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

Two-particle pseudo rapidity correlations are measured in $\sqrt{s_{NN}}$ = 2.76 TeV Pb+Pb, $\sqrt{s_{NN}}$=5.02 TeV p+Pb and $\sqrt{s}$ = 13 TeV pp collisions, with total integrated luminosities of approximately 7 μb$^{−1}$, 28 nb$^{−1}$ and 14 nb$^{−1}$, respectively. The correlation function $C_{N}(η_{1},η_{2})$ is measured using charged particles in the pseudo rapidity range |η|<2.4 with transverse momentum $p_{T}$>0.2 GeV, and it is measured as a function of event multiplicity, defined by the total number of charged particles with |η|<2.5 and $p_{T}$>0.4 GeV. The correlation function contains a significant short-range component, which is estimated and subtracted. The shape and magnitude of this short-range component differ significantly between the opposite-charge pairs and same-charge pairs, and also differ significantly for the three collision systems at similar multiplicity. In contrast, 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 $⟨a_{1}^{2}⟩$ are consistent between the opposite-charge pairs and same-charge pairs, and are similar for the three collision systems at similar multiplicity. The values of $\sqrt{⟨a_{1}^{2}⟩}$ and the magnitude of the short-range component both follow a power-law dependence on the event multiplicity