arxiv: 2604.13932 · v1 · submitted 2026-04-15 · ✦ hep-ph

Recognition: unknown

A dynamical implementation of colour coherence for quenched jets in JEWEL

Korinna Zapp

Authors on Pith no claims yet

Pith reviewed 2026-05-10 13:26 UTC · model grok-4.3

classification ✦ hep-ph

keywords colour coherencejet quenchingJEWELheavy-ion collisionsangular orderingmedium resolution scalenuclear modification factorjet fragmentation function

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The pith

A dynamical check whether medium scatterings resolve color dipoles implements colour coherence in JEWEL.

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

The paper implements colour coherence effects in the JEWEL event generator for jets evolving in dense media. In each hard parton-medium interaction, the code tests whether the momentum transfer is large enough to resolve the colour dipole formed by the parton and its colour partner. When resolution occurs, coherence is broken and angular ordering is lost for the next splitting. This mechanism suppresses hard radiation and thereby lowers the overall scattering rate, which in turn modifies the observable properties of reconstructed jets.

Core claim

In each interaction between a hard parton and the medium it is checked whether the momentum transfer of the scattering is sufficient to resolve the colour dipole. In this way it is dynamically decided which structures stay coherent. Importantly, scatterings that resolve an individual parton disrupt the colour coherence, which affects the next splitting via the loss of angular ordering. This leads to a suppression of hard radiation, and consequently a reduction in overall scattering rate, which is the dominant source of effects of colour coherence observable in reconstructed jets.

What carries the argument

Dynamical resolution check of colour dipoles via momentum transfer in each parton-medium scattering, which removes angular ordering for subsequent splittings.

If this is right

The nuclear modification factor for jets is reduced because fewer hard scatterings occur after coherence is broken.
Jet fragmentation functions become softer due to the suppression of hard radiation.
Jet-hadron correlations exhibit reduced yield at high associated hadron pT.
The overall effect on reconstructed jets is dominated by the lowered scattering rate rather than by direct medium-induced emissions.

Where Pith is reading between the lines

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

The same resolution criterion could be used to decide coherence in vacuum parton showers outside JEWEL.
Comparing the resulting jet shapes to LHC data on heavy-ion collisions would test whether the medium resolution scale is the main driver of coherence loss.
Extending the check to multi-parton systems might reveal additional coherence patterns not captured by pairwise dipoles.

Load-bearing premise

That the momentum-transfer threshold correctly decides when a scattering resolves a colour dipole and thereby breaks coherence for the next splitting.

What would settle it

A measurement in which the hard-radiation component of jet-hadron correlations shows no suppression when colour coherence is expected to be active would falsify the dynamical criterion.

read the original abstract

Colour coherence affects the radiation pattern of hard partons both in vacuum and in a dense coloured background formed in heavy ion collisions. In vacuum evolution it leads to the well-known phenomenon of angular ordering, and in heavy ion collisions the appearance of a medium resolution scale strongly affects the way in which a fragmenting hard parton interacts with the background medium. In this paper I present the implementation of colour coherence in the JEWEL event generator for jet evolution in a dense medium. In each interaction between a hard parton and the medium it is checked whether the momentum transfer of the scattering is sufficient to resolve the colour dipole. In this way it is dynamically decided which structures stay coherent. Importantly, scatterings that resolve an individual parton disrupt the colour coherence, which affects the next splitting via the loss of angular ordering. This leads to a suppression of hard radiation, and consequently a reduction in overall scattering rate, which is the dominant source of effects of colour coherence observable in reconstructed jets. I discuss these modifications using the examples of nuclear modification factor, jet fragmentation function and jet-hadron correlations.

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.

Desk Editor's Note private letter to a colleague

JEWEL now has a per-scattering coherence check, but the claimed suppression of hard radiation from losing angular ordering does not follow from standard QCD.

read the letter

The paper adds a dynamical test inside JEWEL: for each hard parton-medium scattering, it checks whether the momentum transfer resolves the colour dipole. If it does, coherence breaks and angular ordering is removed from the next splitting. This is presented as the new piece, and it is a concrete implementation rather than a static prescription. That is the main advance for users of the generator who want medium-dependent coherence decisions instead of an overall scale. The approach stays close to the usual dipole resolution picture, which is a reasonable starting point. The implementation itself looks like a straightforward addition that could be turned on or off for comparisons. The soft spot is the direction of the effect. The abstract states that breaking coherence and dropping angular ordering suppresses hard radiation, which then reduces the overall scattering rate and drives changes in R_AA, fragmentation functions, and jet-hadron correlations. Standard shower logic says the opposite: angular ordering restricts wide-angle emissions through interference, so removing it enlarges the allowed phase space and should increase gluon yield, including harder ones. No compensating mechanism such as modified formation times or extra medium-induced interference is mentioned to reverse the sign. Because the observable consequences are all attributed to this chain, the missing justification is load-bearing. This work is mainly for the small group that runs JEWEL on heavy-ion data and wants to test coherence effects inside that specific code. A reader outside that circle will not find new theoretical insight or broad QCD results. It still deserves peer review because the implementation is explicit and the topic is relevant to current jet-quenching modeling; referees can check whether the radiation pattern actually behaves as described once the code is examined.

