CERN Accelerating science

The observation of a long-range, near-side, two-particle correlation (known as the ridge) has been over the past decade a key signature of the hydrodynamic evolution of the hot and strongly interacting matter produced in heavy-ion collisions. Indeed, the Quark-Gluon Plasma appears to behave as a perfect fluid and latest results from LHC experiments in Pb-Pb collisions at 2.76 and 5 TeV show a nice agreement with hydrodynamic expectations, either for inclusive charged hadrons or identified particles. The observation of the ridge in high-multiplicity pp and p-Pb collisions opened up new opportunities of exploring novel QCD dynamics in small colliding systems. While extensive studies of this long-range correlation phenomenon in p-Pb collisions have revealed its collective properties, the nature of the ridge in pp collisions remains unknown. In addition, the underlying mechanism that lies behind the ridge is still not understood, in both p-Pb and pp collisions.

Latest CMS measurements of long-range particle correlations in high-multiplicity pp collisions are presented. Anisotropy Fourier harmonics v2 and v3 are extracted as a function of pT and multiplicity, for charged particles and identified $K^0_s$ and $\Lambda$ hadrons. A multi-particle cumulant technique is also used to explore the collective nature of the long-range correlations. The results are directly compared with data in p-Pb and Pb-Pb collisions, shedding light on the physical origin on the novel collective correlations in small systems.