Observation and studies of jet quenching in PbPb collisions at $\sqrt{s_{NN}}=2.76$ TeV

Jet production in PbPb collisions at a nucleon-nucleon center-of-mass energy of 2.76 TeV was studied with the Compact Muon Solenoid (CMS) detector at the LHC, using a data sample corresponding to an integrated luminosity of $6.7\mu$b${}^{-1}$. 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 cutoff used in this study (jet $p_{\mathrm{T}}=120$ GeV/c) up to the statistical limit of the available data sample (jet $p_{\mathrm{T}}\approx 210$ 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.

Figure 1

(Color online) 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

(Color online) (a) Efficiency curve for the HLT 50 GeV/$c$ 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 60k minimum bias events, after selections as described in the text.

Figure 3

(Color online) (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

(Color online) The top row shows the mean of the ratio of reconstructed to generated jet momenta $\langle {p_{\mathrm{T}}}^{\mathrm{CaloJet}}/{p_{\mathrm{T}}}^{\mathrm{GenJet}}\rangle$ as a function of ${p_{\mathrm{T}}}^{\mathrm{GenJet}}$, while the bottom row shows the relative resolution, i.e., the standard deviation of ${p_{\mathrm{T}}}^{\mathrm{CaloJet}}/{p_{\mathrm{T}}}^{\mathrm{GenJet}}$. The standard pp jet energy corrections are included in ${p_{\mathrm{T}}}^{\mathrm{CaloJet}}$. Filled circles are for the leading jets and open squares are for the subleading jets. The left-hand, center, and right-hand 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

(Color online) Jet reconstruction efficiency as a function of generator level jet $p_{\mathrm{T}}$ 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.

Figure 6

(Color online) Subleading jet $p_{\mathrm{T}}$ vs leading jet $p_{\mathrm{T}}$ distributions with $\Delta {\phi }_{12}>2\pi /3$. The top two rows show results for centrality 30%–100% (left-hand column), 10%–30% (middle column) and 0%–10% (right-hand column), for PbPb data (top row) and reconstructed pythia jets embedded into PbPb data events (middle row). The panel in the bottom row shows the distribution for reconstructed jets from pythia alone.

Figure 7

(Color online) Leading jet $p_{\mathrm{T}}$ distribution for dijet events with subleading jets of $p_{\mathrm{T},2}>50$ GeV/$c$ and $\Delta {\phi }_{12}>2\pi /3$ for 7 TeV pp collisions (a) and 2.76 TeV PbPb collisions in several centrality bins: (b) 50%–100%, (c) 30%–50%, (d) 20%–30%, (e) 10%–20%, and (f) 0%–10%. Data are shown as black points, while the histograms show (a) pythia events and (b)–(f) pythia events embedded into PbPb data. The error bars show the statistical uncertainties.

Figure 8

(Color online) $\Delta {\phi }_{12}$ distributions for leading jets of $p_{\mathrm{T},1}>120$ GeV/$c$ with subleading jets of $p_{\mathrm{T},2}>50$ GeV/$c$ for 7 TeV pp collisions (a) and 2.76 TeV PbPb collisions in several centrality bins: (b) 50%–100%, (c) 30%–50%, (d) 20%–30%, (e) 10%–20%, and (f) 0%–10%. Data are shown as black points, while the histograms show (a) pythia events and (b)–(f) pythia events embedded into PbPb data. The error bars show the statistical uncertainties.

Figure 9

(Color online) Fraction of events with $\Delta {\phi }_{12}>3.026$ as a function of $N_{\mathrm{part}}$, among events with $p_{\mathrm{T},1}>120$ GeV/$c$ and $p_{\mathrm{T},2}>50$ GeV/$c$. The result for reconstructed pythia dijet events (blue filled star) is plotted at $N_{\mathrm{part}}=2$. The other points (from left to right) correspond to centrality bins of 50%–100%, 30%–50%, 20%–30%, 10%–20%, and 0%–10%. The red squares are for reconstruction of pythia $+$ data events and the filled circles are for the PbPb data, with statistical (vertical bars) and systematic (brackets) uncertainties.

