Subjects: Physics >> Nuclear Physics submitted time 2025-04-30
Abstract： The average transverse momentumof identified particle is an important observable in relativistic nucleus nucleus collision experiments. It can reflect the properties of hot matter created in the collisions and the characteristics of soft hadrons. It can also deepen our understanding and help to extract the information about the evolution of the collision system with systematic investigation. By using the phenomenological linear and exponential functions, we study the dependence of the average transverse momentaof identified particles produced at the midrapidity in relativistic nucleus nucleus collisions on the collision centrality, particle mass and the collision energy at given collision conditions, respectively, including the data from Au+Au, Cu+Cu, U+U collisions provided by the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) and from Pb+Pb collisions provided by the ALICE Collaboration at the Large Hadron Collider (LHC). The systematic study shows that the good linear relationship between the average transverse momentaof identified particles and collision centrality is universal. Therefore, it proofs that the collision centrality is a good physical quantity for studying the average transverse momenta of identified particles in relativistic nucleus nucleus collisions. It is also found that the fitting parameters of the linear function for the average transverse momentumwith the collision centrality follow a power-law relationship with the collision energy, which is universal as well. For a given collision energy and collision centrality, it is found that the average transverse momentaof identified particles have a linear relationship with the particle mass qualitatively. Few data points of the identified particles deviate from the linear relationship which may be relevant with the details of their production mechanisms. Meanwhile, it is found that the average transverse momentumof a given identified particle follows a exponential relationship with the logarithm of the collision energy at a given collision centrality, which is also universal.
Subjects: Physics >> Nuclear Physics submitted time 2025-04-17
Abstract：在极端相对论重离子碰撞条件下，精确构建有限重子化学势 区域的量子色动力学（Quantum Chromodynamics, QCD）物质状态方程（Equation of State, EoS）是当前高能核物理研究的核心难题之一。本研究提出一种基于深度学习的准部分子模型，通过构建三个深度神经网络，成功实现了零 条件下QCD状态方程的高精度重建。同时，经深入分析四阶广义磁化率 在不同温度和 下的单调性行为，大致限定了QCD临界点可能存在的区间为 (T, ) = (0.113 ± 0.019 GeV, 0.634 ± 0.11 GeV)。此外，对四阶累积量比 随碰撞能量 的依赖关系计算，其结果不仅与实验数据高度契合，还在 附近发现了极为显著的涨落现象。这一深度学习的准粒子模型，为有限重子密度下 QCD 物质的热力学与输运性质研究提供了全新的自洽理论框架，其推导出的状态方程参数不仅可以直接应用于相对论重离子对撞机束流能量扫描计划中的流体动力学模拟，还为深入探索 QCD 相图结构以及寻找临界点提供新的研究方法。
Subjects: Nuclear Science and Technology >> Radiation Physics and Technology submitted time 2025-04-03
Abstract： [Background] The average transverse momentum of identified particle is an important observable in relativistic nucleus nucleus collision experiments. It can reflect the properties of hot matter created in the collisions and the characteristics of soft hadrons. [Purpose] It can also deepen our understanding and help to extract the information about the evolution of the collision system with systematic investigation. [Methods] By using the phenomenological linear and exponential functions, we studied the dependence of the average transverse momenta of identified particles produced at the midrapidity in relativistic nucleus nucleus collisions on the collision centrality, particle mass and the collision energy at given collision conditions, respectively, including the data from Au+Au, Cu+Cu, U+U collisions provided by the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) and from Pb+Pb collisions provided by the ALICE Collaboration at the Large Hadron Collider (LHC). [Results & Conclusions] The systematic study shows that the good linear relationship between the average transverse momenta of identified particles and collision centrality is universal. Therefore, the collision centrality is a good physical quantity for studying the average transverse momenta of identified particles in relativistic nucleus nucleus collisions. It is also found that the fitting parameters of the linear function for the average transverse momentum with the collision centrality follow a power-law relationship with the collision energy, which is universal as well. For a given collision energy and collision centrality, it is found that the average transverse momenta of identified particles have a linear relationship with the particle mass qualitatively. Few data points of the identified particles deviate from the linear relationship which may be relevant with the details of their production mechanisms. Meanwhile, it is found that the average transverse momentum of a given identified particle follows an exponential relationship with the logarithm of the collision energy at a given collision centrality, which is also universal.
