We show that recent measurements of substructure-dependent jet suppression constrain the value of the resolution length of the droplets of quark-gluon plasma (QGP) formed in heavy ion collisions. This resolution length, $L_{\mathrm{res}}$, is defined such that the medium can only resolve partons within a jet shower that are separated by more than $L_{\mathrm{res}}$. We first use calculations done in the hybrid strong/weak coupling model for jet quenching (Hybrid Model) to reproduce ALICE measurements of the scaled soft-drop angle ${\theta }_g$ for anti-$k_t$ $R=0.2$ jets reconstructed from charged-particle tracks. We find that the narrowing of the ${\theta }_g$-distribution in PbPb collisions compared to pp collisions that is seen in the ALICE data rules out a picture of fully coherent energy loss ($L_{\mathrm{res}}=\infty$) where each entire parton shower loses energy to the plasma as if it were a single unresolved colored object. We then use Hybrid Model calculations to reproduce ATLAS measurements of $d{R}_{12}$, the soft-drop angle obtained by applying the Soft Drop grooming procedure to all charged-particle tracks in $R=1$ jets reconstructed from $R=0.2$ skinny subjets. Our analysis demonstrates that the ATLAS measurements of $R_{\mathrm{AA}}$ for such $R=1$ jets as a function of $d{R}_{12}$ are inconsistent with a picture of fully incoherent energy loss ($L_{\mathrm{res}}=0$) in which every splitting in a parton shower is immediately resolved by the plasma. We find that our Hybrid Model calculations agree best with the ATLAS measurements if QGP has a finite, nonzero, resolution length $L_{\mathrm{res}}\sim (1–2)/(\pi T)$. For the first time, jet substructure measurements are constraining the resolution length of QGP from below, as well as from above.

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• Jet quenching
• Quark & gluon jets
• Quark-gluon plasma
• Relativistic heavy-ion collisions