Evidence for collectivity in pp collisions at the LHC

Abstract

Measurements of two- and multi-particle angular correlations in pp collisions at $\sqrt{s}=5,7$, and 13TeV are presented as a function of charged-particle multiplicity. The data, corresponding to integrated luminosities of $1.0pb^{−1} (5 TeV)$, $6.2pb^{−1} (7 TeV)$, and $0.7pb^{−1} (13 TeV)$, 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 $K^S_0$ and $Λ/ \overline{Λ}$ particles, are extracted from long-range two-particle correlations as functions of particle multiplicity and transverse momentum. For high-multiplicity pp events, a mass ordering is observed for the $v_2$ values of charged hadrons (mostly pions), $K^S_0$, and $Λ/ \overline{Λ}$, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below $p_T≈2GeV/c$. For 13 TeV data, the $v_2$ signals are also extracted from four- and six-particle correlations for the first time in pp collisions, with comparable magnitude to those from two-particle correlations. These observations are similar to those seen in pPb and PbPb collisions, and support the interpretation of a collective origin for the observed long-range correlations in high-multiplicity pp collisions.