# Entropy production in pp and Pb-Pb collisions at energies available at the CERN Large Hadron Collider

## TL;DR

This study measures entropy production in proton-proton and lead-lead collisions at LHC energies using experimental data, compares it with statistical models, and models the entropy evolution to infer temperature profiles.

## Contribution

It provides the first detailed measurement of entropy per unit rapidity in pp and Pb-Pb collisions at LHC energies, integrating experimental data with theoretical modeling.

## Key findings

- Entropy per rapidity is significantly higher in Pb-Pb than in pp collisions.
- The entropy per charged particle ratio aligns with statistical model predictions.
- Modeling suggests specific temperature profiles at early collision stages.

## Abstract

We use experimentally measured identified particle spectra and Hanbury Brown-Twiss radii to determine the entropy per unit rapidity $dS/dy$ produced in $\sqrt{s} = 7$ TeV pp and $\sqrt{s_{\rm NN}} = 2.76$ TeV Pb-Pb collisions. We find that $dS/dy = 11335 \pm 1188$ in 0-10% Pb-Pb, $dS/dy = 135.7 \pm 17.9$ in high-multiplicity pp, and $dS/dy = 37.8 \pm 3.7$ in minimum bias pp collisions and compare the corresponding entropy per charged particle $(dS/dy)/(dN_{\rm ch}/dy)$ to predictions of statistical models. Finally, we use the QCD kinetic theory pre-equilibrium and viscous hydrodynamics to model entropy production in the collision and reconstruct the average temperature profile at $\tau_0 = 1$ fm/$c$ for high multiplicity pp and Pb-Pb collisions.