Study of high-pT charged particle suppression in PbPb compared to pp collisions at sqrt(sNN)=2.76 TeV

TL;DR

This CMS study analyzes high-$p_T$ charged-particle production in PbPb and pp collisions at a common energy (2.76 TeV per nucleon pair) to quantify jet quenching effects via the nuclear modification factors $R_{AA}$ and $R_{CP}$. By leveraging Glauber-model-based $T_{AA}$ normalization and extensive track-c reconstruction in the dense PbPb environment, the work reveals a pronounced suppression ($R_{AA}\approx$ 0.13) at intermediate $p_T$ (6 GeV/$c$) in central events, followed by a gradual rise to $R_{AA}\approx$ 0.5–0.6 at $p_T$ = 40–100 GeV/$c$, indicating reduced energy loss at higher momenta. The results, consistent with ALICE and RHIC trends, provide stringent constraints on jet-quenching models and energy-loss mechanisms, and complement jet-based measurements to illuminate the properties of the quark–gluon plasma.

Abstract

The transverse momentum spectra of charged particles have been measured in pp and PbPb collisions at sqrt(sNN) = 2.76 TeV by the CMS experiment at the LHC. In the transverse momentum range pt = 5-10 GeV/c, the charged particle yield in the most central PbPb collisions is suppressed by up to a factor of 5 compared to the pp yield scaled by the number of incoherent nucleon-nucleon collisions. At higher pt, this suppression is significantly reduced, approaching roughly a factor of 2 for particles with pt in the range pt=40-100 GeV/c.

Figures (7)

• Figure 1: (a) Probability distribution of the total HF energy for minimum bias events (black line), Jet65-triggered (blue-shaded region), and Jet80-triggered (red-shaded region) events. (b) Distribution of the events in bins of fractional cross section for minimum bias (black line), Jet65-triggered (blue-shaded region), and Jet80-triggered (red-shaded region) events. By convention, 0% denotes the most central events and 100% the most peripheral.
• Figure 2: (a) Upper panel: Corrected transverse energy $E_{\mathrm{T}}$ of leading jets with $|\eta|<2$ for a minimum bias trigger and two jet triggers normalized to the number of selected minimum bias events $N^{\text{Evt}}_{\mathrm{MB}}$. Lower panel: efficiency curves for the jet triggers with corrected energy thresholds of 65 and 80$\,\text{Ge\spaceV}$. (b) Upper panel: The three trigger contributions to the charged particle transverse momentum spectrum and their sum (filled circles) for the 0--5% most central events. Open squares show the minimum bias spectrum for all values of leading-jet $E_{\mathrm{T}}$. Lower panel: the ratio of the combined spectrum to the minimum bias spectrum.
• Figure 3: (a) Upper panel: Invariant charged particle differential yield for $|\eta|<1.0$ in $\mathrm{p}$$\mathrm{p}$ collisions at $\sqrt{s}=2.76$$\,\text{Te\spaceV}$ compared with the predictions of four tunes Bartalini:2009xxSjostrand:2007gsSkands:2009zmBuckley:2009bj of the pythia MC generator and with the CMS interpolated spectrum using data at 0.9 and 7$\,\text{Te\spaceV}$ppreferenceCMS. Lower panel: the ratio of the measured spectrum to the predictions of the four pythia tunes and to the interpolated spectrum. The grey band corresponds to the statistical and systematic uncertainties of the measurement added in quadrature. (b) Upper panel: Inclusive charged particle invariant differential cross sections, scaled by $(\sqrt{s})^{4.9}$, for $|\eta|<1.0$ as a function of the scaling parameter $x_{\mathrm{T}}$ for CMS data at 0.9 and 7$\,\text{Te\spaceV}$ppreferenceCMS and this analysis at 2.76$\,\text{Te\spaceV}$. The result is the average of the positive and negative charged particles. Lower panel: ratios of the differential cross sections measured at 0.9, 2.76, and 7$\,\text{Te\spaceV}$ to those predicted by NLO calculations Arleo:2010kw. The bands show the variations in the predictions when changing the factorization scales from 0.5 to 2.0 $p_{\mathrm{T}}$.
• Figure 4: Upper panel: Invariant charged particle differential yield in PbPb collisions at 2.76$\,\text{Te\spaceV}$ in bins of collision centrality (symbols), compared to that of $\mathrm{p}$$\mathrm{p}$ at 2.76$\,\text{Te\spaceV}$, normalized by the corresponding $\mathrm{p}$$\mathrm{p}$ invariant cross sections scaled by the nuclear overlap function (dashed lines). The spectra for different centrality bins have been scaled by the arbitrary factors shown in the figure, for easier viewing. The statistical uncertainty is smaller than the marker size for most of the points. Lower panel: The average relative systematic uncertainties of the PbPb differential yields for the 0--10% and 10--90% centrality intervals, as a function of $p_{\mathrm{T}}$.
• Figure 5: Nuclear modification factor $R_{\mathrm{AA}}$ (filled circles) as a function of $p_{\mathrm{T}}$ for six PbPb centralities. The error bars represent the statistical uncertainties and the yellow boxes represent the $p_{\mathrm{T}}$-dependent systematic uncertainties. An additional systematic uncertainty from the normalization of $T_{\mathrm{AA}}$ and the $\mathrm{p}$$\mathrm{p}$ integrated luminosity, common to all points, is shown as the shaded band around unity in each plot.