Figure 10

(Color online) Dijet asymmetry ratio $A_J$ for leading jets of $p_{\mathrm{T},1}>$ 120 GeV/$c$, subleading jets of $p_{\mathrm{T},2}>$50 GeV/$c$, and $\Delta {\phi }_{12}>2\pi /3$ for 7 TeV pp collisions (a) and 2.76 TeV PbPb collisions in several centrality bins: (b) 50%–100%, (c) 30%–50%, (d) 20%–30%, (e) 10%–20%, and (f) 0%–10%. Data are shown as black points, while the histograms show (a) pythia events and (b)–(f) pythia events embedded into PbPb data. The error bars show the statistical uncertainties.

Figure 11

(Color online) Fraction of all events with a leading jet with $p_{\mathrm{T},1}>120$ GeV/$c$ for which a subleading jet with $A_J<0.15$ and $\Delta {\phi }_{12}>2\pi /3$ was found, as a function of $N_{\mathrm{part}}$. The result for reconstructed pythia dijet events (blue filled star) is plotted at $N_{\mathrm{part}}$= 2. The other points (from left to right) correspond to centrality bins of 50%–100%, 30%–50%, 20%–30%, 10%–20%, and 0%–10%. The red squares are for reconstruction of pythia $+$ data events and the filled circles are for the PbPb data, with statistical (vertical bars) and systematic (brackets) uncertainties.

Figure 12

(Color online) Mean value of the fractional imbalance $(p_{\mathrm{T},1}-p_{\mathrm{T},2})/p_{\mathrm{T},1}$ as a function of leading jet $p_{\mathrm{T}}$ for three centrality bins. The PbPb data are shown as circles with vertical bars and brackets indicating the statistical and systematic uncertainties, respectively. Results for pythia are shown with blue stars, and pythia $+$ data with red squares. The dotted-dashed line to guide the eye is drawn at the value for pure pythia for the lowest $p_{\mathrm{T}}$ bin.

Figure 13

(Color online) Distribution of the transverse momentum sum of tracks for three $p_{\mathrm{T}}$ ranges, as a function of the distance $\Delta R$ to the leading and subleading jet axes. Results for the 0%–30% centrality selection are shown for pythia $+$ hydjet (upper row) and PbPb data (lower row). For each figure, the requirements on the dijet asymmetry $A_J$ are given. Note that events with $A_J>0.22$ are much rarer in the pythia+hydjet sample than in the data. Vertical bars are statistical and systematic uncertainties, combined in quadrature, the systematic contributions being 20%, independent of the bin.

Figure 14

(Color online) Average missing transverse momentum ${\displaystyle \langle \neg p_{\mathrm{T}}^{\parallel }\rangle }$ for tracks with $p_{\mathrm{T}}>0.5$ GeV/$c$, projected onto the leading jet axis (solid circles). The ${\displaystyle \langle \neg p_{\mathrm{T}}^{\parallel }\rangle }$ values are shown as a function of dijet asymmetry $A_J$ for 30%–100% centrality (left-hand side) and 0%–30% centrality (right-hand side). For the solid circles, vertical bars and brackets represent the statistical and systematic uncertainties, respectively. Colored bands show the contribution to ${\displaystyle \langle \neg p_{\mathrm{T}}^{\parallel }\rangle }$ for five ranges of track $p_{\mathrm{T}}$. The top and bottom rows show results for pythia $+$ hydjet and PbPb data, respectively. For the individual $p_{\mathrm{T}}$ ranges, the statistical uncertainties are shown as vertical bars.

Figure 15

(Color online) Average missing transverse momentum ${\displaystyle \langle \neg p_{\mathrm{T}}^{\parallel }\rangle }$ for tracks with $p_{\mathrm{T}}>0.5$ GeV/$c$, projected onto the leading jet axis (solid circles). The ${\displaystyle \langle \neg p_{\mathrm{T}}^{\parallel }\rangle }$ values are shown as a function of dijet asymmetry $A_J$ for 0%–30% centrality, inside ($\Delta R<0.8$) one of the leading or subleading jet cones (left-hand side) and outside ($\Delta R>0.8$) the leading and subleading jet cones (right-hand side). For the solid circles, vertical bars and brackets represent the statistical and systematic uncertainties, respectively. For the individual $p_{\mathrm{T}}$ ranges, the statistical uncertainties are shown as vertical bars.