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

We present a first measurement of two-particle angular correlations for charged hadrons emitted from photon-proton, $\gamma$p, interactions over a wide range of pseudorapidity and full azimuth. The $\gamma$p events were produced within ultra-peripheral pPb collisions at $\sqrt{s_{_{\text{NN}}}}=8.16~\mathrm{TeV}$ and were selected by requiring a large rapidity gap in the lead-going direction and no neutron emission from the lead nucleus. The results are compared to a sample of minimum-bias pPb events with same multiplicity. The observed azimuthal correlations at large relative pseudorapidity are used to extract the first, second and third-order two-particle anisotropy harmonics, ${{V_{1\Delta}}}$, ${{V_{2\Delta}}}$, and ${{V_{3\Delta}}}$ as a function of track multiplicity and transverse momentum ${p_{\mathrm{T}}}$. 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 ${v_2}({p_{\mathrm{T}}})$ is larger for $\gamma$p events than for minimum-bias hadronic pPb collisions at the same multiplicity.