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arxiv: 2606.20061 · v1 · pith:XI72Z3K7new · submitted 2026-06-18 · ⚛️ nucl-ex

Modification of jet-energy flow in heavy-ion collisions

ALICE Collaboration This is my paper

Pith reviewed 2026-06-26 15:06 UTC · model grok-4.3

classification ⚛️ nucl-ex
keywords jet energy flowheavy-ion collisionsquark-gluon plasmajet quenchingPb-Pb collisionsparton shower modificationALICE
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The pith

Heavy-ion collisions suppress jet energy flow at larger radii, narrowing the distribution relative to proton-proton collisions.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper introduces measurements of the jet-energy flow observable in Pb-Pb collisions at 5.02 TeV for jets with 60-80 GeV/c and in pp collisions at 13 TeV for 40-60 and 60-80 GeV/c. It establishes that most energy stays near the jet axis in both systems, yet Pb-Pb data show a statistically significant drop in energy at larger distances from the axis compared with the pp reference. This narrowing points to medium-induced changes in how partons shower inside the quark-gluon plasma. The result supplies a new infrared-safe probe that can distinguish between models of jet modification.

Core claim

The measurements establish a clear suppression of jet-energy flow at larger radii in Pb-Pb collisions relative to pp, with a significance of 3.5-4.5 sigma, indicating a narrowing of the radial energy distribution. The pp reference is reproduced by PYTHIA 8, HERWIG, JEWEL and JETSCAPE with only minor tail deviations, while the Pb-Pb modification is described well by JEWEL without recoil but deviates from JEWEL with recoil and from JETSCAPE.

What carries the argument

The jet-energy flow (Δp_T) observable, which quantifies the radial distribution of transverse momentum around the jet axis in an infrared- and collinear-safe manner.

If this is right

  • The narrowing implies that the quark-gluon plasma preferentially affects energy carried by partons at larger angles from the jet axis.
  • Models that include medium recoil or response produce trends with radius that are disfavored by the data.
  • Most parton energy remains concentrated in the jet core even after passage through the medium.
  • The observable isolates medium effects on the parton shower while remaining insensitive to certain reconstruction biases.
  • JEWEL without recoil captures the relative modification, while versions with recoil and JETSCAPE do not.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The result suggests that softer particles at wider angles are more readily modified by the medium, which could be tested with identified-particle correlations inside jets.
  • Repeating the measurement at matched collision energies would isolate purely medium-induced effects from any residual initial-state differences.
  • The narrowing pattern may constrain the angular scale at which jet quenching becomes active, offering input for hydrodynamic or transport-model calculations of the plasma.
  • Extensions to higher jet momenta or to photon-jet events could reveal whether the narrowing persists or saturates with increasing parton energy.

Load-bearing premise

The 60-80 GeV/c pp sample at 13 TeV can be scaled and compared directly to the Pb-Pb sample at 5.02 TeV without residual initial-state or fragmentation differences that would mimic medium-induced narrowing.

What would settle it

A direct measurement of the same jet-energy flow observable in pp collisions at 5.02 TeV to test whether the scaled reference matches the Pb-Pb baseline after energy adjustment.

Figures

Figures reproduced from arXiv: 2606.20061 by ALICE Collaboration.