Referee Report

1 major / 0 minor

Summary. The paper implements a dynamical criterion for colour coherence in the JEWEL Monte Carlo generator for jets in heavy-ion collisions. In each parton-medium scattering, the momentum transfer is checked against the colour dipole resolution scale; resolved scatterings are taken to break coherence, which removes angular ordering from the subsequent splitting. The resulting suppression of hard radiation is stated to reduce the overall medium scattering rate and thereby modify jet observables such as the nuclear modification factor, fragmentation functions, and jet-hadron correlations.

Significance. If the sign and magnitude of the effect are correctly reproduced, the work supplies a parameter-free, physically motivated extension of angular ordering to medium-modified showers. This addresses a long-standing gap in jet-quenching generators and could improve the description of coherence-sensitive observables without additional tuning.

major comments (1)

[Abstract / algorithm description] Abstract and the description of the algorithm: the central claim that removal of angular ordering suppresses hard radiation (and thereby the scattering rate) is load-bearing for all reported modifications to R_AA, fragmentation functions and jet-hadron correlations. Standard QCD shower logic predicts that loss of angular ordering enlarges the allowed emission phase space and increases (rather than suppresses) the yield of gluons, including harder ones. No compensating mechanism (modified formation times, medium-induced interference, or explicit phase-space calculation) is provided to reverse this sign; the manuscript must supply either an analytic derivation or a controlled numerical test demonstrating why the net effect is suppression.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and for the constructive feedback. We value the recognition of the potential importance of a dynamical colour-coherence implementation in JEWEL. We address the single major comment below and will revise the manuscript accordingly.

read point-by-point responses

Referee: Abstract and the description of the algorithm: the central claim that removal of angular ordering suppresses hard radiation (and thereby the scattering rate) is load-bearing for all reported modifications to R_AA, fragmentation functions and jet-hadron correlations. Standard QCD shower logic predicts that loss of angular ordering enlarges the allowed emission phase space and increases (rather than suppresses) the yield of gluons, including harder ones. No compensating mechanism (modified formation times, medium-induced interference, or explicit phase-space calculation) is provided to reverse this sign; the manuscript must supply either an analytic derivation or a controlled numerical test demonstrating why the net effect is suppression.

Authors: We agree that the referee has identified a point that requires explicit clarification. In vacuum QCD the loss of angular ordering indeed enlarges the emission phase space. In the present medium-modified implementation the coherence-breaking decision is made per scattering on the basis of the local resolution scale; this changes the colour flow for the subsequent vacuum-like splitting and, through the interplay with the medium scattering and induced radiation already present in JEWEL, produces a net suppression of hard emissions. The manuscript reports the resulting modifications to observables but does not isolate the sign-reversing mechanism. We will therefore add a dedicated numerical test (new figure and accompanying text) that compares the parton-level emission spectra and scattering rates with and without the dynamical coherence criterion, thereby demonstrating the suppression explicitly. revision: yes

Circularity Check

0 steps flagged

No circularity: implementation criterion is independent of target observables

full rationale

The paper implements a momentum-transfer resolution check for colour dipoles inside the existing JEWEL framework and states that resolved scatterings remove angular ordering from the subsequent splitting. No equations, fitted parameters, or results are shown to reduce by construction to the inputs or to prior self-citations; the central mechanism is a dynamical decision rule grounded in standard QCD dipole resolution and is applied to standard observables (R_AA, fragmentation functions) without tautological redefinition. The derivation chain therefore remains self-contained.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The work rests on established QCD colour coherence and the existing JEWEL framework; no new free parameters, ad-hoc entities, or unproven axioms are introduced in the abstract description.

axioms (2)

standard math Standard QCD colour coherence leading to angular ordering in vacuum
Invoked as the baseline behaviour that is modified by medium scatterings.
domain assumption Existence of a medium resolution scale that can resolve colour dipoles
Used to decide dynamically whether coherence is broken in each scattering.

pith-pipeline@v0.9.0 · 5482 in / 1286 out tokens · 48753 ms · 2026-05-10T13:26:01.515647+00:00 · methodology

discussion (0)

Reference graph

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