Evidence for collectivity in pp collisions at the LHC

TLDR

The study measures two‑ and multi‑particle angular correlations in pp collisions at 5, 7, and 13 TeV as a function of charged‑particle multiplicity. Using CMS data at the LHC with integrated luminosities of 1.0, 6.2, and 0.7 pb⁻¹ at 5, 7, and 13 TeV, the authors extracted second‑ and third‑order azimuthal anisotropy harmonics (v₂, v₃) of unidentified charged particles and v₂ of K⁰ₛ and Λ/Λ̄ from long‑range two‑particle correlations as functions of multiplicity and transverse momentum. High‑multiplicity pp events show a mass ordering of v₂, with lighter particles exhibiting stronger anisotropy below ~2 GeV/c, and 13 TeV data reveal comparable v₂ from four‑ and six‑particle correlations; these results mirror those in pPb and PbPb collisions and support a collective origin of the long‑range correlations.

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.