CMS-PAS-HIN-16-010

CMS-PAS-HIN-16-010
Observation of particle-species dependence and collectivity in azimuthal correlations of pp collisions at the LHC
CMS Collaboration
May 2016

Abstract: Measurements of two- and multi-particle angular correlations in pp collisions at $\sqrt{s} =$ 5, 7, and 13 TeV are presented as a function of charged-particle multiplicity. The data, corresponding to an integrated luminosity of 1.0 pb $^{-1}$ (5 TeV), 6.2 pb $^{-1}$ (7 TeV), and 0.7 pb $^{-1}$ (13 TeV), were collected using the CMS detector at the LHC. The second-order ( $v_2$ ) and third-order ( $v_3$ ) azimuthal anisotropy harmonics of unidentified charged particles, as well as $v_2$ of $\mathrm{K_S}^0$ and $\Lambda$ / $\overline{\Lambda}$ particles, are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum, after correcting for the contribution of back-to-back jet correlations. A clear particle-species dependence of $v_2$ is observed for high-multiplicity pp events (e.g., with more than 105 charged particles produced within $\|\eta\|<$ 2.4 ) at $\sqrt{s} =$ 13 TeV. For $p_{\mathrm{T}}<$ 2.5 GeV/ $c$ , a mass ordering is observed for the $v_2$ values of charged hadrons (mostly pions), $\mathrm{K_S}^0$ , and $\Lambda$ / $\overline{\Lambda}$ , with lighter particle species exhibiting a stronger azimuthal anisotropy signal. This observation is similar to what is seen in pPb and PbPb collisions. For 13 TeV data, the $v_2$ signals are extracted from four- and six-particle correlations for the first time in pp collisions, with comparable magnitude to those from two-particle correlations. This observation supports the collective nature of the long-range correlations in high-multiplicity pp collisions.
Links: CDS record (PDF) ; inSPIRE record ; CADI line (restricted) ; These preliminary results are superseded in this paper, PLB 765 (2017) 193. The superseded preliminary plots can be found here.

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