Constraining the Resolution Length of Quark-Gluon Plasma with New Jet Substructure Measurements
Arjun Srinivasan Kudinoor🇺🇸 · Daniel Pablos🇪🇸 · Krishna Rajagopal🇺🇸
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, , is defined such that the medium can only resolve partons within a jet shower that are separated by more than . We first use Hybrid Model calculations to reproduce ALICE measurements of the scaled Soft Drop angle for anti- jets reconstructed from charged-particle tracks. We find that the narrowing of the -distribution in PbPb collisions compared to pp collisions that is seen in the ALICE data rules out a picture of fully coherent energy loss () 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 , the Soft Drop angle obtained by applying the Soft Drop grooming procedure to all charged-particle tracks in jets reconstructed from skinny subjets. Our analysis demonstrates that the ATLAS measurements of for such jets as a function of are inconsistent with a picture of fully incoherent energy loss () 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 . For the first time, jet substructure measurements are constraining the resolution length of QGP from below, as well as from above.