Measurement of the radius dependence of charged-particle jet suppression in Pb-Pb collisions at $\sqrt{s_{\rm NN}}$ = 5.02 TeV

The ALICE Collaboration reports a new differential measurement of inclusive jet suppression using pp and Pb$-$Pb collision data at center-of-mass energy per nucleon-nucleon collision $\sqrt{s_{\rm NN}} = 5.02$ TeV. Charged-particle jets are reconstructed using the anti-$k_{\rm T}$ algorithm with resolution parameters $R =$ 0.2, 0.3, 0.4, 0.5, and 0.6 in pp collisions and $R =$ 0.2, 0.4, 0.6 in central (0$-$10%), semi-central (30$-$50%), and peripheral (60$-$80%) Pb$-$Pb collisions. The analysis uses a novel approach based on machine learning to mitigate the influence of jet background in central heavy-ion collisions, which enables measurements of inclusive jet suppression for jet $p_{\rm T} \geq 40$ GeV/$c$ in central collisions at a resolution parameter of $R = 0.6$. This is the lowest value of jet $p_{\rm T}$ achieved for inclusive jet measurements at $R=0.6$ at the LHC, and is an important step for discriminating different models of jet quenching in the quark-gluon plasma. The transverse momentum spectra, nuclear modification factors, and derived cross section and nuclear modification factor ratios for different jet resolution parameters of charged-particle jets are presented and compared to model predictions. A mild dependence of the nuclear modification factor ratios on collision centrality and resolution parameter is observed. The results are compared to a variety of jet quenching models with varying levels of agreement, demonstrating the effectiveness of this observable to discriminate between models.


Submitted to: PLB
e-Print: arXiv:2303.00592 | PDF | inSPIRE
Figure group

Figure 1

Residual $\pT$-distributions of embedded jet probes of known transverse momentum into Pb$-$Pb collision data. Left: Comparison of the distributions for the area-based and ML-based background estimators. Note the lines connecting the points do not represent a fit and are only present in order to guide the eye. Right: Radius dependence of the width of the distributions where the error bars come from the uncertainty in calculating the width.

Figure 2

Left: Comparison of toy model modifications for $R = 0.4$ jets using the $R^{\rm toy}_{\rm AA}$ as defined in Eq. (4). Right: The fragmentation functions at low jet $p_{\rm T}$ ($ 40 < p_{\rm T, true} < 100 \text{ GeV}/c$, lower right panel) and high jet $p_{\rm T}$ ($ 100 < p_{\rm T, true} < 200 \text{ GeV}/c$, upper right panel) for 0$-$10% central Pb$-$Pb collisions. In the fractional collinear and fractional large angle case, $f_{\rm loss} = 25\%$ and $p_{\rm loss} = 100\%$. In the medium response case, $f_{\rm loss} = 10\%$ and $p_{\rm loss} = 50\%$ The ratio of the fragmentation functions measured in Pb$-$Pb and pp collisions are shown for jets with $R=0.4$ and $\pt>100$ GeV/$c$  (ATLAS), and for jets with $R=0.3$ and $p_{\rm T} > 30$ GeV/$c$ recoiling from a photon with $E_{\rm T}>60$ GeV/$c$  (CMS).

Figure 3

The $\pT$-differential inclusive charged-particle jet yield distributions as a function of $\pT$ for different values of $R$ in three centrality classes: Top Left: 0$-$10%, top right: 30$-$50%, bottom left: 60$-$80%. The peripheral spectra were measured using the area-based method for the background correction. All other reported spectra were corrected with the ML-based background estimator. In the bottom right panel, the production cross sections in pp collisions are shown. The vertical bars denote statistical uncertainties and the vertical extent of the boxes denotes systematic uncertainties. Note that the data points are plotted horizontally at the bin center.

Figure 5

Nuclear modification factors for jets with $R=0.6$ in 0$-$10% (left) and 30$-$50% (right) central Pb$-$Pb collisions outlining the impact of the various fragmentation models on the final result. Note that the systematic uncertainties, described in Sec. 6, are drawn both with and without the fragmentation uncertainties in the empty and filled boxes, respectively.

Figure 8

Double ratio of jet nuclear modification factors using $R_\mathrm{AA}^{R=0.2}$ as denominator and using $R = 0.4$ (left) and $R = 0.6$ (right) as the numerator compared to model predictions for central (top row) and semi-central (bottom row) collisions. Note that a comparison to JEWEL with recoils was omitted from the top right plot as its prediction is out of scale.

Figure A.1

The $\delta p_{\rm T}$ distributions for $R = 0.4$ (left) and $R = 0.6$ (right) jets in central (0-10\%) collisions using the area-based (dashed lines) and ML-based (solid markers) corrections.