Measurement of the azimuthal anisotropy of charged particles produced in $$\sqrt{s_{_\text {NN}}}$$ s NN = 5.02 TeV Pb+Pb collisions with the ATLAS detector

Abstract

Measurements of the azimuthal anisotropy in lead–lead collisions at = 5.02 TeV are presented using a data sample corresponding to 0.49 integrated luminosity collected by the ATLAS experiment at the LHC in 2015. The recorded minimum-bias sample is enhanced by triggers for “ultra-central” collisions, providing an opportunity to perform detailed study of flow harmonics in the regime where the initial state is dominated by fluctuations. The anisotropy of the charged-particle azimuthal angle distributions is characterized by the Fourier coefficients, – , which are measured using the two-particle correlation, scalar-product and event-plane methods. The goal of the paper is to provide measurements of the differential as well as integrated flow harmonics over wide ranges of the transverse momentum, 0.5 60 GeV, the pseudorapidity, 2.5, and the collision centrality 0–80%. Results from different methods are compared and discussed in the context of previous and recent measurements in Pb+Pb collisions at = 2.76 and 5.02 . In particular, the shape of the dependence of elliptic or triangular flow harmonics is observed to be very similar at different centralities after scaling the and values by constant factors over the centrality interval 0–60% and the range 0.5 5 GeV.

Disentangling contributions to small-system collectivity via scans of light nucleus-nucleus collisions

“…5 compares the v n (p T ) data for n = 2,3 at two energies in large A+A system and p+A system. It is well-known that v n (p T ) for charged hadrons has very little √ s NN dependence from RHIC to LHC [47], as well as from 39 to 200 GeV at RHIC [48,49], this is confirmed by the left panel which compares the Pb+Pb [50] with Au+Au data [51] in 30-40% centrality. However, a comparison of v n (p T ) between p+Pb [52] and p+Au [14] central data suggests a very different story.…”

Measurement of the azimuthal anisotropy of charged particles in 5.02 TeV Pb+Pb and 5.44 TeV Xe+Xe collisions with ATLAS experiment

“…Secondly, the peak v n is reached at different p T from one centrality to another. ATLAS performed a scaling procedure [3], which yields universal shapes for the flow coefficients across different centrality classes. In the procedure, the x-and y-axes in each centrality class were scaled to best agree with the 0-60% centrality reference shape.…”

“…Figure 3: Left: the v 2 (p T ) obtained with the 2PC method for different centrality classes. Right: the corresponding scaled-v 2 (p T ) in the same centrality classes [3].…”

“…This report presents the recent measurements of v n harmonics obtained by the ATLAS experiment in √ s NN = 5.02 TeV Pb+Pb [3] and √ s NN = 5.44 TeV Xe+Xe [4] collisions. The data sample used in Pb+Pb analysis corresponds to 0.49 nb −1 , while Xe+Xe measurement is based on 3 µb −1 integrated luminosity collected by the ATLAS detector [5] during heavy-ion LHC runs in 2015 and 2017, respectively.…”

“…Both effects are studied in Xe+Xe collisions. Using the large sample of Pb+Pb data, ATLAS measured the v n harmonics in a wide range of centralities from ultra-central to peripheral collisions, and also in a broad p T range up to 60 GeV [3]. Figure 1 shows v n (p T ) integrated over |η| < 2.5 obtained with the SP method in three centrality intervals: 0-0.1% (ultra-central), 30-40% (mid-central) and 60-70% (peripheral).…”