Evidence for collectivity in pp collisions at the LHC

TL;DR

This paper presents measurements of azimuthal anisotropy in proton-proton collisions at various energies, revealing collective behavior similar to larger collision systems, indicating possible collective phenomena in small systems.

Contribution

It provides the first extraction of four- and six-particle correlations in pp collisions at 13 TeV, demonstrating collectivity in small systems with detailed particle species analysis.

Findings

Mass ordering observed in v2 for different particle species.

v2 signals from four- and six-particle correlations are comparable to two-particle results.

Collective behavior in high-multiplicity pp collisions similar to pPb and PbPb systems.

Abstract

Measurements of two- and multi-particle angular correlations in pp collisions at sqrt(s) = 5, 7, and 13 TeV are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of 1.0 inverse picobarn (5 TeV), 6.2 inverse picobarns (7 TeV), and 0.7 inverse picobarns (13 TeV), were collected using the CMS detector at the LHC. The second-order (v[2]) and third-order (v[3]) azimuthal anisotropy harmonics of unidentified charged particles, as well as v[2] of K0 short and Lambda/anti-Lambda particles, are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum. For high-multiplicity pp events, a mass ordering is observed for the v[2] values of charged hadrons (mostly pions), K0 short, and Lambda/anti-Lambda, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below pt of about 2 GeV/c. For 13 TeV data, the v[2] signals are also extracted from four- and six-particle correlations for the first time in pp collisions, with comparable magnitude to those from two-particle correlations. These observations are similar to those seen in pPb and PbPb collisions, and support the interpretation of a collective origin for the observed long-range correlations in high-multiplicity pp collisions.