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

The CMS collaboration

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The first measurements of the Fourier coefficients (V) 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=8.16TeV 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 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, V is negative, V is positive, and V consistent with 0. For each multiplicity and pT range, V is larger for γp events. The γp data are consistent with model predictions that have no collective effects.

Original languageEnglish
Article number137905
JournalPhysics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics
StatePublished - 10 Sep 2023


  • CMS
  • Collectivity
  • Photon-proton


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