First $k_{\mathrm{T}}$ scan of the Lund jet plane in heavy-ion collisions to test the factorization of the vacuum and medium parton shower

Abstract

This note presents the first relative transverse momentum ($k_{\mathrm{T}}$) scan of the Lund jet plane for high-energy jets in lead-lead (Pb-Pb) collisions at the LHC at a nucleon-nucleon center-of-mass energy of 5.02 TeV. We report the fully corrected angular distribution of the primary emissions with the highest $k_{\mathrm{T}}$ in two different ranges of $k_{\mathrm{T}}$ for anti-$k_{\mathrm{T}}$ jets with distance parameter $R=0.4$ and transverse momentum in the range $200 < p_{\mathrm{T}}^\text{jet} < 1000$ GeV. The analysis uses Pb-Pb and pp data samples with integrated luminosities of $1.7\,\text{nb}^{-1}$ and $301\,\text{pb}^{-1}$, respectively, collected with the CMS experiment in 2018 and 2017. A $k_{\mathrm{T}}$ scan of the Lund plane allows us to explore the scale dependence of jet quenching phenomena. Our measurement was designed to test the validity of the assumed factorization between the early vacuum and QGP-induced stages of jet evolution in heavy-ion collisions, which is an underlying assumption of several jet quenching models and has not yet been experimentally proven. The reported angular distribution of emissions at high $k_{\mathrm{T}}$ have a similar shape in pp and Pb-Pb, and this is consistent with the emissions being part of the early and vacuum-like regime of the jet evolution in Pb-Pb