Long-range and short-range dihadron angular correlations in central PbPb collisions at $ \sqrt {{{s_{\text{NN}}}}} $  = 2.76 TeV

Abstract

First measurements of dihadron correlations for charged particles are presented for central PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV over a broad range in relative pseudorapidity (∆η) and the full range of relative azimuthal angle (∆φ). The data were collected with the CMS detector, at the LHC. A broadening of the away-side (∆φ ≈ π) azimuthal correlation is observed at all ∆η, as compared to the measurements in pp collisions. Furthermore, long-range dihadron correlations in ∆η are observed for particles with similar φ values. This phenomenon, also known as the "ridge", persists up to at least |∆η| = 4. For particles with transverse momenta (p T ) of 2-4 GeV/c, the ridge is found to be most prominent when these particles are correlated with particles of p T = 2-6 GeV/c, and to be much reduced when paired with particles of p T = 10-12 GeV/c.

Heavy-ion Results of the CMS Experiment

“…the long-range (large ∆η) near-side (∆φ ≈ 0) two-particle correlations. A similar correlation structure has also been observed in 2.76 TeV PbPb collisions at the LHC [28,29]. Unexpectedly, the effect appeared in very high-multiplicity pp collisions at 7 TeV as well [30].…”

Observation of near-side angular correlations in pPb collisions at 5.02TeV with CMS

“…To further investigate these long-range near-side pA correlations and to provide a quantitative comparison to measurements in pp collisions, one-dimensional (1D) ∆φ distributions are formed by averaging the two-dimensional signal and background distributions over the range of 2 < |∆η| < 4 [1,3,4]. The minimum of the correlation function in the azimuthal range of 0.1 < |∆φ| < 2 is first found and the corresponding value, C ZYAM , is subtracted from the 1-D correlation function [6].…”

“…The two-particle correlations analysis was performed in bins of track multiplicity in order to match the analysis strategy in high-multiplicity pp collisions [1], and using the procedure established in [3,4]. Charged particles, which have their trajectories reconstructed in the silicon tracker, are denoted as "tracks", and multiplicity, N offline trk , is defined as the number of tracks having |η| < 2.4 and p T > 0.4 GeV/c and passing a set of quality cuts described in more detail in [5].…”

Mean transverse momenta correlations in hadron-hadron collisions in MC toy model with repulsing strings

“…More dihadron results from the MC toy model and the discussion can be found in the contribution called "Constraints on the percolation model from anomalous centrality evolution of two-particle correlations" in the same proceedings. The dihadron analysis method, used here, is the same as, for example, in [9]. Charged particles with p T > 0.15 GeV/c in |η| < 2 are analysed.…”