The first measurements of the Fourier coefficients (V n ) 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 V n 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 1 is negative, V 2 is positive,
and V 3 consistent with 0. For each multiplicity and pT range, V 2 is larger for γp events. The γp data
are consistent with model predictions that have no collective effects.