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 integrated luminosities of 1.0 inverse picobarn (5 TeV), 6.2 inverse picobarns (7 TeV), and 0.7 inverse picobarns (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 K0 short and Lambda/anti-Lambda 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), K0 short, and Lambda/anti-Lambda, with lighter particle species exhibiting a stronger azimuthal anisotropy signal below pt of about 2 GeV/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.
Highlights
The observation of long-range two-particle azimuthal correlations at large relative pseudorapidity (|∆η|) in high final-state particle multiplicity proton-proton [1,2,3] and proton-lead [4,5,6,7] collisions at the LHC has opened up new opportunities for studying novel dynamics of particle production in small, high-density quantum chromodynamic (QCD) systems
This paper extends the characterization of long-range correlation phenomena in high-multiplicity pp collisions by presenting a detailed study of two- and multi-particle azimuthal correlations with unidentified charged particles, as well as correlations of reconstructed K0S and Λ/Λ particles at various LHC collision energies
√ The v2 values reported by the ATLAS collaboration for pp collisions at s = 13 TeV in Ref. [2] have a different multiplicity dependence than the results presented in this paper
Summary
The observation of long-range two-particle azimuthal correlations at large relative pseudorapidity (|∆η|) in high final-state particle multiplicity (high-multiplicity) proton-proton (pp) [1,2,3] and proton-lead (pPb) [4,5,6,7] collisions at the LHC has opened up new opportunities for studying novel dynamics of particle production in small, high-density quantum chromodynamic (QCD) systems. Various theoretical models have been proposed to interpret the origin of the observed long-range correlations in small collision systems [30,31,32,33,34,35,36,37] These include initial-state gluon correlations without final-state interactions [33, 34] or, similar to what is thought to occur in AA systems, hydrodynamic flow that develops in a conjectured high-density medium [35,36,37]. A better understanding of the underlying particle correlation mechanisms leading to these observations requires more detailed study of the properties of the v2 and higher-order harmonics in pp collisions Their dependence on particle species, and other aspects related to their possible collective nature, are the key to scrutinize various theoretical interpretations.
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