly sensitive to the strength of the interaction.

Λ3H and Λ4H Lifetime, Yield, Directed Flow Measurements in Au+Au Collisions at √SNN = 3 GeV With the STAR Detector

In this proceedings, the lifetime and yields of Λ3H and Λ4H in Au+Au collisions at √SNN = 3 GeV are presented. The measured yields are compared to measurements at other energies and theoretical models, and the physics implications are discussed. We also report the first observation of the Λ3H and Λ4H directed flow in 5 - 40% centrality. The directed flow of Λ3H and Λ4H are compared with those of the copiously produced particles such as p, Λ, d, t, 3He, and 4He. These results shed light on light hyper-nuclei production in heavy-ion collisions in the high baryon density region.

Effects of centrality fluctuation and deuteron formation on the proton number cumulant in Au+Au collisions at = 3 GeV from the JAM model * *Supported by the National Key Research and Development Program of China (2020YFE0202002, 2018YFE0205201), the National Natural Science Foundation of China (11828500, 11828501, 11575069, 11890711, 11861131009)

We studied the effects of centrality fluctuation and deuteron formation on the cumulant ( ) and correlation functions ( ) of protons up to the sixth order in the most central ( < 3 fm) Au+Au collisions at = 3 GeV in a microscopic transport model (JAM). The results are presented as a function of rapidity acceptance within the transverse momentum 0.4 < p T < 2 GeV/ c. We compared the results obtained by the centrality bin width correction (CBWC) using charged reference particle multiplicities with the CBWC using impact parameter bins. It was found that, at low energies, the centrality resolution for determining the collision centrality using charged particle multiplicities is not sufficient to reduce the initial volume fluctuation effect for higher-order cumulant analysis. New methods need to be developed to classify events with high centrality resolution for heavy-ion collisions at low energies. Finally, we observed that the formation of deuterons suppresses the higher-order cumulants and correlation functions of protons and found it to be similar to the efficiency effect. This work can serve as a noncritical baseline for the QCD critical point search in the high baryon density region.

QCD phase diagram at finite isospin and baryon chemical potentials with the self-consistent mean field approximation * *Supported by National Natural Science Foundation of China (12075117, 11535005, 11775118, 11690030, 11905104) and National Major state Basic Research and Development of China (2016YEF0129300)

The self-consistent mean field approximation of the two-flavor NJL model, with a free parameter to reflect the competition between the "direct" channel and the "exchange" channel, is employed to study the QCD phase structure at finite isospin chemical potential , finite baryon chemical potential and finite temperature T, and especially to study the location of the QCD critical point. Our results show that in order to match the corresponding lattice results of isospin density and energy density, the contributions of the "exchange" channel need to be considered in the framework of the NJL model, and a weighting factor should be taken. It is also found that for fixed isospin chemical potentials, the lower temperature of the phase transition is obtained with increasing in the plane, and the largest difference of the phase transition temperature with different 's appears at . At the temperature of the QCD critical end point (CEP) decreases with increasing , while the critical baryon chemical potential increases. At high isospin chemical potential ( MeV), the temperature of the QCD tricritical point (TCP) increases with increasing , and in the low temperature regions the system will transition from the pion superfluidity phase to the normal phase as increases. At low density, the critical temperature of the QCD phase transition with different 's rapidly increases with at the beginning, and then increases smoothly around MeV. In the high baryon density region, the increase of the isospin chemical potential will raise the critical baryon chemical potential of the phase transition.

Study of the QCD phase structure at HIAF

High-energy nuclear collisions offer a unique opportunity for studying the properties of the nuclear matter under extreme conditions, the evolution of the universe soon after the big bang as well as the inner structure of stars. Quantum chromodynamics (QCD), the theory for strong interaction, has predicted the existence of the new form of matter, where the de-confined quarks and gluons move “freely” in a volume that is much larger than that of an ordinary nucleon, the quark-gluon plasma (QGP). After more than thirty years of searching, scientists have collected circumstantial evidences of QGP from the relativistic heavy-ion collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC). Thermodynamically one might expect phase boundary or/and critical point that accompany the new phase of matter. Nowadays the QCD phase structure, especially in the high baryon density region, has become the focus in order to search for the QCD phase boundary and possible critical point. In this article, the experimental status on the QCD phase structure will be reviewed. Discussion will include the collectivity, chirality and criticality in high-energy nuclear collisions. These will be followed by the discussion of the main physics programs for the future experiment. The CSR-External-target Experiment (CEE) at both HIRFL and HIAF accelerator complexes, will be discussed.

A Study of the Properties of the QCD Phase Diagram in High-Energy Nuclear Collisions

With the aim of understanding the phase structure of nuclear matter created in high-energy nuclear collisions at finite baryon density, a beam energy scan program has been carried out at Relativistic Heavy Ion Collider (RHIC). In this mini-review, most recent experimental results on collectivity, criticality and heavy flavor productions will be discussed. The goal here is to establish the connection between current available data and future heavy-ion collision experiments in a high baryon density region.

Effects of resonance weak decay and hadronic rescattering on the proton number fluctuations in Au+Au collisions at sNN=5GeV from a microscopic hadronic transport (JAM) model

Proton number fluctuation is sensitive observable to search for the QCD critical point in heavy-ion collisions. In this paper, we studied rapidity acceptance dependence of the proton cumulants and correlation functions in most central Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 5$ GeV from a microscopic hadronic transport model (JAM). At mid-rapidity, we found the effects of resonance weak decays and hadronic re-scattering on the proton cumulants and correlation functions are small, but those effects get larger when further increasing the rapidity acceptance. On the other hand, we found the baryon number conservation is a dominant background effect on the rapidity acceptance dependence of proton number fluctuations. It leads to a strong suppression of cumulants and cumulant ratios, as well as the negative proton correlation functions. We also studied those two effects on the energy dependence of cumulant ratios of net-proton distributions in most central Au+Au collisions at $\sqrt{s_\mathrm{NN}} = 5-200$ GeV from JAM model. This work can serve as a non-critical baseline for future QCD critical point search in heavy-ion collisions at high baryon density region.

