CMS-PAS-HIN-18-008

CMS-PAS-HIN-18-008
Search for elliptic azimuthal anisotropies in $\gamma$ p interactions within ultra-peripheral pPb collisions at ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 8.16 TeV
CMS Collaboration
July 2020

Abstract: 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 ${\sqrt {\smash [b]{s_{_{\mathrm {NN}}}}}}=$ 8.16 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.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PLB. The superseded preliminary plots can be found here.

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