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

Two-particle pseudorapidity correlations are measured in √ ^sNN = 2.76TeV Pb + Pb, √ ^sNN = 5.02TeV p + Pb, and √s = 13 TeV pp collisions at the Large Hadron Collider (LHC), with total integrated luminosities of approximately 7μb ^–1, 28 nb ^–1, and 65 nb ^–1, respectively. The correlation function C _N(η ₁,η ₂) is measured as a function of event multiplicity using charged particles in the pseudorapidity range |η| < 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 + < ^a2 ₁ > ^1/2η ₁η ₂ in all collision systems over the full multiplicity range. The values of < a ² ₁ > ^1/2 are consistent for the opposite-charge pairs and same-charge pairs, and for the three collision systems at similar multiplicity. The values of < a ² ₁ > ^1/2 and the magnitude of the short-range component both follow a power-law dependence on the event multiplicity. Here, the short-range component in p + Pb collisions, after symmetrizing the proton and lead directions, is found to be smaller at a given η than inmore » pp collisions with comparable multiplicity.