Search for azimuthal anisotropies in $gamma$p interactions within ultra-peripheral pPb collisions at $\sqrt{s_{NN}}$ = 8.16 TeV

About

In recent years there has been a great effort to search for collectivity in small collision systems. Ultra-peripheral pPb events offer the possibility to study photon-proton collisions at center of mass energies of several hundred GeV, and provide a new arena for the search of collectivity at very small systems. The CMS experiment has collected a large dataset of photon-proton collisions by selecting very asymmetric pPb events which are characterized by a large rapidity gap in the lead going side and no neutron emission from the lead nucleus. These events are compared to a sample of minimum-bias pPb events with the same multiplicity. The observed azimuthal correlations at large relative pseudorapidity are used to extract the first, second, and third-order two-particle anisotropy harmonics, $V_{1\Delta}$, $V_{1\Delta}$, and $V_{1\Delta}$ as a function of track multiplicity and transverse momentum $p_T$. For both the $\gamma$p and minimum-bias hadronic pPb samples, significant negative $V_{1\Delta}$ and positive $V_{2\Delta}$ values are observed, while the $V_{3\Delta}$ values are consistent with zero. The single-particle second-order harmonic $v_2(p_T)$ is larger for $gamma$p events than for minimum-bias hadronic pPb collisions at the same multiplicity. These results will be discussed within the context of other recent results to shed light on the emergence of collectivity in small systems..

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