First Demonstration that Quark-Gluon Plasma has a Nonzero Resolution Length

TL;DR

The paper tackles whether the quark-gluon plasma exhibits a finite jet-resolution length $L_{ m res}$ that governs whether nearby jet prongs interact coherently or independently with the medium. It employs the Hybrid Strong/Weak Coupling Model to simulate jet quenching, varying $L_{ m res}$ across $0$, $1/\(\pi T\)$, $2/\(\pi T\)$, and $\infty$, and compares to ALICE Soft Drop theta_g data, ATLAS Hard Group $\Delta R_{12}$ measurements, and ATLAS $dR_{12}$-dependent observables. The data exclude both fully coherent ($L_{ m res}=\infty$) and fully incoherent ($L_{ m res}=0$) pictures, with finite nonzero $L_{ m res}$ around $(1-2)/\(\pi T\)$ providing the best description, indicating a finite coherence length in the QGP. This result offers the first strong experimental indication of a nonzero $L_{ m res}$ and outlines a path toward precise determination with higher-statistics data, while highlighting the need to extend the model to include elastic scatterings for a comprehensive description of jet substructure in heavy-ion collisions.

Abstract

We report on our investigation in arXiv:2509.08881 of how recent jet substructure measurements constrain the resolution length $L_{\rm res}$ of the quark-gluon plasma formed in heavy-ion collisions. $L_{\rm res}$ is defined such that high-energy partons within a jet shower are resolved by the medium if and only if they are separated by a distance greater than $L_{\rm res}$. Using the Hybrid Model, we reproduce ALICE data on the scaled Soft Drop angle $θ_g$ for $R=0.2$ charged-particle jets and ATLAS data on the Hard Group angle $ΔR_{12}$ for $R=1$ jets reclustered from skinny $R = 0.2$ inclusive subjets. We find that the narrowing of the $θ_g$-distribution in PbPb collisions observed by ALICE and the suppression of $R=1$ jets with multiple skinny subjets in PbPb collisions observed by ATLAS rule out $L_{\rm res} = \infty$, where each entire parton shower loses energy to the plasma coherently as if it were a single colored object. We then compare Hybrid Model calculations to ATLAS measurements of $R_{\rm AA}$ of $R=1$ jets reclustered from $R=0.2$ subjets, as a function of the Soft Drop angle $dR_{12}$ obtained by grooming all charged-particle tracks associated with each $R=1$ jet. We demonstrate, for the first time, that the ATLAS data is inconsistent with $L_{\rm res} = 0$, where the plasma resolves every splitting in a parton shower. Our results agree best with the data when QGP possesses a finite, nonzero $L_{\rm res}\sim (1-2)/(πT)$.

Figures (4)

• Figure 1: Ratio of the differential cross sections of jets in PbPb collisions to jets in pp collisions, as a function of the scaled Soft Drop angle $\theta_g$ for anti-$k_t$$R = 0.2$ charged-particle jets with $60 < p_T^{\rm ch-jet} < 80$ GeV and $|\eta^{\rm ch-jet}| < 0.7$, reconstructed using the Soft Drop grooming procedure with parameters $z_{\rm cut} = 0.2$ and $\beta = 0$. The colored bands show the results of Hybrid Model calculations with $L_{\rm res} = 0$ (red), $1/(\pi T)$ (purple), $2/(\pi T)$ (blue) and $\infty$ (green). ALICE experimental measurements from Ref. ALargeIonColliderExperiment:2021mqf are depicted using point markers, upon which the vertical bars indicate statistical uncertainties and the shaded boxes indicate systematic uncertainties extracted from Ref. ALargeIonColliderExperiment:2021mqf.
• Figure 2: $R_{\rm AA}$ as a function of $\Delta R_{12}$ for reclustered large-radius $R = 1.0$ jets with $200 < p_T < 251$ GeV. The left-most bin denotes the value of $R_{\rm AA}$ for $R = 1.0$ jets that contain only a single skinny subjet, placed arbitrarily at 0.1; all bins with $\Delta R_{12}\geq 0.2$ show $R_{\rm AA}$ for $R=1.0$ jets composed of multiple skinny subjets with the hardest splitting given by $\Delta R_{12}$, as described in the text. The colored bands show the results of Hybrid Model calculations with $L_{\rm res} = 0$ (red), $1/(\pi T)$ (purple), $2/(\pi T)$ (blue) and $\infty$ (green). ATLAS experimental measurements from Ref. ATLAS:2023hso are depicted using point markers. The vertical bars on ATLAS' experimental data points indicate statistical uncertainties and the shaded boxes indicate systematic uncertainties ATLAS:2023hso.
• Figure 3: $R_{\rm AA}$ as a function of $dR_{12}$ for $R = 1.0$ jets with $200 < p_T < 251$ GeV and $|\eta| < 1.3$ constructed from skinny $R = 0.2$ subjets using the Hard Group procedure, whose associated charged-particle tracks were then reclustered using the $k_t$ algorithm and the Soft Drop grooming procedure with parameters $z_{\rm cut} = 0.15$ and $\beta = 0$. The colored bands show the results of Hybrid Model calculations with $L_{\rm res} = 0$ (red), $1/(\pi T)$ (purple), $2/(\pi T)$ (blue) and $\infty$ (green). ATLAS experimental measurements from Ref. ATLAS:2025lfb are depicted using point markers, upon which the vertical bars indicate statistical uncertainties and the shaded boxes indicate systematic uncertainties extracted from Ref. ATLAS:2025lfb.
• Figure 4: $R_{\rm AA}$ as a function of $dR_{12}$ for $R = 1.0$ jets with $200 < p_T < 251$ GeV and $|\eta| < 1.3$ constructed from skinny $R = 0.2$ subjets using the Hard Group procedure, whose associated charged-particle tracks were then reclustered using the $k_t$ algorithm and the Soft Drop grooming procedure with parameters $z_{\rm cut} = 0.15$ and $\beta = 0$. The orange band shows the results of Hybrid Model calculations with elastic $2 \rightarrow 2$ scatterings, and $L_{\rm res} = 0$. ATLAS experimental measurements from Ref. ATLAS:2025lfb are depicted using point markers, upon which the vertical bars indicate statistical uncertainties and the shaded boxes indicate systematic uncertainties extracted from Ref. ATLAS:2025lfb.