Abstract
This paper describes the measurements of flow harmonics $v_2$-$v_6$ in 3 $\mu\mathrm{b}^{-1}$ of Xe+Xe collisions at $\sqrt{s_{\mathrm{NN}}}=5.44$ TeV performed using the ATLAS detector at the LHC. Measurements of the centrality, multiplicity and $p_{\mathrm{T}}$ dependence of the $v_n$ obtained using two-particle correlations and the scalar product technique are presented. The measurements are also performed using a template-fit procedure, which was developed to remove non-flow correlations in small collision systems. This non-flow removal is shown to have a significant influence on the measured $v_n$ at high $p_{\mathrm{T}}$, especially in peripheral events. Comparisons of the measured $v_n$ with measurements in Pb+Pb collisions and $p$+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02$ TeV are also presented. The $v_n$ values in Xe+Xe collisions are observed to be larger than those in Pb+Pb collisions for $n=2$, 3 and 4 in the most central events. However, with decreasing centrality or increasing harmonic order $n$, the $v_n$ values in Xe+Xe collisions become smaller than those in Pb+Pb collisions. The $v_n$ in Xe+Xe and Pb+Pb collisions are also compared as a function of the mean number of participating nucleons, $\langle N_\text{part} \rangle$, and the measured charged-particle multiplicity in the detector. The $v_3$ values in Xe+Xe and Pb+Pb collisions are observed to be similar at the same $\langle N_\text{part}\rangle$ or multiplicity, but the other harmonics are significantly different. Comparisons of the $v_n$ measurements with theoretical calculations are also made.
Highlights
Heavy-ion collisions, such as those at the Relativistic Heavy Ion Collider (RHIC) [1,2,3,4] and at the Large Hadron Collider (LHC) [5,6,7,8,9,10], produce a state of matter, with deconfined quarks and gluons, commonly called quark-gluon plasma (QGP)
Events containing more than one inelastic interaction were removed by exploiting the correlation between the transverse energy measured in the forward calorimeters (FCal) ( ETFCal ) and the number of tracks associated with the primary vertex
Because of “jet quenching” effects present in heavy-ion collisions [44,45,46,47], it is possible that the assumption made in the template-fit procedure, that the shape of the dijet correlation in φ does not change from low- to high-multiplicity events, may not be valid
Summary
Heavy-ion collisions, such as those at the Relativistic Heavy Ion Collider (RHIC) [1,2,3,4] and at the Large Hadron Collider (LHC) [5,6,7,8,9,10], produce a state of matter, with deconfined quarks and gluons, commonly called quark-gluon plasma (QGP). Where φ is the azimuthal angle of the particle momentum and vn and n are the magnitude and phase of the nth-order anisotropy. Measurements of the vn and their comparisons with calculations based on relativistic hydrodynamics have shown that the QGP produced in heavy-ion collisions behaves like a nearly perfect fluid, characterized by a very low ratio of shear viscosity to entropy density, η/s, close to the conjectured lower limit of h/4π kB [13]. Phases [21] of different-order anisotropies These highprecision measurements have led to significant improvement in constraining the value of η/s [22,23]. The initial entropy production in heavy-ion collisions, is to compare the vn measured across collision systems of different sizes [24].
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