Figure 1
Figure 1. Figure 1: Sketch of the matching process between jets of different resolution parameter. On the left, a case where the jet axes are completely aligned and on the right a more general case where there is significant overlap between the jets but the axes themselves are not aligned. 3.2 Corrections 3.2.1 Background subtraction In pp collisions, background contributions from the underlying event (UE) are negligible in t… view at source ↗
Figure 2
Figure 2. Figure 2: JEF distributions in 40 < p ch jet T < 60 GeV/c (left) and 60 < p ch jet T < 80 GeV/c (right) for each of the seven jet pairs with radii of [0.05, 0.1], [0.1, 0.15], [0.15, 0.2], [0.2, 0.25], [0.25, 0.3], [0.3, 0.35], and [0.35, 0.4] in pp collisions at √ s = 13 TeV. For certain radius-pair ∆pT distributions, the statistical uncertainties are smaller than the marker size. 5.1.1 Comparison to MC models The … view at source ↗
Figure 3
Figure 3. Figure 3: JEF distribution in 60 < p ch jet T < 80 GeV/c for three jet pairs with radii of [0.05, 0.1] (top left), [0.15, 0.20] (top right), and [0.2, 0.25] (bottom) compared to PYTHIA, HERWIG, JEWEL pp [41] and JETSCAPE (PP19 tune) [43] in pp collisions at √ s = 13 TeV. The bottom panels of each figure show the ratio of MC to data, and the gray band represents the systematic uncertainty of the data. 5.1.2 Mean ener… view at source ↗
Figure 4
Figure 4. Figure 4: Mean JEF in the 40 < p ch jet T < 60 GeV/c (left) and 60 < p ch jet T < 80 GeV/c (middle) jet-pT intervals, and its jet-pT dependence (right), as a function of the smaller jet radius, Rjet1 , compared with PYTHIA, HERWIG, JEWEL, and JETSCAPE predictions for pp collisions at √ s = 13 TeV. 5.2 Jet-energy flow in Pb–Pb The fully corrected JEF distributions for central Pb–Pb collisions (0–10% centrality class)… view at source ↗
Figure 5
Figure 5. Figure 5: JEF distributions in 60 < p ch jet T < 80 GeV/c for jet pairs with radii [0.05, 0.1], [0.1, 0.15], [0.15, 0.2], and [0.2, 0.25], in Pb–Pb collisions at √ sNN = 5.02 TeV. The lines represent the statistical uncertainties, and the boxes represent the systematic uncertainties. illustrating the expected dependence on the collision energy. The significance of the data points at the largest ∆pT relative to the P… view at source ↗
Figure 6
Figure 6. Figure 6: JEF distributions in the 60 < p ch jet T < 80 GeV/c interval for pp collisions at √ s = 13 TeV and Pb–Pb collisions at √ sNN = 5.02 TeV are shown for jet pairs with radii [0.05, 0.1] (top left), [0.1, 0.15] (top right), [0.15, 0.2] (bottom left), and [0.2, 0.25] (bottom right). The bottom panels show the ratio of JEF in Pb–Pb collisions to pp collisions, compared to JETSCAPE and JEWEL (with recoil on and o… view at source ↗
Figure 7
Figure 7. Figure 7: Top: Comparison of the mean energy flow for jet pairs with radii between Rjet = 0.05 and Rjet = 0.4 (Rjet = 0.25 for Pb–Pb) in the 60 < p ch jet T < 80 GeV/c interval in ALICE pp (Pb–Pb) collisions, compared to models. In the model calculations √ s = 13 TeV is used for pp collisions and √ sNN = 5.02 TeV for Pb–Pb collisions. Bottom: Ratio of the relative difference of Pb–Pb with respect to pp, compared to … view at source ↗
read the original abstract

The ALICE Collaboration presents the first measurements of the jet-energy flow ($\Delta p_{\rm T}$) observable in proton-proton and heavy-ion collisions. Jets are excellent probes for the quark$-$gluon plasma, a deconfined state of matter produced in heavy-ion collisions. The jet-energy flow observable characterizes the radial distribution of energy from the jet axis in an infrared and collinear-safe way and is sensitive to medium-induced parton-shower modifications. Inclusive charged jets are measured in Pb$-$Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV for the transverse-momentum interval 60$-$80 GeV/$c$. For pp collisions at $\sqrt{s}=13$ TeV, measurements include the 40$-$60 and 60$-$80 GeV/$c$ intervals, where the latter serves as the reference for investigating medium-induced modifications. Results show that most parton energy is concentrated in the jet core, with a clear suppression of energy flow in heavy-ion collisions at larger radii (significance 3.5$-$4.5$\sigma$) indicating a narrowing of the energy flow. While all models -- PYTHIA 8, HERWIG, JEWEL, and JETSCAPE -- reproduce the pp results with only small deviations in the tails, the relative modification in Pb$-$Pb collisions is well described by JEWEL without recoil. Conversely, JEWEL with recoil (medium response) and JETSCAPE show significant deviations, exhibiting increasing or more constant trends with radius that are disfavored by the data.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

1 major / 1 minor

Summary. The manuscript reports the first measurements of the infrared- and collinear-safe jet-energy flow observable Δp_T for inclusive charged jets. In Pb-Pb collisions at √s_NN = 5.02 TeV (60–80 GeV/c) a statistically significant (3.5–4.5σ) suppression of Δp_T is observed at larger radii relative to a pp reference at √s = 13 TeV (60–80 GeV/c), interpreted as medium-induced narrowing of the jet energy flow. The 40–60 GeV/c pp interval at 13 TeV is also presented. All tested models reproduce the pp data; only JEWEL without recoil describes the Pb-Pb modification, while JEWEL with recoil and JETSCAPE are disfavored.

