Two-particle azimuthal correlations in $\gamma$p interactions using pPb collisions at $\sqrt{s_{\mathrm{NN}}}$ = 8.16 TeV

Abstract

The first measurements of the Fourier coefficients ($V_{n\Delta }$) of the azimuthal distributions of charged hadrons emitted from photon-proton ($\gamma$p) interactions at the LHC 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 TeV using the CMS detector. The high energy lead ions produce a flux of photons that can interact with the oncoming proton. This $\gamma$p 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 ($p_{\mathrm{T}}$) and are compared to corresponding hadronic minimum bias pPb results. For a given multiplicity range, the mean $p_{\mathrm{T}}$ of charged particles is smaller in $\gamma$p than in pPb collisions. For both the $\gamma$p and pPb samples, $V_{1\Delta }$ is negative, $V_{2\Delta }$ is positive, and $V_{3\Delta }$ consistent with 0. For each multiplicity and $p_{\mathrm{T}}$ range, $V_{2\Delta }$ is larger for $\gamma$p events. The $\gamma$p data are consistent with model predictions that have no collective effects.

arXiv commentReplaced with the published version. Added the journal reference and the DOI. All the figures and tables can be found at http://cms-results.web.cern.ch/cms-results/public-results/publications/HIN-18-008 (CMS Public Pages)

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