Advances in High Energy Physics

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Particle Production in High Energy Collisions 2020

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Research Article | Open Access

Volume 2021 |Article ID 6663846 | https://doi.org/10.1155/2021/6663846

Shaista Khan, Bushra Ali, Anuj Chandra, Shakeel Ahmad, "Event-by-Event Particle Ratio Fluctuations at LHC Energies", Advances in High Energy Physics, vol. 2021, Article ID 6663846, 14 pages, 2021. https://doi.org/10.1155/2021/6663846

Event-by-Event Particle Ratio Fluctuations at LHC Energies

Academic Editor: Raghunath Sahoo
Received01 Jan 2021
Revised04 Jul 2021
Accepted10 Aug 2021
Published12 Oct 2021

Abstract

A Monte Carlo study of identified particle ratio fluctuations at LHC energies is carried out in the framework of HIJING model using the fluctuation variable . The simulated events for Pb-Pb collisions at and 5.02 TeV and Xe-Xe collisions at are analyzed. From this study, it is observed that the values of , , and follow the similar trends of energy dependence as observed in the most central collision data by NA49, STAR, and ALICE experiments. It is also observed that for all the three combinations of particles for semicentral and central collisions, the model predicted values of for Pb-Pb collisions at agree fairly well with those observed in the ALICE experiment. For peripheral collisions, however, the model predicted values of are somewhat smaller, whereas for and it predicts larger values as compared to the corresponding experimental values. The possible reasons for the observed differences are discussed. The values scaled with charged particle density when plotted against exhibit a flat behaviour, as expected from the independent particle emission sources. For and combinations, a departure from the flat trend is, however, observed in central collisions in the case of low window when the effect of jet quenching or resonances is considered. Furthermore, the study of dependence on particle density for various collision systems (including proton-proton collisions) suggests that at LHC energies values for a given particle pair are simply a function of charged particle density, irrespective of system size, beam energy, and collision centrality.

1. Introduction

Fluctuations associated to a physical quantity measured in an experiment, in general, depend on the property of the system and are expected to provide useful clue about the nature of the system under study [13]. As regards the heavy-ion (AA) collisions, the system created is assumed to be a hot and dense fireball of hot partonic and (or) hadronic matter [1, 2]. One of the main aims of studying AA collisions at relativistic energies is to search for the existence of partonic matter in the early stage of the created fireball. Fluctuations associated to a thermal system are supposed to be related to various susceptibilities [1, 2, 4] and would serve as an indicator of the possible phase transition. Moreover, the presence of large event-by-event (ebe) fluctuations, if observed, might be signal for the presence of distinct classes of events, one with and one without QGP formation [57]. Therefore, the search for the phase transition from hadronic matter to QGP still remains a topic of interest of high-energy physicists [810]. Correlations and ebe fluctuations of dynamical nature are believed to be associated with the critical phenomena of phase transition and their studies would lead to the local and global differences between the events produced under similar initial conditions [11].

ebe fluctuations in hadronic and heavy-ion collisions have been investigated at widely different energies using several different approaches, for example, normalized factorial moments [1215], multifractals [16, 17], -order rapidity spacing [1820], erraticity [2123], and intensive and strongly intensive quantities (defined in terms of multiplicity, transverse momentum, , etc.) [2426]. Furthermore, ebe fluctuations in conserved quantities like strangeness, baryon number, and electric charge have emerged as new tools to estimate the degree of equilibration and criticality of the measured systems [2731]. The dynamical net charge fluctuations have been investigated by STAR and ALICE experiments [28, 29] in terms of variable [32], which is an excellent probe because of its robustness against detector efficiency losses [29]. The other measures of the net charge fluctuations, like the variance of charge , variance of charge ratio , and the -measure [29, 31, 33, 34], are prone to the measurement conditions [32, 35].

It has, however, been pointed out [36] that large systematic uncertainties, like volume fluctuations due to impact parameter variations, are associated in such measurements, while the multiplicity ratio fluctuations are sensitive to the density fluctuations instead of volume fluctuations [37]. Thus, the variable , defined by considering the particle species pair, rather than defining it in terms of combinations of like and unlike charges, has been used as a tool to probe the properties of QGP [33, 38]. Since it is speculated that the phase transition, if occurs, would result in increase and divergence of fluctuations and could be related to ebe fluctuations of a suitably chosen observable. An enhanced fluctuation in the particle ratio is expected during a phase transition at critical point (CP). For example, , , and fluctuations could be related to baryon number fluctuations, strangeness fluctuations, and baryon-strangeness correlations [38, 39].

