Observation of Associated Near-side and Away-side Long-range Correlations in sqrt(s_NN)=5.02 TeV Proton-lead Collisions with the ATLAS Detector

TL;DR

This paper reports the observation of long-range azimuthal correlations in proton-lead collisions at 5.02 TeV, indicating possible collective effects similar to those seen in heavy-ion collisions.

Contribution

It provides the first measurement of long-range near-side and away-side correlations in p+Pb collisions at the LHC, highlighting potential collective phenomena.

Findings

Long-range correlations grow with event activity (SumE_T).

Correlation shapes are consistent with cos(2Delta-phi) modulation.

Results suggest final-state collective effects in high-multiplicity p+Pb events.

Abstract

Two-particle correlations in relative azimuthal angle (Delta-phi) and pseudorapidity (Delta-eta) are measured in sqrt(s_NN) = 5.02 TeV p+Pb collisions using the ATLAS detector at the LHC. The measurements are performed using approximately 1 mub^-1 of data as a function of p_T and the transverse energy (SumE_T) summed over 3.1 < eta < 4.9 in the direction of the Pb beam. The correlation function, constructed from charged particles, exhibits a long-range (2<|Delta-eta|<5) near-side (Delta-phi ~ 0) correlation that grows rapidly with increasing SumE_T. A long-range away-side (Delta-phi ~ pi) correlation, obtained by subtracting the expected contributions from recoiling dijets and other sources estimated using events with small SumE_T, is found to match the near-side correlation in magnitude, shape (in Delta-eta and Delta-phi) and SumE_T dependence. The resultant Delta-phi correlation is approximately symmetric about pi/2, and is consistent with a cos(2Delta-phi) modulation for all SumE_T ranges and particle p_T. The amplitude of this modulation is comparable in magnitude and p_T dependence to similar modulations observed in heavy-ion collisions, suggestive of final-state collective effects in high multiplicity events.