Observation of particle-species dependence and collectivity in azimuthal correlations of pp collisions at the LHC

Measurements of two- and multi-particle angular correlations in pp collisions at $\sqrt{s} = 5$, $7$, and $13~\mathrm{TeV}$ are presented as a function of charged-particle multiplicity. The data, corresponding to an integrated luminosity of $1.0~\mathrm{pb}^{-1}$ ($5~\mathrm{TeV}$), $6.2~\mathrm{pb}^{-1}$ ($7~\mathrm{TeV}$), and $0.7~\mathrm{pb}^{-1}$ ($13~\mathrm{TeV}$), 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 $\text{K}_{\text{S}}^{0}$ 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 $\sqrt{s} = 13~\mathrm{TeV}$. For $p_\mathrm{T}<2.5~\mathrm{GeV}/c$, a mass ordering is observed for the $v_2$ values of charged hadrons (mostly pions), $\text{K}_{\text{S}}^{0}$, 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 $13~\mathrm{TeV}$ 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.