Mapping the QCD phase diagram with statistics-friendly distributions

We demonstrate that the multiplicity distribution of a system located in the vicinity of a first-order phase transition can be successfully measured in terms of its factorial cumulants with a surprisingly small number of events. This finding has direct implications for the experimental search of a QCD phase transition conjectured to be located in the high baryon density region of the QCD phase diagram.

QCD phase diagram in chiral imbalance with self-consistent mean field approximation

We employ a new self-consistent mean field approximation of NJL model, which introduces a free parameter $\alpha$ ($\alpha$ reflects the weight of different interaction channels), to study the effects of the chiral chemical potential $\mu_5$ on QCD phase structure, especially the location of the QCD critical end point (CEP). We find that, at a high temperature, the critical temperature of QCD phase transition smoothly increases with $\mu_5$ at the beginning, and then decreases rapidly. At low temperature and high baryon density region, the increase of the chiral chemical potential will reduce the critical chemical potential of phase transition. The temperature of the CEP shows a non-monotonic dependence on the chiral chemical potential with a long plateau around the maximum. At $\mu_5=0$, we found that the CEP will disappear when the $\alpha$ value is larger than 0.71 and will reappear when the $\mu_5$ increases. This study is important for exploring the QCD phase structure and the location of CEP in chiral imbalanced systems.

The STAR BES-II and Forward Rapidity Physics and Upgrades

The second phase of the Beam Energy Scan at RHIC, BES-II, is scheduled to start in 2019 and will explore the high baryon density region of the QCD phase diagram with precision measurements. The detector upgrades will enhance some of the key measurements by extending STAR's kinematic reach, like the kurtosis of net-protons distribution which could pinpoint the position of a critical point. The upgrades currently under way comprise: the replacement of the inner TPC sectors, the Event Plane Detector (EPD) and the end-cap TOF. Building on these upgrades STAR is planning to further enhance its detector capabilities by installing a Forward Calorimeter System (FCS) integrating an electromagnetic and hadronic calorimeter and a Forward Tracking System (FTS) combining 3 Silicon mini-strip disks and 4 Small-Strip Thin Gap Chamber (sTGC) wheels. The forward upgrades are motivated by studying the initial state of nucleons and nuclei and the exploration of cold QCD physics in the very high and low x regions. This contribution will highlight these upgrades and some of the physics opportunities that they allow.

JAM: an event generator for high energy nuclear collisions

We review recent developments of an event generator JAM microscopic transport model to simulate high energy nuclear collisions, especially at high baryon density regions. Recent developments focus on the collective effects: implementation of nuclear potentials, equation of state (EoS) modified collision term, and dynamical integration of fluid dynamics. With these extensions, we can discuss the EoS dependence of the transverse collective flows.

The Projectile Spectator Detector for measuring the geometry of heavy ion collisions at the CBM experiment on FAIR

The Compressed Baryonic Matter (CBM) experiment at the future Facility for Antiproton and Ion Research (FAIR) is aimed to explore the QCD phase diagram in the region of high baryon densities . The Projectile Spectator Detector (PSD) is the forward hadron calorimeter that will be used in the CBM to measure the event centrality as well as the reaction plane orientation in heavy-ion collisions in the beam energy range of 2–11 A GeV with beam interaction rates up to 10 MHz. The supermodule consisting of 9 modules has been assembled to explore the performance of the PSD. Its response has been studied with proton beams in the momentum range 2–10 GeV/c at the CERN Proton Synchrotron (PS) T9 and T10 beam lines. For the first time, the energy resolution and the linearity of the response have been measured at such low energies. The PSD operation under radiation conditions at the CBM, as well as the results of one PSD module measurements with irradiated photodetectors are discussed. • The PSD hadron calorimeter properties have been measured at test beams. • A simulation of the PSD operation under radiation conditions is conducted. • Degradation of photodetectors as a result of neutron irradiation is discussed.

Beam energy dependence of the squeeze-out effect on the directed and elliptic flow in Au + Au collisions in the high baryon density region

We present a detailed analysis of the beam energy dependence of the mechanisms for the generation of directed and elliptic flows in Au+Au collisions focusing on the role of hadronic rescattering and spectator shadowing within a microscopic transport model JAM with different equation of state. A systematic study of the beam energy dependence is performed for Au+Au collisions at $\sqrt{s_{NN}} =2.3 - 62.4$ GeV. The transition of the dynamical origin of the directed flow is observed. We find that the initial Glauber type nucleon-nucleon collisions generate negative $v_1$ for nucleons at midrapidity due to the presence of spectator matter, and this negative nucleon $v_1$ is turned to be positive by the meson-baryon interactions at the beam energy region of $\sqrt{s_{NN}} < 30$ GeV. In contrast, above 30 GeV there is no spectator shadowing at midrapidity, and initial nucleon-nucleon collisions do not generate directed flow, but subsequent rescatterings among produced particles generate negative $v_1$ for nucleons. It is demonstrated that negative pion-directed flows are mostly generated by the interaction with the spectator matter. It is also shown that the squeeze-out effect is largely suppressed in the case of softening, which leads to the enhancement of elliptic flow around $\sqrt{s_{NN}}=5-7$ GeV. The elliptic flow at midrapidity above 10 GeV is not influenced by the squeeze-out due to spectator matter, while its effect is seen at the forward rapidity range of $y/y_\mathrm{c.m.}>0.5$, which decreases as beam energy increases.