Sensitivity of jet substructure to jet-induced medium response

TL;DR

This paper demonstrates that modern jet substructure measurements can reveal jet-induced medium response effects in heavy ion collisions, providing new signatures like modifications in $z_g$, $ riangle R_{12}$, and subjet girth.

Contribution

It introduces novel jet substructure observables sensitive to recoil effects, offering a new way to identify medium response signals in heavy ion collision data.

Findings

Recoil effects influence the $z_g$ distribution in data.

Recoil effects cause an increase in $ riangle R_{12}$ with separation.

Recoil effects enhance the girth of the subleading subjet as $z_g$ decreases.

Abstract

Jet quenching in heavy ion collisions is expected to be accompanied by recoil effects, but unambiguous signals for the induced medium response have been difficult to identify so far. Here, we argue that modern jet substructure measurements can improve this situation qualitatively since they are sensitive to the momentum distribution inside the jet. We show that the groomed subjet shared momentum fraction $z_g$, and the girth of leading and subleading subjets signal recoil effects with dependencies that are absent in a recoilless baseline. We find that recoil effects can explain most of the medium modifications to the $z_g$ distribution observed in data. Furthermore, for jets passing the Soft Drop Condition, recoil effects induce in the differential distribution of subjet separation $\Delta R_{12}$ a characteristic increase with $\Delta R_{12}$, and they introduce a characteristic enhancement of the girth of the subleading subjet with decreasing $z_g$. We explain why these qualitatively novel features, that we establish in \textsc{Jewel+Pythia} simulations, reflect generic physical properties of recoil effects that should therefore be searched for as telltale signatures of jet-induced medium response.