Simultaneous description of hadron and jet suppression in heavy-ion collisions

We present a global fit to all data on the suppression of high-energy jets and high-energy hadrons in the most central heavy-ion collisions at the CERN Large Hadron Collider (LHC) for two different collision energies, within a hybrid strong-weak coupling quenching model. Even though the measured suppression factors for hadrons and jets differ significantly from one another and appear to asymptote to different values in the high-energy limit, we obtain a simultaneous description of all these data after constraining the value of a single model parameter. We use our model to investigate the origin of the difference between the observed suppression of jets and hadrons and relate it, quantitatively, to the observed modification of the jet fragmentation function in jets that have been modified by passage through the medium produced in heavy-ion collisions. In particular, the observed increase in the fraction of hard fragments in medium-modified jets, which indicates that jets with the fewest hardest fragments lose the least energy, corresponds quantitatively to the observed difference between the suppression of hadrons and jets. We argue that a harder fragmentation pattern for jets with a given energy after quenching is a generic feature of any mechanism for the interaction between jets and the medium that they traverse that yields a larger suppression for wider jets. We also compare the results of our global fit to LHC data to measurements of the suppression of high-energy hadrons in BNL Relativistic Heavy Ion Collider (RHIC) collisions, and find that with its parameter chosen to fit the LHC data, our model is inconsistent with the RHIC data at the $3\sigma$ level, suggesting that hard probes interact more strongly with the less hot quark-gluon plasma produced at RHIC.

Figure 1

Best values of ${\kappa }_{\mathrm{sc}}$ for $T_c=145$ MeV (first three panels) and 170 MeV (last three panels). The individual red (orange) error bars show the $1\sigma$ ($2\sigma$) uncertainties in the value of ${\kappa }_{\mathrm{sc}}$ obtained by fitting separately to each one of ten data sets, nine from the LHC and one from RHIC. “H” stands for charged hadrons (LHC, PbPb collisions, $\sqrt{s_{NN}}$ specified in TeV) or ${\pi }^0$ (PHENIX, AuAu collisions, $\sqrt{s_{NN}}$ again specified in TeV) in the 0–5% centrality bin, while “J” stands for calorimetrically reconstructed jets, with the anti-$k_t$ radius [72] in parentheses, in the 0–10% centrality bin. First panel of each set of three corresponds to $L_{\mathrm{res}}=0$, second one to $L_{\mathrm{res}}=2/\left(\pi T\right)$, and the third panel shows the goodness (${\chi }^2$ per degree of freedom) of each fit. The horizontal red (orange) lines show the $1\sigma$ ($2\sigma$) range of values of ${\kappa }_{\mathrm{sc}}$ obtained via a global fit to all nine LHC data sets, and the (almost indistinguishable) purple and green horizontal lines show the goodness of these global fits for $L_{\mathrm{res}}=0$ and $L_{\mathrm{res}}=2/\left(\pi T\right)$.

Figure 2

Results for $R_{\mathrm{AA}}^{\mathrm{had}}$ and $R_{\mathrm{AA}}^{\mathrm{jet}}$ from our model with its parameter fixed via the global fit, compared to CMS [52] and ATLAS [55] data. Error bars on the experimental data points show only the uncorrelated error. The corrected data points (darker red and blue) have been obtained from the uncorrected data points (paler red and blue) by shifting them according to the best fit value of the correlated error correction, obtained following the procedure from Ref. [61] as detailed in the Supplemental Material [62]. Colored bands show results from the hybrid model with $L_{\mathrm{res}}=2/\left(\pi T\right)$, with the bands spanning results obtained with $T_c=145$ and 170 MeV, in each case using the one sigma range of values of ${\kappa }_{\mathrm{sc}}$ obtained from the global fit in Fig. 1.

Figure 3

The ratio of fragmentation functions for jets in PbPb collisions with 125 GeV $<p_T^{\mathrm{jet}}<160$ GeV to those for jets with the same $p_T$ in pp collisions, with hybrid model predictions for $L_{\mathrm{res}}=0$ and $2/\left(\pi T\right)$ compared to ATLAS data [67]. In this observable, we do see some evidence favoring $L_{\mathrm{res}}=2/\pi T$ over $L_{\mathrm{res}}=0$. The disagreement between the hybrid model predictions and data at small $z$, on the right, points to the need to improve the current hybrid model implementation [13] of the wakes that jets deposit in the medium.

Figure 4

Hybrid model results for $R_{\mathrm{AA}}^{\mathrm{had}}$ (blue) and $R_{\mathrm{AA}}^{\mathrm{jet}}$ (red) in LHC collisions with $\sqrt{s_{\mathrm{NN}}}=2.76$ TeV. The dashed yellow line shows the $R_{\mathrm{AA}}^{\mathrm{had}}$ obtained by convolving the quenched jet spectrum with the hybrid model PbPb jet fragmentation function, shown in the inset. The dotted yellow line shows the $R_{\mathrm{AA}}^{\mathrm{had}}$ obtained by convolving the quenched jet spectrum with an unmodified fragmentation function.