Transverse momentum and pseudorapidity dependence of particle production in Xe-Xe collision at 5.44 TeV

Through the collision-system configuration, the Tsallis statistics is combined with a multisource thermal model. The improved model is used to investigate the transverse momentum and pseudorapidity of particles produced in Xe-Xe collisions at 5.44 TeV. We discuss detailedly the thermodynamic properties, which are taken from the transverse momentum distributions of pion, kaon and proton for different centralities. The pseudorapidity spectra of charged particles for different centralities are described consistently in the model. And, the model result can estimate intuitively the longitudinal configuration of the collision system. PACS numbers: 13.85.-t, 14.40.-n, 12.40.Ee


Introduction
e important goal of the ultrarelativistic heavy-ion collisions is to find and study the Quark-Gluon Plasma (QGP), which is a new matter state of strongly interacting quarks and gluons at high temperature and density [1][2][3]. From 2010 to 2019, the Large Hadron Collider (LHC) has mainly carried out p-p, p-Pb, and Pb-Pb collision experiments at various collision energies, which can provide different types of collision-system configurations. In 2017, the LHC performed a different kind of hadron collision at high energy, i.e. the first Xe 129 ion collisions at 푠 = 5.44 TeV [4][5][6][7]. Since the nucleons of the Xe 129 nucleus is fewer than that of Pb 208 nucleus, the investigation of Xe-ion collisions can roughly bridge or connect the gap between and Pb ion collisions. As a good intermediate-size system, the Xe-Xe colliding system brings a wonderful opportunity to discuss the colliding-system size dependence of multiparticle production in high-energy nuclear collisions [8,9]. e nucleus collisions at high energies offer numerous experimental data about charged particle production, such as pions, kaons, and protons. e particle production in the collision contains the interaction effects between hard and so QCD processes. e feature discussion of the particle distribution can be used to infer the evolution and dynamics of different collision systems at different center of mass energies.
With respect to the final-state observables in these collisions, the particle transverse momentum and pseudorapidity multiplicity are two key measurements to understand the particle-production process and the matter evolution under the extreme conditions. e transverse momentum spectra are very important because they can provide essential information about QGP created in the collisions. e charged-particle pseudorapidity multiplicity is related to the early geometry of the collision system and is of great interest to investigate the properties of the collision-system evolution. Recently, the ALICE Collaboration measured charged-particle transverse momentum spectra and multiplicity density in Xe-Xe collisions at 푠 = 5.44 TeV at the LHC [5,6]. In this work, the transverse momentum spectra are analyzed in an improved multisource thermal model, where the Tsallis statistics [10][11][12][13] is imported. Combined with the collision picture, we also discuss the charged-particle pseudorapidity density for different collision centralities. e investigation of the particle production in different collision systems can help us understand the matter evolution in the different collisions.

