Two-particle azimuthal correlations in γp interactions using pPb collisions at

The first measurements of the Fourier coefficients (${V}_{n\Delta }$) 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 $\sqrt{{s}_{{}_{\mathrm{NN}}}}=8.16\phantom{\rule{0.25em}{0ex}}\text{TeV}$ 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 ${V}_{n\Delta }$ 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, ${V}_{1\Delta }$ is negative, ${V}_{2\Delta }$ is positive, and ${V}_{3\Delta }$ consistent with 0. For each multiplicity and ${p}_{T}$ range, ${V}_{2\Delta }$ is larger for γp events. The γp data are consistent with model predictions that have no collective effects.