Physics Preliminary Summary: Measurements of the groomed jet radius and groomed momentum fraction in pp and Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV

We report jet substructure measurements in pp and Pb-Pb collisions at $\sqrt{s_{\rm{NN}}}$ = 5.02 TeV with the ALICE detector. The groomed jet momentum fraction, $z_\rm{g}$, and the groomed jet radius, $\theta_\rm{g} \equiv R_\rm{g}/R$, are measured using the Soft Drop grooming algorithm. Charged-particle jets were reconstructed at midrapidity with the ALICE tracking detectors using the anti-$k_\rm{T}$ 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 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. We observe a narrowing of the $\theta_\rm{g}$ ($R_{\rm{g}}$) distribution in Pb-Pb collisions compared to pp collisions, which provides direct evidence of the modification of the angular scale of jets in the quark-gluon plasma. We observe no significant modification of the $z_\rm{g}$ distribution in Pb-Pb collisions compared to pp collisions. These results are compared to a variety of theoretical models of jet quenching, and provide constraints on jet energy loss mechanisms and coherence effects in the quark-gluon plasma