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arxiv: 2509.24316 · v1 · submitted 2025-09-29 · ✦ hep-ph

Energy loss of heavy-flavor quarks in color string medium

Pith reviewed 2026-05-18 13:23 UTC · model grok-4.3

classification ✦ hep-ph
keywords heavy-flavor quarksenergy losscolor stringsproton-proton collisionselastic scatteringfluctuating mediumquark-gluon plasmasmall systems
0
0 comments X

The pith

Heavy-flavor quarks lose less transverse momentum in a fluctuating color-string medium than in hydrodynamic descriptions of small systems.

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

The paper calculates the energy loss of charm quarks produced in hard scatterings as they move through the medium created in proton-proton collisions at LHC energies. It models that medium as a set of color strings whose longitudinal oscillations and varying overlaps generate fluctuations in color field energy density. Those fluctuations set the rate at which the quarks scatter elastically with gluons. The resulting loss is substantially smaller than the loss obtained when the same collisions are described with an expanding hydrodynamic medium. A reader would care because the difference bears directly on whether tiny collision systems form an equilibrated quark-gluon plasma or retain string-like, non-equilibrated dynamics.

Core claim

The central claim is that charm quarks traversing the non-equilibrated medium formed by fluctuating color strings from multi-pomeron exchanges in minimum-bias proton-proton collisions experience significantly lower momentum loss than they do in an expanding hydrodynamic scenario, with the loss computed event by event via a hybrid simulation that accounts for the dynamic initialization of the medium at each time step.

What carries the argument

The varying overlaps of longitudinally oscillating color strings, which produce fluctuations in color field energy density that directly govern the elastic scattering rate of heavy-flavor quarks with gluons inside the string volume.

If this is right

  • The transverse-momentum dependence of momentum loss for charm quarks follows from event-by-event propagation through the fluctuating string environment.
  • The lower energy loss stems from the non-equilibrated, dynamically initialized medium rather than a smooth hydrodynamic expansion.
  • String-overlap fluctuations create a different scattering environment that reduces the average interaction rate compared with equilibrated descriptions.
  • The result bears on whether signals of heavy-quark suppression in small systems indicate full quark-gluon plasma formation or string-like dynamics.

Where Pith is reading between the lines

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

  • If the string description holds, heavy-flavor observables in proton-proton data could serve as a baseline that distinguishes string dynamics from hydrodynamic flow without assuming equilibrium.
  • The same fluctuating-medium setup could be used to predict other observables such as azimuthal anisotropies in small systems while remaining outside the hydrodynamic regime.
  • Testing the model against measured heavy-meson spectra at the LHC would directly probe whether string overlaps capture the dominant energy-loss mechanism in tiny collision volumes.

Load-bearing premise

The premise that energy-density fluctuations arising from string overlaps set the elastic scattering rates of heavy quarks with gluons without requiring additional equilibration of the medium.

What would settle it

A measurement of charm-quark transverse-momentum loss in high-multiplicity proton-proton events that matches the higher values predicted by hydrodynamic models rather than the lower values from the string model would falsify the central result.

Figures

Figures reproduced from arXiv: 2509.24316 by Daria Prokhorova, Evgeny Andronov, Shuzhe Shi.

Figure 1
Figure 1. Figure 1: Transverse momentum loss of a charm quark as a function of its initial transverse mo [PITH_FULL_IMAGE:figures/full_fig_p012_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: Transverse momentum loss of a charm quark as a function of its initial transverse momen [PITH_FULL_IMAGE:figures/full_fig_p013_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: Transverse momentum loss of a charm quark as a function of its initial transverse momen [PITH_FULL_IMAGE:figures/full_fig_p013_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Transverse momentum loss of a charm quark as a function of its initial transverse momen [PITH_FULL_IMAGE:figures/full_fig_p014_4.png] view at source ↗
read the original abstract

