Measurement of the groomed jet radius and momentum splitting fraction in pp and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV

This article presents groomed jet substructure measurements in pp and Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV with the ALICE detector. The Soft Drop grooming algorithm provides access to the hard parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet momentum splitting fraction, $z_{\rm g}$, and the (scaled) groomed jet radius, $\theta_{\rm g}$. Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with resolution parameters $R = 0.2$ and $R = 0.4$. In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A narrowing of the $\theta_{\rm g}$ distribution in Pb$-$Pb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quark$-$gluon plasma. No significant modification of the $z_{\rm g}$ distribution in Pb$-$Pb collisions compared to pp collisions is observed. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quark$-$gluon plasma.

 

Phys. Rev. Lett. 128 (2022) 102001
HEP Data
e-Print: arXiv:2107.12984 | PDF | inSPIRE
CERN-EP-2021-151
Figure group

Figure 1

Graphical representation of the angularly ordered Cambridge-Aachen reclustering of jet constituentsand subsequent Soft Drop grooming procedure~,with the identified splitting denoted in black and the splittings that were groomed away in light blue.

Figure 2

Unfolded zg distributions for charged-particle jets in pp collisions compared to those in Pb–Pb collisions at √sNN = 5.02 TeV with zcut = 0.2 for 0–10% centrality for R = 0.2 (left) and 30–50% centrality for R = 0.4 (right). The distributions are normalized to the inclusive jet cross section in the 60 < pT, ch jet < 80 GeV/c interval, and ftagged indicates the fraction of splittings that were tagged to pass the SD condition in the selected pT, ch jet interval. The ratios in the bottom panel are compared to the following theoretical predictions: JETSCAPE [63], JEWEL [62, 64], Caucal et al. [34, 65], Chien et al. [33], Qin et al. [35], and Pablos et al. [36, 66, 67]. Further details can be found in Ref. [50].

Figure 3

Unfolded θg distributions for charged-particle jets in pp collisions compared to those in Pb–Pb collisions at √sNN = 5.02 TeV with zcut = 0.2 for 0–10% centrality for R = 0.2 (left) and 30–50% centrality for R = 0.4 (right). The distributions are normalized to the inclusive jet cross section in the 60 < pT, ch jet < 80 GeV/c interval, and ftagged indicates the fraction of splittings that were tagged to pass the SD condition in the selected pT, ch jet interval. The ratios in the bottom panel are compared to the following theoretical predictions: JETSCAPE [63], JEWEL [62, 64], Caucal et al. [34, 65], Pablos et al. [36, 66, 67], and Yuan et al. [31]. Further details can be found in Ref. [50].