Dependence on pseudorapidity and centrality of charged hadron production in PbPb collisions at a nucleon-nucleon centre-of-mass energy of 2.76 TeV

TL;DR

This study reports the charged-hadron multiplicity density in PbPb collisions at $\sqrt{s_{NN}}=2.76$ TeV, using two independent CMS pixel-based methods (hit-counting and tracklets) to measure $dN_{ch}/d\eta$ across centrality and pseudorapidity. Centrality is defined via HF energy and connected to $N_{part}$ through a Glauber model, with UPC effects carefully controlled. The key result at mid-rapidity for the most central 0–5% is $dN_{ch}/d\eta|_{\eta=0}=1612\pm55$ (syst.), and the $N_{part}$-normalized densities show weak $\eta$ dependence and a centrality shape consistent with saturation-based descriptions. The measurements, in good agreement with ALICE, provide important initial-condition constraints for hydrodynamic evolution and the energy dependence of particle production, informing models of the partonic structure and QCD dynamics in ultra-relativistic heavy-ion collisions.

Abstract

A measurement is presented of the charged hadron multiplicity in hadronic PbPb collisions, as a function of pseudorapidity and centrality, at a collision energy of 2.76 TeV per nucleon pair. The data sample is collected using the CMS detector and a minimum-bias trigger, with the CMS solenoid off. The number of charged hadrons is measured both by counting the number of reconstructed particle hits and by forming hit doublets of pairs of layers in the pixel detector. The two methods give consistent results. The charged hadron multiplicity density dN(ch)/d eta, evaluated at eta=0 for head-on collisions, is found to be 1612 +/- 55, where the uncertainty is dominated by systematic effects. Comparisons of these results to previous measurements and to various models are also presented.

Figures (6)

• Figure 1: Distribution of the total transverse energy in the HF used to determine the centrality of the PbPb interactions. The centrality boundaries for each 5% centrality interval are shown by the dashed lines.
• Figure 2: Left: Distribution of the angle-corrected pixel-cluster charge in units of equivalent kilo-electrons from 2.76 TeV PbPb data and simulation. Right: Pixel-cluster length along the beam direction in units of pixel cells for hits from the first layer of the BPIX, as a function of $\eta$ after the event selection. The solid red line shows the selection on the minimum cluster length used in the analysis.
• Figure 3: The ( left) $\Delta\eta$ and ( right) $\Delta\phi$ distributions for reconstructed tracklets in minimum-bias collisions from the first and the second pixel layers in data and simulation.
• Figure 4: Left: $dN_{\rm ch}/d\eta|_{\eta=0}$ as a function of centrality class in 2.76 TeV PbPb collisions from this experiment (solid circles) and from ALICE (open squares) ALICE_dNdeta. The inner green band shows the measurement uncertainties affecting the scale of the measured distribution from this analysis, while the outer grey band shows the full systematic uncertainty, i.e. affecting both the scale and the slope. Right: Measured $dN_{\rm ch}/d\eta/(N_{\rm part}/2)$ distributions from this analysis as a function of $\eta$ in various centrality bins.
• Figure 5: Left: Measured $(dN_{\rm ch}/d\eta|_{\eta=0})/(N_{\rm part}/2)$ as a function of the number of participants in 2.76 TeV PbPb collisions from this analysis and the ALICE experiment ALICE_dNdeta, from RHIC RHIC_av at 200 GeV and 19.6 GeV, and from extrapolated pp results from CMS CMS_dNdeta_pp_1 and ALICE ALICE_pp. Systematic uncertainties affecting the scale of the measurements from this analysis are shown as inner green error bands and the total systematic uncertainties as an outer grey band, while the error bars indicate statistical uncertainties. The black stars are shifted slightly to the right for better visibility. The ALICE and the averaged RHIC results are from ALICE_dNdeta and RHIC_av, respectively. Right: Results from this analysis are compared with model predictions of $(dN_{\rm ch}/d\eta|_{\eta=0})/(N_{\rm part}/2)$ as a function of the number of participants in 2.76 TeV PbPb collisions. The model predictions are taken from Refs. HIJING2.0, Albacete, and DPMJET.