Event-by-event fluctuations of mean transverse momentum in Pb–Pb and Xe–Xe collisions with ALICE

Event-by-event fluctuations of the mean transverse momentum of charged particles produced in Pb–Pb and Xe–Xe collisions at sNN=5.02 TeV and sNN=5.44 TeV, respectively, are studied as a function of the charged-particle multiplicity using the ALICE detector at the LHC. Dynamical fluctuations are observed in both collision systems which indicate correlated particle emission. The central Pb–Pb and Xe–Xe collisions show a significant reduction of the fluctuation in comparison to peripheral collisions and are in qualitative agreement with previous measurements in Pb–Pb collisions at sNN=2.76 TeV. The results are compared with the HIJING model. A clear deviation from simple superposition of independent nucleon-nucleon collisions scenario, where the final state particles are produced from superposition of particle emitting sources, is observed.


Introduction
Fluctuations of various observables in ultrarelativistic heavy-ion collisions have been extensively studied as they provide important indications to the formation of a quark-gluon-plasma (QGP).The collisions made at the LHC produce a large number of particles, even in a single event.Such large statistics in a single collision could be used to study the fluctuations of different observables, such as mean transverse momenta, on an event-by-event basis.These fluctuation analyses can also reveal collective effects and the onset of thermalization of a system.Thus, they are proposed as one of the key observables for the investigation of the hot and dense matter generated in heavy-ion collisions [1,2,3].
Here, we report on the event-by-event fluctuations of the mean transverse momentum, p T in Pb-Pb collisions at √ s NN = 5.02 TeV and Xe-Xe collisions at √ s NN = 5.44 TeV.The analysis focuses on non-statistical fluctuations of p T on an event-by-event basis which is represented by the two particle correalator, ∆p T,i , ∆p T,j .

Analysis details
The reported results in this proceedings are obtained from Pb-Pb collisions at √ s NN = 5.02 TeV and Xe-Xe collisions at √ s NN = 5.44 TeV recorded with the ALICE detector [4] during Run 2 of the LHC.A minimum-bias trigger condition is used for both Pb-Pb and Xe-Xe collisions.The centrality is defined based on the total charge deposited in both forward V0A and V0C detectors.The main detectors used for tracking and estimating the position of the arXiv:2211.14796v1[nucl-ex] 27 Nov 2022 collision vertex along the beam direction, V z , are the Inner Tracking System (ITS) and the Time Projection Chamber (TPC).Only events with at least one charged track contributing to the reconstruction of V z , and with |V z | within 10 cm from the nominal interaction point, are accepted.To ensure uniform tracking efficiency within the TPC, we restrict the acceptance to |η| < 0.8.The tracking efficiency drops at very low transverse momentum, therefore we only consider tracks with p T greater than 0.15 GeV/c.At the same time, we restrict the analysis to the domain of soft physics by requiring track p T lower than 2.0 GeV/c.

Two-particle correlator
The mean transverse momentum of an event, p T , is calculated on an event-by-event basis as where p Ti is the transverse momentum of i th particle in the event k and N ch,k is the chargedparticle multiplicity of the event.We further calculate the p T by averaging the p T over events within the same multiplicity bin.The two-particle correlator is given by: Here, p i and p j are the transverse momenta of i th and j th particle of an event in a particular multiplicity bin, where N ch (N ch −1) represents the number of particle pairs.The above equation can be further simplified and can be rewritten as, where,

Results and discussion
The left panel of Fig. 1 shows the event-by-event fluctuations of p T in Pb-Pb collisions at √ s NN = 2.76 [5] and 5.02 TeV, represented by the dimensionless quantity ∆p Ti ∆p Tj / p T .A significant dynamical fluctuation is observed for both collision energies, which decreases with increasing multiplicity.
One of the key effects in Pb-Pb collisions is the stronger decrease of the p T fluctuations in high multiplicity bins which may be related to radial flow and other final state effects.The central Pb-Pb collisions show a significant reduction of the fluctuation in comparison to peripheral collisions and the value of the correlator is found to be higher for Pb-Pb collisions at experimental data shows a clear deviation from the HIJING model as well as from the power law fit, indicating a deviation from a simple superposition scenario.This indicates that the final state particle production in heavy-ion collisions at the LHC can not be described by a mere superposition of independent particle-emitting sources.TeV.A clear deviation from a simple superposition scenario, where the final state particles are produced from a superposition of independent particle emitting sources, is observed.

√ s NN = 5 .Figure 1 .
Figure 1.Left panel: Event-by-event fluctuations of mean transverse momentum in Pb-Pb collisions at 2.76 and 5.02 TeV and their model comparisons with HIJING, right panel: eventby-event fluctuations of mean transverse momentum in Pb-Pb collisions at √ s NN = 5.02 TeV and Xe-Xe collisions at √ s NN = 5.44 TeV Event-by-event p T fluctuations of charged particles produced in Pb-Pb and Xe-Xe collisions at √ s NN = 5.02 TeV and √ s NN = 5.44 TeV are studied as a function of the charged-particle multiplicity.Significant dynamical fluctuations are observed in both collision systems which indicate correlated particle emission.The central Pb-Pb collisions show a significant reduction of the fluctuation in comparison to peripheral collisions.The results are compared with the HIJING model as well as with previous measurements in Pb-Pb collisions at √ s NN = 2.76