Measurement of the azimuthal anisotropy of charged particles produced in s NN = 5.02 TeV Pb+Pb collisions with the ATLAS detector

Abstract

Measurements of the azimuthal anisotropy in lead-lead collisions at $\sqrt{s_{{}_{\text{NN}}}}$ = 5.02 TeV are presented using a data sample corresponding to 0.49 ${\mathrm{nb}}^{-1}$ 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, $v_2$ - $v_7$ , 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 $v_n$ over wide ranges of the transverse momentum, 0.5 $<p_T<$ 60 GeV, the pseudorapidity, $\left|\eta \right|<$ 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 $\sqrt{s_{{}_{\text{NN}}}}$ = 2.76 $\mathrm{TeV}$ and 5.02 $\mathrm{TeV}$ . In particular, the shape of the $p_T$ dependence of elliptic or triangular flow harmonics is observed to be very similar at different centralities after scaling the $v_n$ and $p_T$ values by constant factors over the centrality interval 0-60% and the $p_T$ range 0.5 $<p_T<$ 5 GeV.

Fig. 1

The $\mathrm{\Sigma }{E}_{\mathrm{T}}^{\mathrm{FCal}}$ distribution in $\sqrt{s_{{}_{\text{NN}}}}$ = 5.02 $\mathrm{TeV}$ Pb+Pb data for events selected by the minimum-bias trigger. The $\mathrm{\Sigma }{E}_{\mathrm{T}}^{\mathrm{FCal}}$ thresholds for several centrality intervals are marked with vertical lines and labelled on the plot. Also shown are the number of events over the 0–1% and 0–0.1% centrality intervals selected by the ultra-central triggers

Fig. 2

Two-particle correlation functions $C(\mathrm{\Delta }\eta ,\mathrm{\Delta }\varphi )$ in 5.02 $\mathrm{TeV}$ Pb+Pb collisions for $2<p_{\mathrm{T}}^{a,b}<3\mathrm{GeV}$. The left, middle and right panels correspond to the 0–5%, 30–40% and 60–70% centrality classes, respectively. The distributions are truncated to suppress the peak at $\mathrm{\Delta }\eta =\mathrm{\Delta }\varphi =0$ to show the long-range correlations in greater detail

Fig. 3

One-dimensional two-particle correlation functions $C(\mathrm{\Delta }\varphi )$ in 5.02 $\mathrm{TeV}$ Pb+Pb collisions for $2<p_{\mathrm{T}}^{a,b}<3\mathrm{GeV}$ (points). The solid-black line indicates a fit to Eq. (2) containing harmonics $v_{n,n}$ up to $n=6$. The dashed grey line shows the contribution of the $v_{1,1}$. The contributions of the $v_{2,2}$–$v_{6,6}$ are indicated by the coloured lines ($v_{2,2}$-red, $v_{3,3}$-blue, $v_{4,4}$-magenta, $v_{5,5}$-orange, $v_{6,6}$-green), and can be identified by the number of peaks that they have. Each panel corresponds to a different centrality class. The y-axis range for the different panels is different

Fig. 4

The dependence of the correction factor in the SP method, $\sqrt{⟨Q_n^N{Q}_n^{P\ast }⟩}$ (left panel), and EP method, $\sqrt{\langle \frac{Q_n^N{Q}_n^{P\ast }}{|Q_n^N||Q_n^P|}\rangle }$ (right panel), for all measured harmonics as a function of $\mathrm{\Sigma }{E}_{\mathrm{T}}^{\mathrm{FCal}}$ binned according to the centrality bins definition

Fig. 5

The $v_n$ obtained with the SP method as a function of transverse momentum $p_{\mathrm{T}}$, integrated over $|\eta |<2.5$ in 11 centrality intervals, from the most central at the top left panel to the most peripheral at the bottom right panel. Results are averaged over the intervals indicated by horizontal error bars. The vertical error bars indicate statistical uncertainties; the shaded boxes indicate systematic uncertainties

Fig. 6

The $v_n$ values obtained with the 2PC method as a function of $p_{\mathrm{T}}^b$ for $0.5<p_{\mathrm{T}}^a<5\mathrm{GeV}$. Each panel represents a different centrality interval. The vertical error bars indicate statistical uncertainties. The shaded bands indicate systematic uncertainties