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Subjects: Physics >> Nuclear Physics submitted time 2025-04-02
Abstract： The average transverse momentumof identified particle is an important observable in relativistic nucleus nucleus collision experiments. It can reflect the properties of hot matter created in the collisions and the characteristics of soft hadrons. It can also deepen our understanding and help to extract the information about the evolution of the collision system with systematic investigation. By using the phenomenological linear and exponential functions, we study the dependence of the average transverse momentaof identified particles produced at the midrapidity in relativistic nucleus nucleus collisions on the collision centrality, particle mass and the collision energy at given collision conditions, respectively, including the data from Au+Au, Cu+Cu, U+U collisions provided by the STAR Collaboration at the Relativistic Heavy Ion Collider (RHIC) and from Pb+Pb collisions provided by the ALICE Collaboration at the Large Hadron Collider (LHC). The systematic study shows that the good linear relationship between the average transverse momentaof identified particles and collision centrality is universal. Therefore, it proofs that the collision centrality is a good physical quantity for studying the average transverse momenta of identified particles in relativistic nucleus nucleus collisions. It is also found that the fitting parameters of the linear function for the average transverse momentumwith the collision centrality follow a power-law relationship with the collision energy, which is universal as well. For a given collision energy and collision centrality, it is found that the average transverse momentaof identified particles have a linear relationship with the particle mass qualitatively. Few data points of the identified particles deviate from the linear relationship which may be relevant with the details of their production mechanisms. Meanwhile, it is found that the average transverse momentumof a given identified particle follows a exponential relationship with the logarithm of the collision energy at a given collision centrality, which is also universal.
Subjects: Physics >> Nuclear Physics submitted time 2023-06-12 Cooperative journals: 《核技术》
Abstract： In high-energy heavy ion collisions, quarks and gluons are released from the colliding nucleus to form a new state of nuclear matter called deconfined quark gluon plasma (QGP). To study the transition from normal nuclear matter or hadron resonance gas to QGP, non-perturbative quantum chromodynamics (QCD) must be solved on supercomputers using the lattice numerical method (lattice Quantum Chromodynamics, lattice QCD). However, lattice QCD only works for zero and small baryon chemical potential regions that can be described by the Taylor expansion and provides the nuclear equation of state (EoS) and QCD transition in these regions. For large baryon chemical potential regions that cannot be described by the Taylor expansion, lattice QCD fails to provide the nuclear EoS and QCD transition owing to the famous sign problem. Machine learning helps to study the nuclear EoS and QCD phase transition. First, machine learning can determine the nuclear EoS and QCD transition using the momentum distribution of final state hadrons in heavy-ion collisions, with data from both heavy-ion collision experiments and relativistic hydrodynamic simulations. Second, it can contribute to the direct solution of the sign problem in lattice QCD. The present paper reviews the applications of machine learning to the study of the QCD phase transition in heavy-ion collisions. This study (1) introduces nuclear EoS and QCD transition as well as the difficulty of the lattice QCD method, (2) analyzes the nuclear EoS using Bayesian analysis, (3) identifies the nuclear EoS and QCD phase transition using different types of deep neural networks (e. g., convolutional neural network, point cloud network, and many-event averaging), (4) searches for critical self-similarity using a dynamical edge convolution-based graph neural network, (5) learns the quasi-particle mass using a physically informed network and auto-differentiation, (6) discards unphysical regions in the nuclear EoS with a critical endpoint using active learning, (7) discusses unsupervised learning for the nuclear liquid-gas phase transition, (8) determines the nuclear symmetry energy in heavy-ion collisions, (9) investigates Mach cones using deep learning assisted jet tomography, and (10) accelerates the sampling of lattice QCD configurations using a physically constrained neural network while solving the sign problem in lattice QCD using deep learning.
Subjects: Physics >> Nuclear Physics submitted time 2023-06-12 Cooperative journals: 《核技术》
Abstract： We review the current status of quantum chromodynamics (QCD) properties in strong magnetic fields from lattice QCD. After a general introduction, we briefly present the implementation of a background magnetic field onto a lattice and discuss the recent progress on QCD properties at zero temperature, QCD transition temperature and inverse magnetic catalysis, and QCD phase structure in strong magnetic fields. Finally, we summarize this study.
Subjects: Physics >> Nuclear Physics submitted time 2023-06-12 Cooperative journals: 《核技术》
Abstract： We aim to study the effects of chemical potential and angular velocity on the critical endpoint of quantum chromodynamics (QCD). We used several probes (drag force, jet quenching parameter, heavy vector meson spectral function) to characterize the phase transition and studied gravitational waves from the holographic QCD phase transition in the early universe. We used different holographic QCD models to discuss the QCD phase transition, energy loss, spectral function, and gravitational waves. We found that the chemical potential and angular velocity changed the location of the critical endpoint, and the drag force and jet quenching parameter were temperature dependent and enhanced near the phase transition temperature. The magnetic field had a nontrivial effect on the spectral function. We conclude that the chemical potential decreases ωc, and the angular velocity decreases µc and the phase transition temperature. The jet quenching parameter and drag force can characterize the phase transition, and the magnetic field promotes the dissociation of heavy vector mesons. Moreover, the energy density of gravitational waves decreases as the gluon condensate increases, and the peak frequency shifts downward with increasing gluon condensate.Exploring the phase structure of QCD is an important task in high-energy heavy ion collision physics, and recently, there has been considerable interest in the QCD phase transition for rotating backgrounds.
