CERN Accelerating science

In heavy-ion collisions, event-by-event measurements play a crucial role in understanding the high-energy nuclear interaction dynamics and the quark-gluon plasma (QGP) properties. Studies of event-by-event fluctuations of observables measured in heavy-ion collisions are of great interest given they probe the phase transition from QGP to hadron gas (HG). Of particular interest are event-by-event fluctuations of the average transverse momentum ($\langle p_{\rm T} \rangle$) of charged particles measured in a specific kinematic range. These fluctuations are expected to be sensitive to energy fluctuations, and arguably, temperature variations of the produced matter in these collisions. Fluctuations in the average transverse momentum act as a proxy to the fluctuation in the temperature of the system. The temperature fluctuations are predicted to sharply increase in the vicinity of the critical point and in this part of the QCD phase diagram, a rapid change in the heat capacity of the medium near the phase transition is expected. $\langle p_{\rm T} \rangle$ fluctuations are also highly sensitive to the presence of collective effects and the onset of thermalization in mesoscopic systems. Measurements of $\langle p_{\rm T} \rangle$ fluctuations are thus an essential tool to achieve a better understanding of the hot and dense matter produced in heavy-ion collisions. To measure the non-statistical average $p_{\rm{T}}$ fluctuations, the analysis is carried out using the two-particle transverse-momentum correlator, $\sqrt{\langle \Delta p_{{\rm T},1}\Delta p_{{\rm T},2} \rangle}/\langle\langle p_{\rm T} \rangle\rangle$. This observable is designed to nullify scenarios exhibiting purely statistical fluctuations. Any non-zero signal detected signifies particle correlations in momentum space, contributing to dynamic average $p_{\rm{T}}$. This thesis comprises the analysis of the event-by-event mean transverse-momentum fluctuations in proton–proton (pp), xenon-xenon (Xe–Xe), and lead–lead (Pb–Pb) collisions in the centre-of-mass energies of 5.02 TeV for pp and Pb-Pb collisions and 5.44 TeV for Xe-Xe collisions. The analysis is performed for charged particles with transverse momenta in the range of 0.15 GeV/c < $p_{\rm{T}}$ < 2 GeV/c. The chosen pseudorapidity acceptance of $|\eta|$ < 0.8 plays a crucial role in ensuring uniform acceptance and efficient track detection within the ALICE Time Projection Chamber, a key detector of this study. The data analysis in this investigation is divided into two main components. Initially, the analysis focuses on studying event-by-event fluctuations in the average transverse momentum, $\langle p_{\rm{T}} \rangle$, of charged particles produced in Pb-Pb, Xe-Xe, and pp collisions. In this part, the study reports a system and energy scan of event-by-event $\langle p_{\rm{T}} \rangle$ fluctuations as a function of the charged particle pseudorapidity density. The observed $\sqrt{\langle \Delta p_{{\rm T},1}\Delta p_{{\rm T},2} \rangle}/\langle\langle p_{\rm T} \rangle\rangle$ exhibits a non-vanishing strength and demonstrated an approximate power-law dependence on the produced charged particle density. These findings validate and support previous observations of non-Poissonian fluctuations in AA collisions. In the system size scan, the magnitude of the correlator is observed to decrease by more than one order of magnitude with respect to the produced particle multiplicity density measured in pp, Xe-Xe, and Pb-Pb collisions as the multiplicity increases from low to high values. The measured evolution of the two-particle correlator with $\langle \rm{d}N_{\rm ch}/d\eta\rangle$ for Pb-Pb and Xe-Xe collisions is compared with calculations based on the HIJING and AMPT model using the default mode and the mode with string melting. The magnitude of the two-particle correlator computed with HIJING exhibits a simple power law behavior. This power law dependence and exponent value are consistent with the behaviour expected for a system consisting of a simple superposition of nucleon-nucleon collisions without re-scattering of the secondaries as modelled by HIJING. However, one observes that while the evolution of the correlator measured in both Pb--Pb and Xe--Xe approximately follows the HIJING power-law fit in the range $10< \langle \rm{d}\it{N}_{\rm ch}/\rm{d}\eta\rangle<50$, it clearly deviates from this simple trend at $\langle \rm{d}\it{N}_{\rm ch}/\rm{d}\eta\rangle>50$. This indicates that the final state particle production in Xe-Xe and Pb-Pb collisions cannot be described by a mere superposition of independent particle-emitting sources. The observed deviation is likely a result of radial flow, yet exploring the potential for increased correlations stemming from fluctuations linked to jet production is an intriguing factor. The aim is to discern whether fluctuations in the number of jets compared to the "baseline" regime (those devoid of jets) might amplify fluctuations, potentially influencing the magnitude of the correlator. This curiosity led to explore how the strength of the correlator varies in pp collisions, ranging from jet-like to isotropic events, constituting the second part of the analysis. Hence, the evolution of $\sqrt{\langle \Delta p_{{\rm T},1}\Delta p_{{\rm T},2} \rangle}/\langle\langle p_{\rm T} \rangle\rangle$ with $\langle \rm{d}\it{N}_{\rm ch}/\rm{d}\eta\rangle$ in pp collisions at $\sqrt{s}$ = 5.02 TeV for different spherocity classes is studied in this thesis. Events characterized by the presence of jets exhibits more significant mean transverse momentum fluctuations compared to isotropic events. In high-multiplicity collisions, the presence of jets increases the correlator's strength by around 20\%. This enhancement primarily stems from particles emitted within a "narrow" cone by jets, showing a higher average correlation among themselves compared to other particles. Consequently, the existence of jets notably increases the correlator's strength within these events. Observing an increase in the correlator strength for events with jet-like characteristics relative to spherocity-integrated pp collisions suggests that the outcomes witnessed in A-A collisions might not be solely due to jet suppression. Hence, the presence of jets in central A-A collisions isn't expected to significantly contribute to the strength of the correlator.