Significance. If the central result holds, the work supplies a new, infrared-safe observable that directly constrains the radial redistribution of jet energy in the QGP and provides useful model discrimination. The explicit comparison of multiple Monte Carlo implementations (PYTHIA 8, HERWIG, JEWEL, JETSCAPE) is a strength of the analysis.

major comments (1)
  1. [Results and discussion of the Pb-Pb to pp comparison] The central claim of medium-induced narrowing rests on the direct comparison of the 60–80 GeV/c Pb-Pb sample at 5.02 TeV to the 60–80 GeV/c pp sample at 13 TeV. Because parton distributions, initial-state radiation, and fragmentation functions retain a mild √s dependence even at fixed jet p_T, any residual difference in the large-radius tails of the vacuum baseline would shift the extracted significance. The manuscript does not present an explicit same-energy (5.02 TeV) pp reference or a dedicated study of the residual √s dependence after the standard scaling; this assumption is therefore load-bearing for the 3.5–4.5σ result.
minor comments (1)
  1. [Abstract and Methods] The abstract states that details of background subtraction, unfolding, and systematic uncertainties are not provided; the full manuscript should make these procedures and the associated covariance matrices explicit so that the quoted significance can be independently verified.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the positive assessment of the work's significance and for the detailed comment on the Pb-Pb to pp comparison. We address the point below and will strengthen the manuscript accordingly.

read point-by-point responses

  1. Referee: The central claim of medium-induced narrowing rests on the direct comparison of the 60–80 GeV/c Pb-Pb sample at 5.02 TeV to the 60–80 GeV/c pp sample at 13 TeV. Because parton distributions, initial-state radiation, and fragmentation functions retain a mild √s dependence even at fixed jet p_T, any residual difference in the large-radius tails of the vacuum baseline would shift the extracted significance. The manuscript does not present an explicit same-energy (5.02 TeV) pp reference or a dedicated study of the residual √s dependence after the standard scaling; this assumption is therefore load-bearing for the 3.5–4.5σ result.

    Authors: We agree that quantifying any residual √s dependence strengthens the robustness of the result. The 13 TeV pp sample was selected for its high statistical precision. In the revised manuscript we will add a dedicated study using PYTHIA 8 (and, where applicable, HERWIG) comparing Δp_T in pp collisions at 5.02 TeV versus 13 TeV for the same jet-p_T interval. This will explicitly show the size of the difference in the large-radius region and demonstrate that it remains well below the observed medium-induced modification, preserving the reported significance. The medium models are already run at the correct √s for each system, providing further consistency checks. revision: yes

Circularity Check

0 steps flagged

No circularity: direct experimental measurement with external model comparisons

full rationale

This is an experimental paper reporting measurements of the jet-energy flow observable Δp_T in pp and Pb-Pb collisions. The central results are raw distributions and ratios extracted from data, compared to external Monte Carlo generators (PYTHIA 8, HERWIG, JEWEL, JETSCAPE) that are not fitted to the present dataset. No derivation step reduces by construction to a parameter defined from the same data, no self-citation chain carries the load-bearing claim, and the pp reference at 13 TeV is an external baseline (with acknowledged energy mismatch) rather than a self-derived quantity. The analysis is self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work is an experimental measurement paper. No free parameters are fitted to produce the central observable. Standard assumptions of perturbative QCD, jet clustering algorithms, and heavy-ion background subtraction are invoked but not enumerated in the abstract.

axioms (2)
  • domain assumption Standard model of strong interactions and perturbative QCD applies to jet production and fragmentation in vacuum.
    Implicit in the use of PYTHIA and HERWIG as pp baselines.
  • domain assumption Jet reconstruction and background subtraction procedures in Pb-Pb collisions do not introduce artificial radial narrowing.
    Required for interpreting the observed suppression as a medium effect.

pith-pipeline@v0.9.1-grok · 5814 in / 1400 out tokens · 22341 ms · 2026-06-26T15:06:33.888099+00:00 · methodology

discussion (0)

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Reference graph

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