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

The first measurements of the Fourier coefficients (V _nΔ) 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 s _NN =8.16TeV 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 V _nΔ coefficients are presented in ranges of event multiplicity and transverse momentum (p _T) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean p _T of charged particles is smaller in γp than in pPb collisions. For both the γp and pPb samples, V _1Δ is negative, V _2Δ is positive, and V _3Δ consistent with 0. For each multiplicity and p _T range, V _2Δ is larger for γp events. The γp data are consistent with model predictions that have no collective effects.