Probing early parton emissions in heavy ion collisions using the Lund jet plane

TL;DR

This study measures the Lund jet plane in lead-lead and proton-proton collisions to investigate whether early jet emissions occur before quark-gluon plasma formation, finding no significant difference in high-$k_T$ emissions between the two collision types.

Contribution

It provides the first comparison of Lund jet plane emissions in heavy-ion and proton-proton collisions to probe early parton emissions relative to QGP formation.

Findings

No significant difference in high-$k_T$ emission angular distributions between PbPb and pp collisions.

Supports the hypothesis that early jet emissions occur before QGP formation.

Results consistent with early parton emissions happening prior to medium interaction.

Abstract

In scattering experiments, high-virtuality partons, i.e., quarks and gluons, initiate a series of additional parton emissions to create collimated sprays of particles known as jets. This paper presents a measurement of the Lund jet plane (LJP) of high-energy jets produced in lead-lead (PbPb) collisions and compares the results to data for proton-proton (pp) collisions. The LJP is formed by iteratively declustering the constituents of a jet into consecutive emissions and recording the relative transverse momentum ($k_\mathrm{T}$) and angle of the resulting emission with respect to its emitter. The angular distributions of two different $k_\mathrm{T}$ slices of the LJP are investigated for jets with radius parameter of 0.4 and transverse momentum in the range 200$-$1000 GeV. The PbPb (pp) data were recorded by the CMS experiment in 2018 (2017) and correspond to an integrated luminosity of 1.7 nb$^{-1}$ (301 pb$^{-1}$) at a nucleon-nucleon center-of-mass energy of 5.02 TeV. The measurement was designed to test whether the earliest jet emissions are produced before the formation of the quark-gluon plasma (QGP) in PbPb collisions. Within the experimental uncertainties, no significant difference is observed between the angular distribution of high-$k_\mathrm{T}$ emissions in \pp and PbPb collisions, which is consistent with these emissions occurring early in the jet evolution, before substantial interaction with the QGP.