The Particle Spectra in the Improved Multisource Thermal Model
In high-energy nucleon or nuclei collisions, the thermodynamic information of the system evolution is very rich. ese identified particles produced in the collisions may be regarded as a multiparticle system. e identified particles emit from Hindawi Advances in High Energy Physics Volume 2020, Article ID 9734320, 5 pages https://doi.org/10.1155/2020/9734320 Advances in High Energy Physics 2 different sources. We can assume that many emission sources are formed in the interacting system [14][15][16][17]. In the stationary reference frame of a considered source, the distribution function of the particle momentum ὔ is given by where , , and is the normalization constant, the temperature and the nonequilibrium degree parameter, respectively. e value is close to 1. For the chemical potential 휇 = 0, the distribution function is When tends to 1, the density function is the standard Boltzmann distribution. e particle momentum function 耠 = 1 can be obtained by the Monte Carlo calculation, ∫ is where ὔ and 푝 ὔ = 푝 ὔ cos휃 ὔ is the energy and longitudinal momentum, respectively. e pseudorapidity and the transverse momentum are where 휃 耠 = arctan [2 푟 2 1 − 푟 2 /(1 − 2푟 2 )] is the particle emission angle and is calculated by the Monte Carlo method. e parameter 2 is a random number distributed evenly in [0, 1]. Due to = ὔ , the distribution function of the particle transverse-momentum in the laboratory reference system frame is In contrast to the transverse momentum, the particle pseudorapidity in the laboratory reference system frame is not easy to calculate. Since ὔ is a result of the source reference frame, one source is only considered in Equation (1). For the calculation of the pseudorapidity, the space scale of the collision system cannot be ignored at the pseudorapidity space. Along the beam, these sources can be grouped into four categories as follows: a projectile leading-particle source with a pseudorapidity shi , a projectile cylinder composed of a series of sources with pseudorapidity shi s (휂 min ≤ 휂 ≤ 휂 max ), a target cylinder composed of a series of sources with pseudorapidity shi s 휂 min ≤ 휂 ≤ 휂 max and a target leading-particle .
. source with a pseudorapidity shi . In the laboratory reference system frame, the Monte Carlo pseudorapidity function of particles from the four parts can be written as By the ὔ distribution function Equation (1), we can obtain the source pseudorapidity ὔ in the stationary reference frame.
en, the pseudorapidity distribution in the laboratory reference frame can be derived from the space scale of the collision system, which is described by the collision Equations (9) and (10).  Tables 1-3 with 휒 2 /ndf and increase with the increase of the collision centrality. e differential cross sections for different collision centralities are governed by the temperature , where the reaction system freezes out and the considered particles will no longer interact. e particles at low region are more close to a thermal equilibrium and the particles at high region are more close to be produced in a hard scatterings, which is determined by pQCD [18,19]. e temperature is used to reflect quantitatively the excitation of emission sources of final-state particles.

Discussions and Conclusions
From pions to protons, these particle masses affect the slope of the transverse momentum spectra. So, the temperature and nonequilibrium degree depend on the final-state particle mass. With increasing particle mass, the temperature increases generally and the nonequilibrium degree decreases. e mass dependence may originate from the deformed nuclei, Xe. With the matter produced in the collision moving at a finite velocity, the Lorentz-boost magnitude of the momentum distribution occurs obviously and is proportional to the particle mass. erefore, the values of , and systems are different. is shows how close the three systems are to the kinetic equilibrium. Figure 2 shows pseudorapidity spectra of charged particles produced in Xe-Xe collisions at 푠 = 5.44 TeV. e filled circles indicate the experimental data [6] for twelve centrality bins (from 0% to 2.5% central collisions to 80-90% peripheral collisions). e lines are the results of the Equations (7)- (10). e heights of the pseudorapidity spectra exhibit strong centrality dependences. It is because the number of observed particles is approximately proportional to the number of collision participant nucleons, which is a function of the impact parameter [20][21][22][23]. e configuration parameters of the thermalized cylinder are shown in Table 4. e max and min slightly increase with collision centralities. e pseudorapidity distributions of the peripheral collision are wider than that of the most central collision. So, the length of the thermalized cylinder at space decreases with the impact parameter. It means the number of thermal sources produced in Xe-Xe collision increases with centralities. e source  Advances in High Energy Physics 4 dependence are discussed. Based on the result, the pseudorapidity spectra of charged particles are reproduced. e model can describe both transverse momentum spectra and pseudorapidity spectra. e configuration of the intermediate-size collision system is quantized visually by the collision picture, which can characterize the primary properties of the collision system.

Data Availability
Our paper is a theoretical investigation. is paper has explained how to calculate the theoretical results in detail.

Conflicts of Interest
e authors declare that they have no conflicts of interest.
contributions from different categories are seen intuitively and the configuration of the collision system is quantized visually. It helps us understand the influence of the collision-system size and the evolution information of the produced matter in the collision [24,25].
In reference [6], the experimental data of the pseudorapidity spectra in Xe-Xe collision at 푠 = 5.44 TeV are first presented. In this paper, the Tsallis statistics is combined with the collision-system configuration, i e., the multisource thermal model. e improved model is used to investigate the particle production in the intermediate-size collision system, Xe-Xe collision [20,26]. By the study of the transverse momentum distributions of , and , the temperature and nonequilibrium degree are obtained.
e centrality dependence and the particle mass T 1: Values of parameters used in Figure 1(a). e unit of is GeV.  [6]. e lines are the results of the Equations (7)-(10).