Substructure grooming of inclusive and photon-tagged jets in heavy-ion collisions

Abstract

Jet substructure provides a powerful probe of partonic interactions within the quark-gluon plasma (QGP) in heavy-ion collisions. In this paper, we present a systematic theoretical study of the groomed substructures for both inclusive jets and photon-tagged jets ($\gamma+$jets) utilizing the Dynamical and Soft-Drop Grooming algorithms in PbPb collisions by employing the SHELL transport model. Our theoretical calculations exhibit a suppression at high $k_{\rm T,g}$, the relative transverse momentum between the two subjets in the groomed substructure, consistent with the recent ALICE measurements. We show that the suppression of high $k_{\rm T,g}$ arises from the combined effects of the reduction of the subleading subjet transverse momentum due to partonic energy loss and the narrowing of the groomed jet radius $R_g$ induced by selection bias. Our findings demonstrate that no enhancement is observed at high $k_{\rm T,g}$, even in the complete absence of selection bias. Furthermore, we propose that the broadening of $R_g$ in photon-tagged jets, which are less susceptible to selection bias compared to inclusive jets, provides relatively direct evidence of the jet substructure broadening. Our analysis reveals that the $R_g$ broadening becomes more pronounced as the jet radius increases, where the medium-induced gluon radiation plays a dominant role in driving such broadening. In particular, we find that as the jet radius increases, the Soft Drop grooming algorithm exhibits a better resolving power for the contribution of the medium response to the jet substructure broadening.