Observation and studies of jet quenching in PbPb collisions at nucleon-nucleon center-of-mass energy = 2.76 TeV

TL;DR

<3-5 sentence high-level summary> CMS reports a robust observation of jet quenching in PbPb collisions at sqrt(s_NN)=2.76 TeV, evidenced by a centrality-dependent dijet energy imbalance that grows with collision centrality and extends to high leading-jet pT. By reconstructing jets from calorimeter energy and correlating them with tracks, the study shows a softened fragmentation pattern for the away-side jet and demonstrates that the missing energy reappears as soft particles at large angles, indicating medium-induced energy loss and redistribution. Comparisons to pp-based generators embedded in PbPb events reveal that the observed balance and angular correlations in data cannot be fully captured by fragmentation in vacuum, highlighting the role of the QGP. The results provide qualitative and quantitative constraints on jet quenching models and the transport properties of the produced medium."

Abstract

Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV was studied with the CMS detector at the LHC, using a data sample corresponding to an integrated luminosity of 6.7 inverse microbarns. Jets are reconstructed using the energy deposited in the CMS calorimeters and studied as a function of collision centrality. With increasing collision centrality, a striking imbalance in dijet transverse momentum is observed, consistent with jet quenching. The observed effect extends from the lower cut-off used in this study (jet transverse momentum = 120 GeV/c) up to the statistical limit of the available data sample (jet transverse momentum approximately 210 GeV/c). Correlations of charged particle tracks with jets indicate that the momentum imbalance is accompanied by a softening of the fragmentation pattern of the second most energetic, away-side jet. The dijet momentum balance is recovered when integrating low transverse momentum particles distributed over a wide angular range relative to the direction of the away-side jet.

Figures (15)

• Figure 1: Example of an unbalanced dijet in a PbPb collision event at $\sqrt{s_{_{NN}}}=2.76$ TeV. Plotted is the summed transverse energy in the electromagnetic and hadron calorimeters vs. $\eta$ and $\phi$, with the identified jets highlighted in red, and labeled with the corrected jet transverse momentum.
• Figure 2: (a) Efficiency curve for the HLT 50 ${\,\text{Ge\spaceV\space/\space}c}$ single-jet trigger, as a function of the corrected leading jet transverse momentum. Error bars shown are statistical. (b) Correlation between the number of pixel hits and HF total energy for a single run containing 60 k minimum bias events, after selections as described in the text.
• Figure 3: (a) Probability distribution of the total HF energy for minimum bias collisions (black open histogram). The five regions correspond to the centrality ranges used in this analysis. Also shown is the HF energy distribution for the subset of events passing the HLT jet trigger (red hatched histogram). (b) Distribution of the fraction of events in the 40 centrality bins for minimum bias (black open histogram) and HLT jet triggered (red hatched histogram) events. The centrality-bin labels run from 100% for the most peripheral to 0% for the most central events.
• Figure 4: The top row shows the mean of the ratio of reconstructed to generated jet momenta, $\langle p_{\mathrm{T}}\xspace^{\rm CaloJet}/p_{\mathrm{T}}\xspace^{\rm GenJet}\rangle$, as a function of $p_{\mathrm{T}}\xspace^{\rm GenJet}$, while the bottom row shows the relative resolution, i.e., the standard deviation of $p_{\mathrm{T}}\xspace^{\rm CaloJet}/p_{\mathrm{T}}\xspace^{\rm GenJet}$. The standard pp jet energy corrections are included in $p_{\mathrm{T}}\xspace^{\rm CaloJet}$. Filled circles are for the leading jets and open squares are for the subleading jets. The left, center, and right columns are for jets in pythia+data events with centrality 50--100%, 20--30%, and 0--10%, respectively. On the jet resolution plots (bottom row), the dashed line is a fit to the leading jet resolution in pp events. The vertical bars denote the statistical uncertainty.
• Figure 5: Jet reconstruction efficiency as a function of generator level jet $p_{\mathrm{T}}\xspace$ for the leading jet (filled circles) and subleading jet (open squares). From left to right three centrality bins are shown: 30--100%, 10--30%, 0--10%. The vertical bars denote the statistical uncertainty.