Measurement of jet suppression in central Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 2.76 TeV

The transverse momentum ($p_{\rm T}$) spectrum and nuclear modification factor ($R_{\rm AA}$) of reconstructed jets in 0-10% and 10-30% central Pb-Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV were measured. Jets were reconstructed from charged and neutral particles, utilizing the ALICE tracking detectors and Electromagnetic Calorimeter (EMCal), with the anti-$k_{\rm T}$ jet algorithm with a resolution parameter of R=0.2. The jet $p_{\rm T}$ spectra are reported in the pseudorapidity interval of $|{\eta}_{\rm jet}|< 0.5$ for $40< p_{\rm T,jet}< 120$ GeV/$c$ in 0-10% and for $30< p_{\rm T,jet}< 100$ GeV/$c$ in 10-30% collisions. Reconstructed jets were required to contain a leading charged particle with $p_{\rm T}>5$ GeV/$c$ to suppress jets constructed from the combinatorial background in Pb-Pb collisions. The effect of the leading charged particle requirement has been studied in both pp and Pb-Pb collisions and has been shown to have negligible effects on the $R_{\rm AA}$ within the uncertainties of the measurement. The nuclear modification factor is obtained by dividing the jet spectrum measured in Pb-Pb by that in pp collisions scaled by the number of independent nucleon-nucleon collisions estimated using a Glauber model. $R_{\rm AA}$ is found to be $0.28\pm0.04$ in 0-10% and $0.35\pm0.04$ in 10-30% collisions, independent of $p_{\rm T,jet}$ within the uncertainties of the measurement. The observed suppression is in fair agreement with expectations from two model calculations with different approaches to jet quenching.

Figures

Figure 1

The $\delta p_{\mathrm{T}}$ distribution for $R=0.2$ with the random-cone and the embedded-track methods in the 10% most central events, with ${p}_{\rm T}^{\rm probe} = 60$ GeV/$c$ for the embedded-track method.
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Figure 2

The spectra of $R=0.2$ jets with a leading track requirement of $5$ GeV/$c$ in $0$-$10$% and $10$-$30$% most central Pb-Pb collisions scaled by $1/N_\mathrm{coll}$ and in inelastic pp collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV. The uncertainties on the normalization are about 11% for the Pb-Pb data from the uncertainty on $N_\mathrm{coll}$ and about 8% for the pp data from the total inelastic cross section.
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Figure 3

Ratio of the jet spectrum with a leading track $p_{\mathrm{T}} > 5$ GeV/$c$ over the inclusive jet spectrum for $R=0.2$ in pp collisions at $\sqrt{s}=2.76$ TeV.
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Figure 4

Ratios of jet spectra with different leading track $p_{\mathrm{T}}$ requirements (``$0$ over $5$'', ``$3$ over $5$'', ``$7$ over $5$''and ``$10$ over $5$'') for $R=0.2$ jets in 0-10% Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV. The solid black lines represent predictions from PYTHIA.
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Figure 5

${R}_{\mathrm{AA}}$ for $R=0.2$ jets with the leading track requirement of $5$ GeV/$c$ in $0$-$10$% (left) and $10$-$30$% (right) most central Pb-Pb collisions compared to calculations from YaJEM and JEWEL. The boxes at ${R}_{\mathrm{AA}}=1$ represent the systematic uncertainty on ${T}_{\mathrm{AA}}$.
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