Abstract
Measurements of two-particle angular correlations between an identified strange hadron (KS0 or Λ/Λ‾) and a charged particle, emitted in pPb collisions, are presented over a wide range in pseudorapidity and full azimuth. The data, corresponding to an integrated luminosity of approximately 35 nb−1, were collected at a nucleon–nucleon center-of-mass energy (sNN) of 5.02 TeV with the CMS detector at the LHC. The results are compared to semi-peripheral PbPb collision data at sNN=2.76 TeV, covering similar charged-particle multiplicities in the events. The observed azimuthal correlations at large relative pseudorapidity are used to extract the second-order (v2) and third-order (v3) anisotropy harmonics of KS0 and Λ/Λ‾ particles. These quantities are studied as a function of the charged-particle multiplicity in the event and the transverse momentum of the particles. For high-multiplicity pPb events, a clear particle species dependence of v2 and v3 is observed. For pT<2 GeV, the v2 and v3 values of KS0 particles are larger than those of Λ/Λ‾ particles at the same pT. This splitting effect between two particle species is found to be stronger in pPb than in PbPb collisions in the same multiplicity range. When divided by the number of constituent quarks and compared at the same transverse kinetic energy per quark, both v2 and v3 for KS0 particles are observed to be consistent with those for Λ/Λ‾ particles at the 10% level in pPb collisions. This consistency extends over a wide range of particle transverse kinetic energy and event multiplicities.
Highlights
Studies of multiparticle correlations provide an important insight into the underlying mechanism of particle production in high-energy collisions of protons and nuclei
2013 pPb run, the identified particle correlation data in pPb collisions are explored over a broad particle multiplicity range, comparable to that covered by 50–100% centrality PbPb collisions
The long-range (| η| > 2) correlations are quantified in terms of azimuthal anisotropy Fourier harmonics motivated by hydrodynamic models
Summary
Studies of multiparticle correlations provide an important insight into the underlying mechanism of particle production in high-energy collisions of protons and nuclei. A key feature of such correlations in ultrarelativistic nucleus–nucleus (AA) collisions is the observation of a pronounced structure on the near side (relative azimuthal angle | φ| ≈ 0) that extends over a large range in relative pseudorapidity (| η| up to 4 units or more) This feature, known as the “ridge”, has been found over a wide range of AA energies and system sizes at both the Relativistic Heavy Ion Collider (RHIC) [1,2,3,4,5] and the Large Hadron Collider (LHC) [6,7,8,9,10] and is interpreted as arising primarily from the collective hydrodynamic flow of a strongly interacting, expanding medium [11,12]. Besides hydrodynamic effects in a high-density system [19,20], an alternate model including gluon saturation in the incoming nucleons has been shown to describe these data [21,22]
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