Jet size dependence of single jet suppression in lead-lead collisions at sqrt(s(NN)) = 2.76 TeV with the ATLAS detector at the LHC

TL;DR

In Pb+Pb collisions at $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV, ATLAS measures inclusive jet suppression via the central-to-peripheral ratio $R_{\mathrm{CP}}$ for anti-$k_T$ jets with radii $R=0.2$–$0.5$. The analysis combines meticulous underlying-event subtraction, track-jet–based UE jet rejection, and unfolding with a MC-overlay response matrix to correct for detector effects. The results show a robust ~2x suppression in central vs peripheral collisions across the studied $p_T$ range, with only weak dependence on $p_T$ and jet radius and a measurable radius-dependent effect at low $p_T$. These findings establish inclusive jet quenching as a direct signature of the medium created in ultra-relativistic heavy-ion collisions and complement previous dijet imbalance studies at the LHC.

Abstract

Measurements of inclusive jet suppression in heavy ion collisions at the LHC provide direct sensitivity to the physics of jet quenching. In a sample of lead-lead collisions at sqrt(s) = 2.76 TeV corresponding to an integrated luminosity of approximately 7 inverse microbarns, ATLAS has measured jets with a calorimeter over the pseudorapidity interval |eta| < 2.1 and over the transverse momentum range 38 < pT < 210 GeV. Jets were reconstructed using the anti-kt algorithm with values for the distance parameter that determines the nominal jet radius of R = 0.2, 0.3, 0.4 and 0.5. The centrality dependence of the jet yield is characterized by the jet "central-to-peripheral ratio," Rcp. Jet production is found to be suppressed by approximately a factor of two in the 10% most central collisions relative to peripheral collisions. Rcp varies smoothly with centrality as characterized by the number of participating nucleons. The observed suppression is only weakly dependent on jet radius and transverse momentum. These results provide the first direct measurement of inclusive jet suppression in heavy ion collisions and complement previous measurements of dijet transverse energy imbalance at the LHC.

Figures (8)

• Figure 1: Results of MC evaluation of jet reconstruction performance in 0--10% and 60--80% collisions as a function of truth jet for $R= 0.2$ (left) and $R = 0.4$ (right) jets. Top: jet energy resolution $\sigma[{\hbox{$\Delta \ET$}}]/\hbox{$E_{\mathrm{T}}^{\mathrm{truth}}$}$ and jet energy scale closure, $\langle \hbox{$\Delta \ET$}\rangle/\hbox{$E_{\mathrm{T}}^{\mathrm{truth}}$}$. Solid curves show parameterizations of the JER using Eq. \ref{['eq:resolparam']}. Bottom: Efficiencies, $\varepsilon$ and $\varepsilon'$, for reconstructing jets before and after application of UE jet removal (see text for explanation), respectively.
• Figure 2: Top: Representative distributions of $\hbox{$E_{\mathrm{T}}^{3\times 4}$} - \langle\hbox{$E_{\mathrm{T}}^{3\times 4}$}\rangle$ (left) and $\hbox{$E_{\mathrm{T}}^{7\times 7}$} - \langle\hbox{$E_{\mathrm{T}}^{7\times 7}$}\rangle$ (right) (see text for definitions) for data (points) and MC (filled histogram) for Pb+Pb collisions with $3.4 \leq \hbox{$\Sigma E_{\mathrm{T}}^{\mathrm{FCal}}$} < 3.5$. The vertical lines indicate $\hbox{$E_{\mathrm{T}}^{3\times 4}$} - \langle\hbox{$E_{\mathrm{T}}^{3\times 4}$}\rangle = 0$ and $\hbox{$E_{\mathrm{T}}^{7\times 7}$} - \langle\hbox{$E_{\mathrm{T}}^{7\times 7}$}\rangle = 0$. Bottom: Standard deviations of the $E_{\mathrm{T}}^{3\times 4}$ and $E_{\mathrm{T}}^{7\times 7}$ distributions, $\sigma[\hbox{$E_{\mathrm{T}}^{3\times 4}$}]$ and $\sigma[\hbox{$E_{\mathrm{T}}^{7\times 7}$}]$, respectively, in data and HIJING MC sample as a function of $\Sigma E_{\mathrm{T}}^{\mathrm{FCal}}$.
• Figure 3: Top: Measured and corrected $R_{\rm CP}$ values for the 0--10% centrality bin as a function of jet for $R = 0.4$ and $R= 0.2$ jets. Bottom: Ratio of corrected to measured $R_{\rm CP}$ values for both jet radii. The error bars on the points represent statistical uncertainties only.
• Figure 4: Contributions to the relative systematic uncertainty on the $R_{\rm CP}$ from various sources for the $R = 0.4$ anti-$k_{t}$ jets in the 0--10% centrality bin. The $k\pm1$ curves denote the uncertainty due to the choice of regularization parameter obtained by unfolding with the fourth and sixth singular values. A constant 5% systematic uncertainty on the jet reconstruction efficiency is assigned for $\pT < 100$ only. The 11% uncertainty in the determination of $R_{\mathrm{coll}}$ is indicated with a shaded box and is -independent.
• Figure 5: $R_{\rm CP}$ values as a function of jet for $R= 0.2$ (left) and $R = 0.4$ (right) anti-$k_{t}$ jets in four bins of collision centrality. The error bars indicate statistical errors from the unfolding, the shaded boxes indicate unfolding regularization systematic errors that are partially correlated between points. The solid lines indicate systematic errors that are fully correlated between all points. The horizontal width of the systematic error band is chosen for presentation purposes only. Dotted lines indicate $\hbox{$R_{\rm CP}$} = 0.5$, and the dashed lines on the top panels indicate $\hbox{$R_{\rm CP}$} = 1$.