System-size dependence of particle production at mid- and forward rapidity with ALICE

TL;DR

This paper presents new measurements of charged-particle and photon production across different collision systems at the LHC, analyzing how particle distributions depend on system size and collision energy, with implications for understanding collision dynamics.

Contribution

It provides the first comprehensive measurements of charged-particle pseudorapidity densities in pp, p-Pb, and Pb-Pb collisions at 5.02 TeV, and studies the evolution of rapidity distribution widths with system size.

Findings

Charged-particle pseudorapidity densities are measured across multiple systems.

The width of rapidity distributions increases with the number of participants.

Performance of the new Inner Tracking System is evaluated for future runs.

Abstract

The pseudorapidity densities of charged particles and inclusive photons produced in high energy nuclear collisions are essential observables to characterise the global properties of the collisions, such as the achieved energy density, and to provide important constraints for Monte Carlo model calculations. In the LHC Run 1 and Run 2 configurations, ALICE had large coverage to measure charged particles over a pseudorapidity range ($-3.4~<~\eta~<~5.0$), combining the data from the Silicon Pixel Detector (SPD) and the Forward Multiplicity Detector (FMD). The inclusive photons are measured at forward rapidity using the Photon Multiplicity Detector (PMD), covering the pseudorapidity range $2.3~<~\eta~<~3.9$. New results on charged-particle pseudorapidity densities measured in pp, p$-$Pb, and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV using Run 1 and Run 2 data are presented. Inclusive photon production is reported for p$-$Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV. The charged-particle rapidity densities are derived from the measured charged-particle pseudorapidity densities, and then parameterized by a normal distribution. This allows us to study the evolution of the width of the rapidity distributions as a function of the number of participants in all three collision systems. The performance of the new Inner Tracking System (ITS) designed for ALICE Run 3 configuration is also discussed for pilot beam pp collisions at $\sqrt{s}$ = 0.9 TeV.