Particle ratio fluctuations in AA collisions have been addressed in a number of studies, e.g., NA49 experiment in Pb-Pb collisions at A GeV [40], STAR experiment in Au-Au collisions at to 200 GeV [41], Cu-Cu collisions at , 62.4, and 200 GeV [42], and several others [33, 36, 38]. At LHC energies, the particle ratio fluctuations have been investigated by the ALICE experiment at only [4345]. It has been reported [43] that for and combinations acquires positive values irrespective of the centrality class, whereas, for combination, the variable changes sign from positive to negative toward more peripheral collisions, indicating the difference in the production mechanisms involved of these pairs. The observed trend of energy dependence of with beam energy [43] suggests that the production dynamics changes significantly from that reported at lower energies. It has also been pointed out [43] that further investigations involving fluctuations with charge and species specific pairs are carried out to characterize the production dynamics and understand the observed sign changes. It was, therefore, considered to undertake the study of particle ratio fluctuations by analyzing the data on Pb-Pb collisions at and 5.02 TeV and Xe-Xe collisions at in the framework of HIJING model. Using the HIJING, the effect of jet quenching and resonance production can also be looked into.

2. Formalism

The particle ratio fluctuations may be studied in terms of the yields of the ratio of particle types and . The particle ratio is estimated by counting the particle types and produced in each event. Using the relative widths of the particle ratio distributions of the data and the corresponding mixed events the observable is defined as [38, 46]where and , respectively, denote the relative widths (standard deviation/mean) of the ratio for the data and mixed events. Yet another variable , which is commonly accepted for studying the particle ratio fluctuations, has been proposed [32]. quantifies the deviation of the fluctuations in the number of particle species and from that expected from Poissonian statistics [46]. This variable does not involve particle ratios directly but is related to as [42, 46].

is defined as [38, 4346]where and , respectively, denote the event multiplicities of particle types and within the given kinematical limits, while the quantities within represent their mean values. It should be mentioned here that the particle type or includes the particle and its antiparticle. basically contrasts the relative strengths of fluctuations of particle types and to the relative strength of correlation between the types . It may be noted that should be zero if particles and are produced in statistically independent way [32, 35, 43]. In practice, however, a nonzero value of is expected because produced particles are partially correlated through the production of resonances, string fragmentation, jet fragmentation, and (or) other mechanisms [29]. A negative value of indicates a correlation, whereas positive value would indicate the presence of anticorrelation between particle types and . The indices are taken as particle pair combinations, such as , , and in the present work to construct .

3. The HIJING Model

The Monte Carlo model HIJING (Heavy-Ion Jet Interaction Generator) was developed to study the role of minijets and particle production in proton-proton (pp), proton-nucleus (pA), and nucleus-nucleus (AA) collisions in a wide range of energies from 5 to 2000 GeV [47, 48]. The HIJING model is commonly used in high-energy heavy-ion collisions for providing the baseline to compare the simulation results with the experimental data. The main feature of the HIJING model is based on pQCD (perturbative QCD) approach considering that the multiple minijet partons produced in collisions are transformed into string fragmentation which, in turn, decays into hadrons. The pQCD process is implemented in HIJING using the PYTHIA [49, 50] model for hadronic collisions. The cross section in pQCD for hard parton scattering is determined using leading order to simulate the higher order corrections. The eikonal formalism is embedded to calculate the number of minijets per inelastic nucleon-nucleon collisions. The soft contributions are modeled by diquark-quark strings with gluon kinks along with the line of the Lund FRITIOF and DPM (Dual Parton Model) [48, 5155]. Besides this, the basic property of the HIJING model is that it considers the nucleus-nucleus collisions as a superposition of nucleon-nucleon collisions. However, the mechanism for final state interactions among the low particles is not included in the HIJING model. Due to which, the phenomena such as collectivity and equilibrium cannot be addressed. Therefore, HIJING is mainly designed to explore the range of possible initial conditions that may occur in high-energy heavy-ion collisions.

Furthermore, HIJING also takes into account other important physics processes like jet quenching [56], multiple scattering, and nuclear shadowing to study the nuclear effects [48]. To study the dependence of moderate and high observables on an assumed energy loss of partons traversing the produced dense matter, a jet quenching approach is incorporated in the HIJING model [48].

