CMS-PAS-HIN-24-016

CMS-PAS-HIN-24-016
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
CMS Collaboration
18 February 2025

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 nb $^{-1}$ and 301 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.
Links: CDS record (PDF) ; CADI line (restricted) ;

Figures & Tables	Summary	References	CMS Publications

Figures
png pdf	Figure 1: Left: schematic diagram of the Cambridge-Aachen primary declusterings, i.e., following the hardest subjet at each step of the declustering process. The softer subjets at each step are used as proxies for the emissions whose kinematics can be mapped onto the LJP. In the analysis, the emission with the highest $k_{\mathrm{T}}$ value is used. Right: distinct regions of the LJP with the expected sensitivities of the jet shower evolution as the jet shower traverses the quark-gluon plasma. The dashed diagonal line represents the characteristic length scale where the formation time of the emissions is longer than the lifetime of the plasma. The dotted vertical line represents the expected manifestation of color coherence effects at a critical angle $\theta_c$ , below which the plasma would not be able to separate this.
png pdf	Figure 2: Particle-level angular distributions ( $\text{ln}(1/\Delta)$ ) of hardest $k_{\mathrm{T}}$ emissions in the ranges $k_{\mathrm{T}} \in [10,20]$ GeV (left) and $k_{\mathrm{T}} \in [20,40]$ GeV (right) in pp collisions. A comparison is made with model predictions by PYTHIA8 CP5 tune, HERWIG 7 CH3 tune, JetMed and Hybrid vacuum (modified PYTHIA8 Monash tune), as well as JETSCAPE, with their ratios to data shown in the bottom panels. The uncertainties on the models are statistical. The shaded region represents the total uncertainty in data.
png pdf	Figure 2-a: Particle-level angular distributions ( $\text{ln}(1/\Delta)$ ) of hardest $k_{\mathrm{T}}$ emissions in the ranges $k_{\mathrm{T}} \in [10,20]$ GeV (left) and $k_{\mathrm{T}} \in [20,40]$ GeV (right) in pp collisions. A comparison is made with model predictions by PYTHIA8 CP5 tune, HERWIG 7 CH3 tune, JetMed and Hybrid vacuum (modified PYTHIA8 Monash tune), as well as JETSCAPE, with their ratios to data shown in the bottom panels. The uncertainties on the models are statistical. The shaded region represents the total uncertainty in data.
png pdf	Figure 2-b: Particle-level angular distributions ( $\text{ln}(1/\Delta)$ ) of hardest $k_{\mathrm{T}}$ emissions in the ranges $k_{\mathrm{T}} \in [10,20]$ GeV (left) and $k_{\mathrm{T}} \in [20,40]$ GeV (right) in pp collisions. A comparison is made with model predictions by PYTHIA8 CP5 tune, HERWIG 7 CH3 tune, JetMed and Hybrid vacuum (modified PYTHIA8 Monash tune), as well as JETSCAPE, with their ratios to data shown in the bottom panels. The uncertainties on the models are statistical. The shaded region represents the total uncertainty in data.
png pdf	Figure 3: Fully corrected hardest $k_{\mathrm{T}}$ emission angular distributions ( $\text{ln}(1/\Delta)$ ) in the ranges $k_{\mathrm{T}} \in [10,20]$ GeV (left) and $k_{\mathrm{T}} \in [20,40]$ GeV (right) for pp and PbPb collisions of centrality 0-30%. The uncertainties are propagated as uncorrelated in the ratio (bottom panels). Each distribution is normalized to the total number of jets within $p_{\mathrm{T}} \in [200,1000]$ GeV and $\|\eta\| <$ 2. The ratio is compared with jet quenching model predictions by JetMed, JETSCAPE and Hybrid, which provides different medium resolution lengths ( $L$ ) and the inclusion/exclusion of backreacting medium particles (wake). The uncertainties on the models are statistical. The shaded regions in the data distributions represent the total uncertainty.
png pdf	Figure 3-a: Fully corrected hardest $k_{\mathrm{T}}$ emission angular distributions ( $\text{ln}(1/\Delta)$ ) in the ranges $k_{\mathrm{T}} \in [10,20]$ GeV (left) and $k_{\mathrm{T}} \in [20,40]$ GeV (right) for pp and PbPb collisions of centrality 0-30%. The uncertainties are propagated as uncorrelated in the ratio (bottom panels). Each distribution is normalized to the total number of jets within $p_{\mathrm{T}} \in [200,1000]$ GeV and $\|\eta\| <$ 2. The ratio is compared with jet quenching model predictions by JetMed, JETSCAPE and Hybrid, which provides different medium resolution lengths ( $L$ ) and the inclusion/exclusion of backreacting medium particles (wake). The uncertainties on the models are statistical. The shaded regions in the data distributions represent the total uncertainty.
png pdf	Figure 3-b: Fully corrected hardest $k_{\mathrm{T}}$ emission angular distributions ( $\text{ln}(1/\Delta)$ ) in the ranges $k_{\mathrm{T}} \in [10,20]$ GeV (left) and $k_{\mathrm{T}} \in [20,40]$ GeV (right) for pp and PbPb collisions of centrality 0-30%. The uncertainties are propagated as uncorrelated in the ratio (bottom panels). Each distribution is normalized to the total number of jets within $p_{\mathrm{T}} \in [200,1000]$ GeV and $\|\eta\| <$ 2. The ratio is compared with jet quenching model predictions by JetMed, JETSCAPE and Hybrid, which provides different medium resolution lengths ( $L$ ) and the inclusion/exclusion of backreacting medium particles (wake). The uncertainties on the models are statistical. The shaded regions 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

We present the first momentum scale (

$k_{\mathrm{T}}$ ) scan of the Lund jet plane (LJP) in heavy ion collisions. This scan provides an experimental approach to validate the factorization hypothesis between vacuum and in-medium cascades. This is an underlying assumption in several jet quenching calculations, which lacks so far experimental confirmation. The unfolded angular distribution of the hardest splittings selected in two exclusive intervals of the splittings' relative momentum

$k_{\mathrm{T}}$ are compared in pp and in PbPb collisions. The comparison of the ratio with models suggests the observable does not depend on the medium response as implemented in the Hybrid model. The ratios are most consistent with vanishing resolution length in the Hybrid model and with the JETSCAPE model, which doesn't have an explicit implementation of color coherence. The ratios show a similar shape of the angular distribution in both collision systems, within the experimental uncertainties. This similarity suggests that the analysis isolates early vacuum-like emissions in PbPb collisions.

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