CMS-PAS-HIN-23-001

CMS-PAS-HIN-23-001
Groomed jet radius and girth of jets recoiling against isolated photons in PbPb and pp collisions at 5.02 TeV
CMS Collaboration
13 August 2023

Abstract: 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 TeV, with integrated luminosities of 301 pb$^{-1}$ and 1.7 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.
Links: CDS record (PDF) ; CADI line (restricted) ; These preliminary results are superseded in this paper, Submitted to PLB. The superseded preliminary plots can be found here.

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