Evidence for Collective Multiparticle Correlations inp−PbCollisions

Abstract

The second-order azimuthal anisotropy Fourier harmonics, v 2 , are obtained in p-Pb and PbPb collisions over a wide pseudorapidity (η) range based on correlations among six or more charged particles. The p-Pb data, corresponding to an integrated luminosity of 35 nb −1 , were collected during the 2013 LHC p-Pb run at a nucleon-nucleon center-of-mass energy of 5.02 TeV by the CMS experiment. A sample of semiperipheral PbPb collision data at ffiffiffiffiffiffiffi ffi s NN p ¼ 2.76 TeV, corresponding to an integrated luminosity of 2.5 μb −1 and covering a similar range of particle multiplicities as the p-Pb data, is also analyzed for comparison. The six-and eight-particle cumulant and the Lee-Yang zeros methods are used to extract the v 2 coefficients, extending previous studies of two-and four-particle correlations. For both the p-Pb and PbPb systems, the v 2 values obtained with correlations among more than four particles are consistent with previously published four-particle results. These data support the interpretation of a collective origin for the previously observed long-range (large Δη) correlations in both systems. The ratios of v 2 values corresponding to correlations including different numbers of particles are compared to theoretical predictions that assume a hydrodynamic behavior of a p-Pb system dominated by fluctuations in the positions of participant nucleons. These results provide new insights into the multiparticle dynamics of collision systems with a very small overlapping region. DOI: 10.1103/PhysRevLett.115.012301 PACS numbers: 25.75.Gz Measurements at the CERN LHC have led to the discovery of two-particle azimuthal correlation structures at large relative pseudorapidity (long range) in protonproton (pp) [1] and proton-lead (p-Pb) [2][3][4][5] collisions. Similar long-range structure has also been observed for ffiffiffiffiffiffiffi ffi [6,7]. The results extend previous studies of relativistic heavy-ion collisions, such as for the copper-copper [8], gold-gold [8][9][10][11][12], and lead-lead (PbPb) [13][14][15][16][17][18] systems, where similar long-range, two-particle correlations at small relative azimuthal angle jΔϕj ≈ 0 were first observed. A fundamental question is whether the observed behavior results from correlations exclusively between particle pairs, or if it is a multiparticle, collective effect. It has been suggested that the hydrodynamic collective flow of a strongly interacting and expanding medium [19][20][21] is responsible for these long-range correlations in central and midcentral heavy-ion collisions. The origin of the observed long-range correlations in collision systems with a small overlapping region, such as for pp and p-Pb collisions, is not clear since for these systems the formation of an extended hot medium is not necessarily expected. Various theoretical models have been proposed to interpret the pp [22,23] and p-Pb results, including initial-state gluon saturation without any final state interactions [24,25] and, similar to what is thought to occur in heavi...

Theory of pp/pA/small systems

“…In p+Pb collisions all higher cumulants are expected to be of similar magnitude, only the second moment is larger. Such a hierarchy has been observed experimentally [22,36,37,38], with v 2 {2} > v 2 {4} v 2 {6} v 2 {8} (Fig. 1, right panel).…”

Development of heavy-flavour flow-harmonics in high-energy nuclear collisions

“…Notice that, within a kinetic description, in order for a system to behave as a fluid the mean-free-path of its constituents has to be much smaller than the system size, λ mfp L. The above condition is only marginally satisfied with perturbative partonic cross-sections and hence the idea of the formation of a strongly-interacting QGP was proposed. A further surprise came in the last few years from the observation of analogous effects (mass-dependent radial, elliptic and triangular flow) also in small systems, like the ones produced in high-multiplicity deuteron-nucleus, proton-nucleus and even proton-proton collisions [43][44][45][46]: in light of the small size of the medium this makes the hydrodynamic interpretation of the experimental measurements in these events quite challenging and alternative explanations have been proposed (see e.g. [47][48][49] Recently some authors proposed a different paradigm to interpret the above experimental observations.…”

Hydrodynamical noise and Gubser flow

“…(5.14) is consistent with a Dirac delta in the transverse plane in the original Milne space-time. 6 The same values of Λ ini fixed by eccentricities in the previous section are kept regarding the colliding systems under consideration. Without losing generality, we take θ 0 = 0.2 and φ 0 = π throughout this work, which corresponds to a peak near the origin on the x-axis.…”

“…Especially in the collision events with sufficiently high multiplicity, it has been noticed that observables related to multi-particle correlations are consistent with a medium collective expansion scenario [6,9,10]. Relativistic hydrodynamics is a natural candidate to simulate and investigate the collective expansion of a QCD medium, even in small colliding systems [11][12][13][14][15][16][17].…”