Skewness and kurtosis of mean transverse momentum fluctuations at the LHC energies

Abstract

The first measurements of skewness and kurtosis of mean transverse momentum ($⟨p_{\mathrm{T}}⟩$) fluctuations are reported in Pb$-$Pb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 5.02 TeV, Xe$-$Xe collisions at $\sqrt{s_{\mathrm{NN}}}$ $=$ 5.44 TeV and pp collisions at $\sqrt{s}=5.02$ TeV using the ALICE detector. The measurements are carried out as a function of system size $⟨\mathrm{d}{N}_{\mathrm{ch}}/\mathrm{d}\eta {⟩}_{|\eta |<0.5}^{1/3}$, using charged particles with transverse momentum ($p_{\mathrm{T}}$) and pseudorapidity ($\eta$), in the range $0.2<p_{\mathrm{T}}<3.0$ GeV/$c$ and $|\eta |<0.8$, respectively. In Pb$-$Pb and Xe$-$Xe collisions, positive skewness is observed in the fluctuations of $⟨p_{\mathrm{T}}⟩$ for all centralities, which is significantly larger than what would be expected in the scenario of independent particle emission. This positive skewness is considered a crucial consequence of the hydrodynamic evolution of the hot and dense nuclear matter created in heavy-ion collisions. Furthermore, similar observations of positive skewness for minimum bias pp collisions are also reported here. Kurtosis of $⟨p_{\mathrm{T}}⟩$ fluctuations is found to be in good agreement with the kurtosis of Gaussian distribution, for most central Pb$-$Pb collisions. Hydrodynamic model calculations with MUSIC using Monte Carlo Glauber initial conditions are able to explain the measurements of both skewness and kurtosis qualitatively from semicentral to central collisions in Pb--Pb system. Color reconnection mechanism in PYTHIA8 model seems to play a pivotal role in capturing the qualitative behavior of the same measurements in pp collisions.

24 pages, 5 captioned figures, 1 table, authors from page 18, published version, figures at http://alice-publications.web.cern.ch/node/9598

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• An Introduction to PYTHIA 8.2
• The Color Glass Condensate
• Alternative ansatz to wounded nucleon and binary collision scaling in high-energy nuclear collisions
• The PHOBOS Glauber Monte Carlo
• Event-by-event mean $p_{\mathrm{T}}$ fluctuations in pp and Pb-Pb collisions at the LHC
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• Bayesian reconstruction of anisotropic flow fluctuations at fixed impact parameter
• High-order fluctuations of temperature in hot QCD matter