Measurement of the centrality dependence of the charged particle pseudorapidity distribution in lead-lead collisions at sqrt(s_NN) = 2.76 TeV with the ATLAS detector

TL;DR

This paper addresses how charged-particle production in ultra-relativistic Pb+Pb collisions depends on collision centrality at the LHC. Using ATLAS data collected with the solenoid off to reach $p_T\sim30$ MeV, the authors reconstruct multiplicities from pixel tracklets and pixel tracks, and quantify centrality via forward calorimeter energy with a Glauber-model-based interpretation of $\langle N_{part}\rangle$. They report $dN_{\mathrm{ch}}/d\eta$ across $|\eta|<2$ and its centrality evolution, find that the mid-rapidity yield per participant pair scales with centrality similarly to lower-energy data, and show that the $dN_{\mathrm{ch}}/d\eta$ shape is largely independent of centrality within systematic uncertainties. The results are consistent with prior LHC measurements by ALICE and CMS and with RHIC trends after appropriate energy scaling, providing strong constraints on bulk particle production mechanisms in heavy-ion collisions.

Abstract

The ATLAS experiment at the LHC has measured the centrality dependence of charged particle pseudorapidity distributions over |eta| < 2 in lead-lead collisions at a nucleon-nucleon centre-of-mass energy of sqrt(s_NN) = 2.76 TeV. In order to include particles with transverse momentum as low as 30 MeV, the data were recorded with the central solenoid magnet off. Charged particles were reconstructed with two algorithms (2-point "tracklets" and full tracks) using information from the pixel detector only. The lead-lead collision centrality was characterized by the total transverse energy in the forward calorimeter in the range 3.2 < |eta| < 4.9. Measurements are presented of the per-event charged particle density distribution, dN_ch/deta, and the average charged particle multiplicity in the pseudorapidity interval |eta|<0.5 in several intervals of collision centrality. The results are compared to previous mid-rapidity measurements at the LHC and RHIC. The variation of the mid-rapidity charged particle yield per colliding nucleon pair with the number of participants is consistent with the lower sqrt(s_NN) results. The shape of the dN_ch/deta distribution is found to be independent of centrality within the systematic uncertainties of the measurement.

Figures (4)

• Figure 1: Tracklet candidate $\Delta\eta$ (left) and $\Delta\phi$ (right) distributions from data (histogram) and reweighted MC (shaded region) for Pb+Pb collisions at $\hbox{$\sqrt{s_{_\mathrm{NN}}}$}=2.76$ TeV. The top panels correspond to $|\eta|<1$ and the bottom panels correspond to $1<|\eta|<2$. Data and MC distributions are normalized to the same area.
• Figure 2: Left: Top: uncorrected track/tracklet $dN_{\mathrm{raw}}/d\eta$ distribution from tracklet Method 1 (points), tracklet Method 2 (squares) and pixel tracking (blue triangles) for 0-10$\%$ centrality events. Middle: corrected tracklet and track $d\hbox{$N_{\mathrm{ch}}$}/d\eta$ distributions. Bottom: ratio of $d\hbox{$N_{\mathrm{ch}}$}/d\eta$ from the tracklet Method 2 (squares) and pixel tracking (triangles) to tracklet Method 1. Right:$d\hbox{$N_{\mathrm{ch}}$}/d\eta$ distributions from tracklet Method 1 for eight 10% centrality intervals. The statistical errors are shown as bars and the systematic errors are shown as shaded bands.
• Figure 3: Top:$\sqrt{s_{_\mathrm{NN}}}$ dependence of the charged particle $d\hbox{$N_{\mathrm{ch}}$}/d\eta$ per colliding nucleon pair $\hbox{$d\hbox{$N_{\mathrm{ch}}$}/d\eta|_{\eta=0}$}/(\hbox{$\langle N_{\mathrm{part}} \rangle$}/2)$ from a variety of measurements in p+p and $\bar{\mathrm{p}}$+p (inelastic and non-single diffractive results from Alver:2010ck and references therein, as well as :2010ir-:2009dt) and central A+A collisions, including the ATLAS 0-6% centrality measurement reported here for $|\eta|<0.5$ and the previous 0-5% centrality ALICE Collaboration:2010cz and CMS CMS:2011mult measurements (points shifted horizontally for clarity). The curves show different expectations for the $\sqrt{s_{_\mathrm{NN}}}$ dependence in A+A collisions: results of a Landau hydrodynamics calculation Carruthers:1973ws (dotted line) , an $s^{0.15}$ extrapolation of RHIC and SPS data proposed by ALICE Collaboration:2010cz (dashed line), a logarithmic extrapolation of RHIC and SPS data from Busza:2007ke (solid line). Bottom:$\hbox{$d\hbox{$N_{\mathrm{ch}}$}/d\eta|_{\eta=0}$}/(\hbox{$\langle N_{\mathrm{part}} \rangle$}/2)$ vs $\langle N_{\mathrm{part}} \rangle$ for 2% centrality intervals over 0-20% and 5% centrality intervals over 20-80%. Error bars represent combined statistical and systematic uncertainties on the $d\hbox{$N_{\mathrm{ch}}$}/d\eta|_{\eta=0}$ measurements, whereas the shaded band indicates the total systematic uncertainty including $\langle N_{\mathrm{part}} \rangle$ uncertainties. The RHIC measurements (see text) have been multiplied by 2.15 to allow comparison with the $\hbox{$\sqrt{s_{_\mathrm{NN}}}$} = 2.76$Te V results. The inset shows the $\langle N_{\mathrm{part}} \rangle<60$ region in more detail.
• Figure 4: Top:$d\hbox{$N_{\mathrm{ch}}$}/d\eta$ distributions from tracklet Method 1, scaled by $d\hbox{$N_{\mathrm{ch}}$}/d\eta|_{\eta=0}$, as a function of the pseudorapidity for the 70-80% centrality interval. The statistical errors are shown as error bars. Bottom: Ratio of $\hbox{$d\hbox{$N_{\mathrm{ch}}$}/d\eta$}/(\hbox{$\langle N_{\mathrm{part}} \rangle$}/2)$ measured in different centrality intervals: 0-10% (squares), 20-30% (triangles), 40-50% (inverted triangles) and 60-70% (crosses) to that measured in peripheral collisions (70-80%). Statistical uncertainties are shown as bars while $\eta$-dependent systematic uncertainties are shown as shaded bands.