Fig. 7

Comparison of the $v_n$ obtained with EP and SP methods as a function of $p_{\mathrm{T}}$ in three centrality bins: 0–5%, 20–30% and 40–50%. The right bottom panel shows the $v_n$ as a function of $N_{\mathrm{part}}$, integrated over $0.5<p_{\mathrm{T}}<60\mathrm{GeV}$. The correspondence of $N_{\mathrm{part}}$ to centrality intervals is provided in Table 2. In the inset the $v_6$ and $v_7$ integrated over $0.5<p_{\mathrm{T}}<60\mathrm{GeV}$ are shown with adjusted scale. For the $v_n(p_{\mathrm{T}})$ comparisons, the results are averaged over the intervals indicated by horizontal error bars. The vertical error bars indicate the quadrature sum of statistical and systematical uncertainties

Fig. 8

Comparison of the $v_n$ obtained with 2PC and SP methods as a function of $p_{\mathrm{T}}$. Each panel shows the comparison for a different order harmonic. The comparisons are shown for three different centrality intervals: 0–5%, 20–30% and 40–50%. The vertical error bars indicate statistical uncertainties only

Fig. 9

Comparisons of the 2PC $v_n$ harmonics measured at $\sqrt{s_{{}_{\text{NN}}}}$ = 2.76 $\mathrm{TeV}$ (Run 1) and at $\sqrt{s_{{}_{\text{NN}}}}$ = 5.02 $\mathrm{TeV}$ (Run 2). The results are plotted as a function of $p_{\mathrm{T}}^b$ for $1<p_{\mathrm{T}}^a<2\mathrm{GeV}$ for two centralities: 0–5% and 20–30%. Each panel corresponds to a different harmonic. Results are averaged over the intervals indicated by horizontal error bars. The vertical error bars indicate statistical uncertainties

Fig. 10

Comparison of the $v_n$ obtained with EP method using Run 1 and Run 2 data as a function of $p_{\mathrm{T}}$. The results are shown in three centrality intervals: 0–5%, 20–30% and 40–50%. Results are averaged over the intervals indicated by horizontal error bars. The vertical error bars indicate statistical uncertainties. The shaded areas indicate systematic uncertainties

Fig. 11

The $v_n$ as a function of pseudorapidity obtained with the SP method, for transverse momentum ranges: $0.8<p_{\mathrm{T}}<1\mathrm{GeV}$ (left column), $2<p_{\mathrm{T}}<3\mathrm{GeV}$ (middle column) and $7<p_{\mathrm{T}}<60\mathrm{GeV}$ (right column) and for centrality intervals: 0–0.1% (top row), 0–5%, 10–20%, 30–40% and 60–70% (bottom row). The vertical error bars indicate statistical uncertainties. The shaded boxes indicate systematic uncertainties

Fig. 12

Integrated $v_n\{\mathrm{SP}\}$ vs. $N_{\mathrm{part}}$ for six $p_{\mathrm{T}}$ranges shown in the panels from lowest $p_{\mathrm{T}}$range at the top left to the highest at the bottom middle, measured using the scalar-product method. In the inset in the bottom-right panel the $v_6$ and $v_7$ integrated over $0.5<p_{\mathrm{T}}<60\mathrm{GeV}$ are shown with adjusted scale. The correspondence of $N_{\mathrm{part}}$ to centrality intervals is provided in Table 2. Results are averaged over the intervals indicated by horizontal error bars. The vertical error bars indicate statistical uncertainties. The shaded areas indicate systematic uncertainties

Fig. 13

Left panels: the $v_2$ ($p_{\mathrm{T}}$) (top) and $v_3$ ($p_{\mathrm{T}}$) (bottom) for different centrality intervals. Right panels: the corresponding scaled-$v_n(p_{\mathrm{T}})$. The error bars indicate statistical and systematic uncertainties added in quadrature and are typically too small to be seen

Fig. 14

Left panel: the x-scale factors for the $v_2(p_{\mathrm{T}})$ and $v_3(p_{\mathrm{T}})$ (see text) as a function of the collision centrality. Also shown for comparison are the ALICE 1/$⟨p_{\mathrm{T}}⟩$ for positively charged protons and pions (scaled by constant factors of 1.4 $\mathrm{GeV}$ and 0.57 $\mathrm{GeV}$, respectively for plotting purposes). Right panel: the y-scale factors. The error bars on the scale factors and on the ALICE data indicate systematic and statistical uncertainties added in quadrature