Non-Gaussian transverse momentum fluctuations from impact parameter fluctuations

Abstract

The transverse momentum per particle, $[p_t]$, fluctuates event by event in ultrarelativistic nucleus-nucleus collisions, for a given multiplicity. These fluctuations are small and approximately Gaussian, but a non-zero skewness has been predicted on the basis of hydrodynamic calculations, and seen experimentally. We argue that the mechanism driving the skewness is that, if the system thermalizes, the mean transverse momentum increases with impact parameter for a fixed collision multiplicity. We postulate that fluctuations are Gaussian at fixed impact parameter, and that non-Gaussianities solely result from impact parameter fluctuations. Using recent data on the variance of $[p_t]$ fluctuations, we make quantitative predictions for their skewness and kurtosis as a function of the collision multiplicity. We predict in particular a spectacular increase of the skewness below the knee of the multiplicity distribution, followed by a fast decrease.

arXiv comment8 pages, 5 figures. v2: Minor revision

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• Bayesian reconstruction of anisotropic flow fluctuations at fixed impact parameter
• Extracting the speed of sound of QCD from transverse momentum fluctuations