Investigations of Anisotropic Flow Using Multiparticle Azimuthal Correlations in pp , p−Pb , Xe-Xe, and Pb-Pb Collisions at the LHC

Abstract

Measurements of anisotropic flow coefficients (v n) and their cross-correlations using two-and multiparticle cumulant methods are reported in collisions of pp at ffiffi ffi s p ¼ 13 TeV, p-Pb at a center-ofmass energy per nucleon pair ffiffiffiffiffiffiffi ffi s NN p ¼ 5.02 TeV, Xe-Xe at ffiffiffiffiffiffiffi ffi s NN p ¼ 5.44 TeV, and Pb-Pb at ffiffiffiffiffiffiffi ffi s NN p ¼ 5.02 TeV recorded with the ALICE detector. The multiplicity dependence of v n is studied in a very wide range from 20 to 3000 particles produced in the midrapidity region jηj < 0.8 for the transverse momentum range 0.2 < p T < 3.0 GeV=c. An ordering of the coefficients v 2 > v 3 > v 4 is found in pp and p-Pb collisions, similar to that seen in large collision systems, while a weak v 2 multiplicity dependence is observed relative to nucleus-nucleus collisions in the same multiplicity range. Using a novel subevent method, v 2 measured with four-particle cumulants is found to be compatible with that from six-particle cumulants in pp and p-Pb collisions. The magnitude of the correlation between v 2 n and v 2 m , evaluated with the symmetric cumulants SCðm; nÞ is observed to be positive at all multiplicities for v 2 and v 4 , while for v 2 and v 3 it is negative and changes sign for multiplicities below 100, which may indicate a different v n fluctuation pattern in this multiplicity range. The observed long-range multiparticle azimuthal correlations in high multiplicity pp and p-Pb collisions can neither be described by PYTHIA 8 nor by impact-parameter-Glasma, MUSIC, and ultrarelativistic quantum molecular dynamics model calculations, and hence, provide new insights into the understanding of collective effects in small collision systems.

A framework for hadronic rescattering in pp collisions

“…Even more spectacular is the smoothly increasing fraction of strange baryon production with increasing charged multiplicity, a trend that lines up with pA data before levelling out at the AA results [4,5]. Further examples include non-vanishing v 2 azimuthal flow coefficients [2,3,6], strong peaks in hadron ratios such as Λ 0 /K 0 S at around p ⊥ ≈ 2 GeV [7], and an p ⊥ strongly increasing with particle mass [8], all suggesting some form of collective flow. A recent overview of relevant observations a e-mail: marius.utheim@thep.lu.se (corresponding author) and related theoretical ideas and challenges can be found in Ref.…”

Fluctuations of anisotropic flow in Pb+Pb collisions at $$ \sqrt{{\mathrm{s}}_{\mathrm{NN}}} $$ = 5.02 TeV with the ATLAS detector

“…which are then used to define flow harmonics v n {2k} such as v n {2} = (c n {2}) 1/2 and v n {4} = (−c n {4}) 1/4 . The four-particle cumulants c 2 {4} and c 3 {4} have been measured at RHIC and the LHC [24][25][26][27][28][29][30][31]. Most models of the initial state of A+A collisions predict a p(v n ) with shape that is close to Gaussian, and these models predict zero or negative values for c n {4} [32,33].…”

Dipole evolution: perspectives for collectivity and γ*A collisions

“…In order to compare to data, all events are hadronized with Pythia 8 after the parton-level eccentricities are calculated. Results are presented and in a single case compared to data from ALICE [82]. Parton level eccentricities were calculated with a p ⊥ weighting, cf.…”

“…the baseline pp sample using the Gaussian (red) and dipole (blue) models. Data points calculated from ALICE figures[82]. (b) The normalized symmetric cumulants N SC(3, 2) for pp, pPb, PbPb collisions (solid, dashed, dotted, respectively) using the Glauber (black), Gaussian (red) and dipole (blue) models.…”