Evidence for transverse-momentum- and pseudorapidity-dependent event-plane fluctuations in PbPb and pPb collisions

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 p T and η 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 √ s NN = 2.76 TeV and pPb collisions at √ s NN = 5.02 TeV, covering a very wide range of multiplicity. Factorization is observed to be broken as a function of both particle p T and η. When measured with particles of different p T , 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 η. 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 η-dependent factorization data provide new insights to the longitudinal evolution of the medium formed in heavy ion collisions.

Theory of pp/pA/small systems

“…Theory of pp/pA/small systems Piotr Bożek factorization is approximately fulfilled for p ⊥ < 3 GeV [47], and the agreement with model calculations is satisfactory [28].…”

Rapidity decorrelation from hydrodynamic fluctuations

“…Analyzing the phase space distributions of final hadrons, we compare our results with experimental data. As for the centrality cut, we do almost the same way as done in the experimental analysis: We categorize all the events into each centrality bin using the charged-hadron multiplicity distribution in 2.9 < |η p | < 5.2 [21,50]. For each event, we define the centrality percentile to the total number of events.…”

“…In the following, V n∆ (η a p , η b p ) is calculated with the rapidity region 0 < η a p < 2.5 and the transverse momentum region 0.3 < p a T < 3.0 for a particle a, and the reference rapidity region 3.0 < η b p < 4.0 for a particle b. These rapidity and transverse momentum regions follow the experimental setup of the CMS Collaboration [50]. Figure 4 shows the factorization ratio in the longitudinal direction, r 2 (η a p , η b p ), in Pb+Pb collisions at √ s N N = 2.76 TeV for 0-5% and 20-30% centralities.…”

“…To investigate this rapidity decorrelation phenomenon caused by the hydrodynamic fluctuations, we consider the observable called factorization ratios which quantify the decorrelation. The factorization ratio was initially proposed as a function of transverse momentum [49] and was later extended in the longitudinal direction as a function of pseudorapidity [50] for the purpose of analyzing the longitudinal dynamics. Since then, the factorization ratios in the longitudinal direction are widely measured in experiments [50][51][52][53][54].…”

Flow and correlation phenomena measurements in pp, pPb and PbPb collisions at CMS

“…4. One must note that the two-particle correlation v n with a rapidity gap is used for the normalization, therefore, the results could be affected by the event-plane decorrelation as measured in [15] within a few percent. Nevertheless, all systems would be affected consistently, so that the conclusions from the results would remain unchanged.…”