Two-particle azimuthal correlations in γp interactions using pPb collisions at √sNN = 8.16 TeV

Abstract

The first measurements of the Fourier coefficients (Vn) of the azimuthal distributions of charged hadrons emitted from photon-proton (γp) interactions are presented. The data are extracted from 68.8 nb−1 of ultra-peripheral proton-lead (pPb) collisions at √sNN = 8.16 TeV using the CMS detector. The high energy lead ions produce a flux of photons that can interact with the oncoming proton. This γp system provides a set of unique initial conditions with multiplicity lower than in photon-lead collisions but comparable to recent electron-positron and electron-proton data. The Vn coefficients are presented in ranges of event multiplicity and transverse momentum (pT) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean pT of charged particles is smaller in γp than in pPb collisions. For both the γp and pPb samples, V1 is negative, V2 is positive, and V3 consistent with 0. For each multiplicity and pT range, V2 is larger for γp events. The γp data are consistent with model predictions that have no collective effects.