CMS-HIN-24-016

CMS-HIN-24-016 ; CERN-EP-2025-260
Probing early parton emissions in heavy ion collisions using the Lund jet plane
CMS Collaboration
9 Febraury 2026
Submitted to Phys. Rev. Lett.
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$ \text{nb}^{-1}$ (301$ \text{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.
Links: e-print arXiv:2602.09271 [hep-ex] (PDF) ; CDS record ; inSPIRE record ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Left: Schematic diagram of the Cambridge--Aachen primary declusterings. The kinematics of the softer (grey) subjets in subsequent numbered emissions at each step from the harder subjet (bold black) is mapped onto the PLJP. Right: Distinct regions of the PLJP with the expected sensitivities of the jet shower evolution as the jet shower traverses the QGP, with maximum path length inside the medium $ L_\text{med} $. Emissions with formation times $ t_\text{form} > L_\text{med} $ are formed outside of the QGP, while emissions with $ t_\text{form} < t_0 $, where $ t_0 $ is the hydrodynamic initialization time, are vacuum-like. The typical values of these timescales are $ t_0\sim $ 0.6 fm and $ L_\text{med}\sim $ 6 fm.
png pdf	Figure 2: Particle-level angular distributions ($ \ln(1/\Delta) $) of the hardest $ k_{\mathrm{T}} $ emissions in the ranges $ k_{\mathrm{T}} \in [10,20] \text{Ge\hspace{-.08em}V} $ (left) and $ k_{\mathrm{T}} \in [20,40] \text{Ge\hspace{-.08em}V} $ (right) in pp collisions. A comparison is made with model predictions by PYTHIA, HERWIG, HYBRID, JEWEL, HYBRID and JETSCAPE, with their ratios to data shown in the bottom panels. The uncertainties in the model predictions are statistical only. The shaded boxes represent the total uncertainty in data.
png pdf	Figure 2-a: Particle-level angular distributions ($ \ln(1/\Delta) $) of the hardest $ k_{\mathrm{T}} $ emissions in the ranges $ k_{\mathrm{T}} \in [10,20] \text{Ge\hspace{-.08em}V} $ (left) and $ k_{\mathrm{T}} \in [20,40] \text{Ge\hspace{-.08em}V} $ (right) in pp collisions. A comparison is made with model predictions by PYTHIA, HERWIG, HYBRID, JEWEL, HYBRID and JETSCAPE, with their ratios to data shown in the bottom panels. The uncertainties in the model predictions are statistical only. The shaded boxes represent the total uncertainty in data.
png pdf	Figure 2-b: Particle-level angular distributions ($ \ln(1/\Delta) $) of the hardest $ k_{\mathrm{T}} $ emissions in the ranges $ k_{\mathrm{T}} \in [10,20] \text{Ge\hspace{-.08em}V} $ (left) and $ k_{\mathrm{T}} \in [20,40] \text{Ge\hspace{-.08em}V} $ (right) in pp collisions. A comparison is made with model predictions by PYTHIA, HERWIG, HYBRID, JEWEL, HYBRID and JETSCAPE, with their ratios to data shown in the bottom panels. The uncertainties in the model predictions are statistical only. The shaded boxes represent the total uncertainty in data.
png pdf	Figure 3: The hardest $ k_{\mathrm{T}} $ emission angular distributions ($ \ln(1/\Delta) $) in the ranges $ k_{\mathrm{T}} \in [10,20] \text{Ge\hspace{-.08em}V} $ (left) and $ k_{\mathrm{T}} \in [20,40] \text{Ge\hspace{-.08em}V} $ (right) for pp and PbPb collisions of centrality 0--30% and their ratios in the lower panels. The ratios are compared with jet quenching model predictions by HYBRID, JETSCAPE, JEWEL and HYBRID, which provides different medium resolution lengths ($ L $) and the inclusion/exclusion of backreacting medium particles (wake). The uncertainties in the model predictions are statistical only. The shaded boxes in the data distributions represent the total uncertainty.
png pdf	Figure 3-a: The hardest $ k_{\mathrm{T}} $ emission angular distributions ($ \ln(1/\Delta) $) in the ranges $ k_{\mathrm{T}} \in [10,20] \text{Ge\hspace{-.08em}V} $ (left) and $ k_{\mathrm{T}} \in [20,40] \text{Ge\hspace{-.08em}V} $ (right) for pp and PbPb collisions of centrality 0--30% and their ratios in the lower panels. The ratios are compared with jet quenching model predictions by HYBRID, JETSCAPE, JEWEL and HYBRID, which provides different medium resolution lengths ($ L $) and the inclusion/exclusion of backreacting medium particles (wake). The uncertainties in the model predictions are statistical only. The shaded boxes in the data distributions represent the total uncertainty.
png pdf	Figure 3-b: The hardest $ k_{\mathrm{T}} $ emission angular distributions ($ \ln(1/\Delta) $) in the ranges $ k_{\mathrm{T}} \in [10,20] \text{Ge\hspace{-.08em}V} $ (left) and $ k_{\mathrm{T}} \in [20,40] \text{Ge\hspace{-.08em}V} $ (right) for pp and PbPb collisions of centrality 0--30% and their ratios in the lower panels. The ratios are compared with jet quenching model predictions by HYBRID, JETSCAPE, JEWEL and HYBRID, which provides different medium resolution lengths ($ L $) and the inclusion/exclusion of backreacting medium particles (wake). The uncertainties in the model predictions are statistical only. The shaded boxes in the data distributions represent the total uncertainty.

Tables
png pdf	Table 1: Lower and upper bounds of fractional uncertainties (in %) for each source in PbPb and pp collisions for both reported $ k_{\mathrm{T}} $ ranges.

Summary

In summary, the primary Lund jet plane in heavy ion collisions has been presented for the first time in slices of relative momentum $ k_{\mathrm{T}} $. This measurement provides an experimental approach to search for the presence of early vacuum-like emissions within jet evolution in heavy ion collisions.The unfolded angular distribution of the hardest splittings selected in two intervals of \$ k_{\mathrm{T}} $ are compared in proton-proton (pp) and lead-lead (PbPb) collisions.The ratio of the two distributions in data is compared with predictions by the HYBRID, JETSCAPE, JEWEL, and JETMED models.Predictions employing both factorized and interleaved treatment of emissions are consistent with the observed ratio. In the $ k_{\mathrm{T}} $ ranges considered, the HYBRID model distributions are unaffected by the inclusion of the backreaction of the medium. The data ratios are most consistent with zero resolution length in the HYBRID model and with the JETSCAPE model, which does not have an explicit implementation of color coherence. The PbPb/pp ratios in data are independent of angle within experimental uncertainties, with values smaller than unity. This observation is consistent with the presence of early vacuum-like emissions in PbPb collisions, which subsequently undergo quenching.

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