Measurement of forward-backward multiplicity correlations in lead-lead, proton-lead and proton-proton collisions with the ATLAS detector

TL;DR

This study measures two-particle pseudorapidity correlations in lead-lead, proton-lead, and proton-proton collisions at the LHC, revealing a universal shape after subtracting short-range effects and a power-law dependence on event multiplicity.

Contribution

It provides the first comprehensive comparison of pseudorapidity correlations across different collision systems at the LHC, highlighting universal features and multiplicity dependence.

Findings

Correlation shape is approximately linear in $ abla imes ext{eta}_1 imes ext{eta}_2$ after short-range subtraction.

The magnitude of correlations follows a power-law with event multiplicity.

Short-range component distribution differs between $pp$ and $p$+Pb collisions.

Abstract

Two-particle pseudorapidity correlations are measured in $\sqrt{s_{\rm{NN}}}$ = 2.76 TeV Pb+Pb, $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV $p$+Pb, and $\sqrt{s}$ = 13 TeV $pp$ collisions at the LHC, with total integrated luminosities of approximately 7 $\mu\mathrm{b}^{-1}$, 28 $\mathrm{nb}^{-1}$, and 65 $\mathrm{nb}^{-1}$, respectively. The correlation function $C_{\rm N}(\eta_1,\eta_2)$ is measured as a function of event multiplicity using charged particles in the pseudorapidity range $|\eta|<2.4$. The correlation function contains a significant short-range component, which is estimated and subtracted. After removal of the short-range component, the shape of the correlation function is described approximately by $1+\langle{a_1^2}\rangle \eta_1\eta_2$ in all collision systems over the full multiplicity range. The values of $\sqrt{\langle{a_1^2}\rangle}$ are consistent between the opposite-charge pairs and same-charge pairs, and for the three collision systems at similar multiplicity. The values of $\sqrt{\langle{a_1^2}\rangle}$ and the magnitude of the short-range component both follow a power-law dependence on the event multiplicity. The $\eta$ distribution of the short-range component, after symmetrizing the proton and lead directions in $p$+Pb collisions, is found to be smaller than that in $pp$ collisions with comparable multiplicity.