Abstract
A systematic study of the factorization of long-range azimuthal two-particle correlations into a product of single-particle anisotropies is presented as a function of pt and eta of both particles, and as a function of the particle multiplicity in PbPb and pPb collisions. The data were taken with the CMS detector for PbPb collisions at sqrt(s[NN]) = 2.76 TeV and pPb collisions at sqrt(s[NN]) = 5.02 TeV, covering a very wide range of multiplicity. Factorization is observed to be broken as a function of both particle pt and eta. When measured with particles of different pt, the magnitude of the factorization breakdown for the second Fourier harmonic reaches 20% for very central PbPb collisions but decreases rapidly as the multiplicity decreases. The data are consistent with viscous hydrodynamic predictions, which suggest that the effect of factorization breaking is mainly sensitive to the initial-state conditions rather than to the transport properties (e.g., shear viscosity) of the medium. The factorization breakdown is also computed with particles of different eta. The effect is found to be weakest for mid-central PbPb events but becomes larger for more central or peripheral PbPb collisions, and also for very high-multiplicity pPb collisions. The eta-dependent factorization data provide new insights to the longitudinal evolution of the medium formed in heavy ion collisions.
Highlights
The goal of experiments with heavy ion collisions at ultrarelativistic energies is to study nuclear matter under extreme conditions
At the significantly higher collision energies achieved at the Large Hadron Collider (LHC), the collective phenomena of this quark gluon plasma have been studied in great detail [5,6,7,8,9,10,11,12,13]
The effect of pT-dependent factorization breakdown for the second-order Fourier harmonic is found to increase with the difference in pT between the two particles
Summary
The goal of experiments with heavy ion collisions at ultrarelativistic energies is to study nuclear matter under extreme conditions. The collective expansion of the hot medium in heavy ion collisions can be described by hydrodynamic-flow models Motivated by such models, the azimuthal distribution of emitted particles can be characterized by the Fourier components of the hadron yield distribution in azimuthal angle φ [14,15,16], dN ∝ 1 + 2 dφ n vn cos[n(φ −. Due to local perturbations in the energy density distribution generating a pressure gradient that drives particles in random directions with differing boosts, the resulting event-plane angles found with final-state particles from different pT ranges may fluctuate with respect to each other ( still correlated with the initial participant plane).
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