In high-energy heavy-ion collisions, the interaction of high jets in the produced transient dense medium is treated as one of the signals of phase transition [56]. Therefore, the rapid variation of (Debye screening) near the phase transition point could lead to a variation of jet quenching phenomenon that could be used as a diagnostic tool of the QGP phase transition [48]. Furthermore, resonances play an important role in studying the net charge fluctuations. Resonances have short lifetime and subsequently decay into stable hadrons. This would affect the final hadron yields and their number fluctuations [57]. Resonance decay kinematics influences charge fluctuations in two different ways. It dilutes the effect of global charge conservation if only one of the decay products falls into the acceptance window. However, if both decay products lie within the acceptance cone, mean charged particle multiplicity will increase but the net charge does not change [58]. Hadron production in HIJING involves a cocktail of resonances that may also give a rough estimate of the strength of correlations between charged and neutral kaons [59]. The present study is an attempt to explore the effect of fluctuations in understanding the dissipative properties of a color-defined medium using the jet quenching, resonance production, and jet/minijet contributions incorporated in the HIJING model [60]. It was found that [6164] the HIJING predicted values of charged particle density when jet quenching and contributions from resonance decays are switched off, which are consistent with the ones observed in Au-Au collisions at 200 GeV per nucleon and Pb-Pb at 2.76 and 5.5 TeV per nucleon.

4. Results and Discussion

MC events corresponding to Pb-Pb at and 5.02 and Xe-Xe collisions at are generated using the HIJING-1.37 [47, 48]. Events are simulated by running the code in three different modes: (i) HIJING default, i.e., resonance- (Res-) off jet quenching- (JQ-) off, (ii) Res-on JQ-off, and (iii) Res-off JQ-on. The number of events simulated in each of these modes is listed in Table 1. The analysis is carried out by considering only those charged particles which have pseudorapidity () and transverse momentum () in the range and  GeV/c, respectively. The ALICE experiment has also used the same -cut, but instead of , they have considered the charged particles with momentum,  GeV/c. It may be mentioned here that for the range considered in the ALICE experiment and also in the present study, for  GeV/c, . In order to examine the effect of jet quenching, a higher range,  GeV/c, is also considered where this effect is expected to be more visible. The centrality of an event is estimated by applying VZERO-A and VZERO-C detector cuts of the ALICE experiment [6567], i.e., by considering the charged particles which have their values in the range or . For this multiplicity, distributions of charged particles having their values within these limits are examined and quantiled to fix the minimum and maximum limits for a centrality class.


Energy (GeV)Type of collisionAnalysis modeNo. of events ()

2760Pb-PbRes-off JQ-on4.4
Res-off JQ-off3.6
Res-on JQ-off2.3
5020Pb-PbRes-off JQ-on3.3
Res-off JQ-off2.5
Res-on JQ-off2.3
5440Xe-XeRes-off JQ-on2.9
Res-off JQ-off2.5
Res-on JQ-off3.7

The values of mean number of participating nucleons and mean charged particle density for different centrality classes are listed in Tables 24. Variations of with for these events are plotted in Figure 1. The values of and reported earlier [6567] are also given in these tables and displayed in the figure. It is interesting to note from Tables 24 and Figure 1 that HIJING default predicts somewhat higher values of and for various centrality classes as compared to those observed in experiments [65, 66]. It may also be noted from the tables and the figure that the values of are higher when jet quenching is turned on, which might be due to the enhanced production of low particles. This may be understood as when a partonic jet is quenched in the dense medium, it would fragment into large number of partons which, in turn, result in the production of low charged particles [68]. It may also be noted that the effect of jet quenching is rather more pronounced in central collisions, as compared to that in peripheral collisions. Enhancement in the values due to resonances may also be seen in the figure.


CentralityHIJING Res-off JQ-offHIJING Res-off JQ-onHIJING Res-on JQ-off
(%)

0-5 () ()
5-10 () ()
10-20 () ()
20-30 () ()
30-40 () ()
40-50 () ()
50-60 () ()
60-70 () ()
70-80 () ()

Errors associated include systematic and statistical errors.

CentralityHIJING Res-off JQ-offHIJING Res-off JQ-onHIJING Res-on JQ-off
(%)

0-5 () ()
5-10 () ()
10-20 () ()
20-30 () ()
30-40 () ()
40-50 () ()
50-60 () ()
60-70 () ()
70-80 () ()

Errors associated include systematic and statistical errors.

CentralityHIJING Res-off JQ-offHIJING Res-off JQ-onHIJING Res-on JQ-off
(%)

0-5 () ()
5-10 () ()
10-20 () ()
20-30 () ()
30-40 (