System size and energy dependence of the mean transverse momentum fluctuations at the LHC

Abstract

Event-by-event fluctuations of the event-wise mean transverse momentum, $⟨p_{\text{T}}⟩$, of charged particles produced in proton–proton (pp) collisions at $\sqrt{s}$ = 5.02 TeV, Xe–Xe collisions at $\sqrt{s_{\text{NN}}}$ = 5.44 TeV, and Pb–Pb collisions at $\sqrt{s_{\text{NN}}}$ = 5.02 TeV are studied using the ALICE detector based on the integral correlator $⟨⟨\mathrm{\Delta }{p}_{\text{T}}\mathrm{\Delta }{p}_{\text{T}}⟩⟩$. The correlator strength is found to decrease monotonically with increasing produced charged-particle multiplicity measured at midrapidity in all three systems. In Xe–Xe and Pb–Pb collisions, the multiplicity dependence of the correlator deviates significantly from a simple power-law scaling as well as from the predictions of the HIJING and AMPT models. The observed deviation from power-law scaling is expected from transverse radial flow in semicentral to central Xe–Xe and Pb–Pb collisions. In pp collisions, the correlation strength is also studied by classifying the events based on the transverse spherocity, $S_0$, of the particle production at midrapidity, used as a proxy for the presence of a pronounced back-to-back jet topology. Low-spherocity (jetty) events feature a larger correlation strength than those with high spherocity (isotropic). The strength and multiplicity dependence of jetty and isotropic events are well reproduced by calculations with the PYTHIA 8 and EPOS LHC models.