Observation of long-range near-side angular correlations in proton-lead collisions at the LHC

TL;DR

This paper reports the first observation of long-range near-side angular correlations, or the ridge, in proton-lead collisions at the LHC, revealing collective-like behavior in small collision systems.

Contribution

It presents the first evidence of ridge-like correlations in pPb collisions, expanding understanding of collective phenomena beyond heavy-ion collisions.

Findings

Long-range correlations observed in high-multiplicity pPb events.

Correlation strength peaks at 1-1.5 GeV transverse momentum.

Correlation magnitude increases with charged particle multiplicity.

Abstract

Results on two-particle angular correlations for charged particles emitted in pPb collisions at a nucleon-nucleon center-of-mass energy of 5.02 TeV are presented. The analysis uses two million collisions collected with the CMS detector at the LHC. The correlations are studied over a broad range of pseudorapidity, eta, and full azimuth, phi, as a function of charged particle multiplicity and particle transverse momentum, pt. In high-multiplicity events, a long-range (2 < abs(Delta(eta)) < 4), near-side (Delta(phi) approximately 0) structure emerges in the two-particle Delta(eta)-Delta(phi) correlation functions. This is the first observation of such correlations in proton-nucleus collisions, resembling the ridge-like correlations seen in high-multiplicity pp collisions at sqrt(s) = 7 TeV and in AA collisions over a broad range of center-of-mass energies. The correlation strength exhibits a pronounced maximum in the range of pt = 1-1.5 GeV and an approximately linear increase with charged particle multiplicity for high-multiplicity events. These observations are qualitatively similar to those in pp collisions when selecting the same observed particle multiplicity, while the overall strength of the correlations is significantly larger in pPb collisions.