# Pseudorapidity distributions of charged hadrons in xenon-xenon collisions at $\sqrt{s_\mathrm{NN}} =$ 5.44 TeV

## TL;DR

This paper reports measurements of charged hadron pseudorapidity distributions in xenon-xenon collisions at 5.44 TeV, revealing details about particle production and collision geometry effects at the LHC.

## Contribution

It provides the first detailed pseudorapidity distribution measurements for xenon-xenon collisions at this energy, highlighting differences from lead-lead collisions and challenging existing models.

## Key findings

- Charged-hadron pseudorapidity density at midrapidity is 1187 ± 36 for central collisions.
- Rapidity distribution is flat around y=0, independent of rapidity within the measured range.
- Existing Monte Carlo models fail to describe both the pseudorapidity and rapidity distributions simultaneously.

## Abstract

Measurements of the pseudorapidity distributions of charged hadrons produced in xenon-xenon collisions at a nucleon-nucleon centre-of-mass energy of $\sqrt{s_\mathrm{NN}} =$ 5.44 TeV are presented. The measurements are based on data collected by the CMS experiment at the LHC. The yield of primary charged hadrons produced in xenon-xenon collisions in the pseudorapidity range $|\eta|$ $<$ 3.2 is determined using the silicon pixel detector in the CMS tracking system. For the 5% most central collisions, the charged-hadron pseudorapidity density in the midrapidity region $|\eta|$ $<$ 0.5 is found to be 1187 $\pm$ 36 (syst), with a negligible statistical uncertainty. The rapidity distribution of charged hadrons is also presented in the range $|y|$ $<$ 3.2 and is found to be independent of rapidity around $y =$ 0. Existing Monte-Carlo event generators are unable to simultaneously describe both results. Comparisons of charged-hadron multiplicities between xenon-xenon and lead-lead collisions at similar collision energies show that particle production at midrapidity is strongly dependent on the collision geometry in addition to the system size and collision energy.