## Abstract

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.

Original language | English |
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Article number | 137905 |

Journal | Physics Letters, Section B: Nuclear, Elementary Particle and High-Energy Physics |

Volume | 844 |

DOIs | |

State | Published - 10 Sep 2023 |

## Keywords

- CMS
- Collectivity
- Photon-proton

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