Groomed jet radius and girth of jets recoiling against isolated photons in PbPb and pp collisions at $5.02~\mathrm{TeV}$

We report the first measurements of the groomed jet radius and the girth of jets in events with an isolated photon recoiling from a jet in lead-lead (PbPb) and proton-proton (pp) collisions at the LHC. The analysis uses pp and PbPb data samples collected with the CMS detector in 2017 and 2018, both at a nucleon-nucleon center-of-mass energy of $5.02~\mathrm{TeV}$, with integrated luminosities of $301~\mathrm{pb}^{-1}$ and $1.7~\mathrm{nb}^{-1}$, respectively. Measurements of inclusive jets point to a narrowing of the structure of jets in PbPb collisions relative to pp collisions. A limitation of such measurements is that the comparison is done at the same reconstructed jet transverse momentum, meaning the scattered parton transverse momentum $p_\mathrm{T}$ in pp and PbPb collisions differs due to the jet $p_\mathrm{T}$ loss that happens in the latter case. Since photons do not interact strongly with the quark gluon plasma (QGP), their $p_\mathrm{T}$ can be used instead as a proxy of the $p_\mathrm{T}$ of the parton that initiates the recoiling jet shower. We find that jets that more closely balance the photon $p_\mathrm{T}$ are narrower in PbPb than in pp collisions. On the other hand, jets that balance less the photon $p_\mathrm{T}$ have an angular structure consistent with the pp reference for the same photon energy. Our measurement indicates, with high confidence, that isolated photons in conjunction with jets provide a better controlled assessment of the modification of the angular scale of jets and of its sensitivity to microscopic properties of the QGP relative to inclusive jet measurements.