Subjects: Physics >> Nuclear Physics submitted time 2023-06-12 Cooperative journals: 《核技术》
WU Yuanfang
LI Xiaobing
CHEN Lizhu
LI Zhiming
XU Mingmei
PAN Xue
ZHANG Fan
ZHANG Yanhua
ZHONG Yuming
Abstract： The goal of relativistic heavy-ion collisions is to determine the phase boundary of quantum chromodynamics (QCD) phase transitions. Critically sensitive observables are suggested to be higher-order cumulants of conserved charges. The non-monotonous behavior of higher cumulants was observed at the relativistic heavy-ion collider (RHIC). However, it remains unclear whether these non-monotonous behaviors are critically related. We studied the influences of non-critical fluctuations, finite system size, and limited evolution time to determine if they cause non-monotonous behavior. First, we examined the minimum statistics required for measuring the fourth cumulant. The minimum statistic obtained using the centrality bin width correction (CBWC) method was 25 M. We suggest using a 0.1% centrality bin in the CBWC method instead of each Nch. With a 0.1 centrality bin width, 1 M statistics are sufficient. We then pointed out the statistical fluctuations from the limited number of final particles. By assuming the independent emission of each positive (or negative) charged particle, the statistical fluctuations of positive (or negative) charged particles were presented by a Poisson distribution, and the statistical fluctuations of net-charged particles were their evolution. The obtained statistical fluctuations for net protons, net electronic charges, and net baryons were consistent with those from the Hadron Resonance Gas model. In addition, the measured cumulants at RHIC/STAR are dominated by these Poisson-like statistical fluctuations. At the end of this section, we suggest the pooling method of mixed events and demonstrate that the sample of mixed events accurately presents the contributions of the background. Dynamic cumulants were defined as the cumulant of the original sample minus that of the mixed sample. Dynamical cumulants were shown to simultaneously reduce the influence of the statistical fluctuations, centrality bin width effects, and detector efficiency. Second, because the system is finite, the correlation length at the critical point is not developed to infinity in contrast to the system at thermal limits. Using a Monte Carlo simulation of the three-dimensional three-state Potts model, we demonstrated the fluctuations of the second- and fourth-order generalized susceptibilities near the temperatures of the external fields of the first-, second-, and crossover regions. Both the second- and fourth-order susceptibilities showed similar peak-like and oscillationlike fluctuations in the three regions. Therefore, non-monotonic fluctuations are associated with the second-order phase transition and the first-order phase and crossover in a finite-size system. The exponent of finite-size scaling (FSS) characterizes the order of transitions or crossover. To determine the parameters of the phase transition using the FSS, we studied the behavior of a fixed point in the FSS. To quantify the behavior of the fixed point, we define the width of the scaled observables of different sizes at a given temperature and scaling exponent ratio. The minimum width reveals the position of the fixed point in the plane of the temperature and scaling exponent ratio. The value of this ratio indicates the nature of the fixed point, which can be a critical, first-order phase transition line point, or crossover region point. To demonstrate the effectiveness of this method, we applied it to three typical samples produced by a three-dimensional three-state Potts model. The results show that the method is more precise and effective than conventional methods. Possible applications of the proposed method are also discussed. Finally, because of the limited evolution time, some processes in relativistic heavy-ion collisions may not reach thermal equilibrium. To estimate the influence of the nonequilibrium evolution, we used the three-dimensional Ising model with the Metropolis algorithm to study the evolution from nonequilibrium to equilibrium on the phase boundary. The order parameter exponentially approaches its equilibrium value, as suggested by the Langevin equation. The average relaxation time is defined. The relaxation time is well represented by the average relaxation time, which diverges as the zth power of the system size at a critical temperature, similar to the relaxation time in dynamical equations. During nonequilibrium evolution, the third and fourth cumulants of the order parameter could be positive or negative depending on the observation time, which is consistent with the calculations of dynamical models at the crossover side. The nonequilibrium evolution at the crossover side lasts briefly, and its influence is weaker than that at the firstorder phase transition line. These qualitative features are instructive for experimentally determining the critical point and phase boundary in quantum chromodynamics.
Subjects: Physics >> Nuclear Physics submitted time 2023-06-12 Cooperative journals: 《核技术》
Abstract： Experimental evidences at the relativistic heavy ion collisions (RHIC) and large hadron collider (LHC) have demonstrated the formation of quark gluon plasma (QGP) in ultra-relativistic heavy-ion collisions at a small baryon chemical potential, where the phase transition from hadronic matter to QGP is suggested to be a crossover from state-of-the-art lattice quantum chromodynamics (QCD) calculations. It has been conjectured that there is a firstorder phase transition and a critical point at a finite μB region in the QCD phase diagram. This study reviewed recent progress in searching for the QCD critical point from RHIC-STAR experiments.
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