The paper presents preliminary estimates of heavy-flavor (HF) quark energy loss during its propagation through the non-equilibrated medium formed in minimum bias proton-proton (p+p) collisions at LHC energies. The study is inspired by the ongoing hot debates on whether tiny droplets of Quark-Gluon Plasma can be created in collisions of small systems. In this work, we model a p+p event with a fluctuating number of color strings originated from multi-pomeron exchanges. Considered longitudinal oscillations of strings dynamically initialize medium at each time step. Their varying overlaps create fluctuations in the color field energy density that governs the elastic scattering rate of HF quarks with the gluons present within the string volume. We calculate the transverse momentum dependence of the momentum loss for charm (anti-)quarks that are produced in initial hard scatterings and traverse the described environment. The simulation is performed using a developed hybrid approach on an event-by-event basis. Our results show significantly lower HF quarks energy loss compared to that obtained in the expanding hydrodynamic scenario of the new EPOS4HQ model.

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 / 2 minor

Summary. The manuscript presents preliminary estimates of heavy-flavor quark energy loss in a non-equilibrated medium formed by fluctuating color strings in minimum-bias pp collisions at LHC energies. Longitudinal string oscillations dynamically initialize the medium at each time step, with overlaps producing fluctuations in color-field energy density that govern the elastic scattering rate of HF quarks with gluons. Using an event-by-event hybrid simulation, the transverse-momentum dependence of momentum loss for charm quarks is computed and compared to the expanding hydrodynamic scenario of the EPOS4HQ model, with the central claim being significantly lower energy loss in the string medium.

Significance. If the energy-density-to-scattering-rate mapping can be placed on a firm footing, the result would be relevant to debates on QGP formation in small systems by indicating that non-equilibrated string dynamics with fluctuations yield less suppression than hydrodynamics. The event-by-event treatment of string overlaps is a methodological strength that captures realistic medium fluctuations.

major comments (1)
  1. [hybrid approach description] The functional dependence of the elastic scattering rate on the fluctuating color-field energy density is not derived, perturbatively matched, or calibrated (see the model description following the statement that 'Their varying overlaps create fluctuations in the color field energy density that governs the elastic scattering rate'). This mapping is load-bearing for the headline quantitative claim of significantly lower energy loss relative to EPOS4HQ; different choices of the functional form (e.g., rate ∝ ε or rate ∝ ε²) could shift the reported difference by an amount comparable to the claimed effect without changing the underlying string dynamics.
minor comments (2)
  1. [Results] The abstract refers to 'preliminary estimates' but the results section should explicitly state the number of events, statistical uncertainties, and any systematic variations in the string parameters to allow assessment of the robustness of the 'significantly lower' conclusion.
  2. Clarify whether the charm-quark production points and initial p_T spectra are taken from the same hard-scattering generator used in the EPOS4HQ comparison or from an independent source.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for the careful reading of our manuscript and for the constructive feedback on the hybrid model description. We appreciate the positive assessment of the event-by-event string overlap treatment and its relevance to small-system QGP debates. We address the major comment below and outline planned revisions.

read point-by-point responses
  1. Referee: The functional dependence of the elastic scattering rate on the fluctuating color-field energy density is not derived, perturbatively matched, or calibrated (see the model description following the statement that 'Their varying overlaps create fluctuations in the color field energy density that governs the elastic scattering rate'). This mapping is load-bearing for the headline quantitative claim of significantly lower energy loss relative to EPOS4HQ; different choices of the functional form (e.g., rate ∝ ε or rate ∝ ε²) could shift the reported difference by an amount comparable to the claimed effect without changing the underlying string dynamics.

    Authors: We agree that the mapping from color-field energy density to elastic scattering rate is a central modeling assumption whose justification merits explicit discussion. In the current implementation the rate is taken proportional to the local energy density ε, reflecting the expectation that higher energy density within overlapping strings corresponds to a greater number of gluons available for scattering with the heavy-flavor quark. This linear ansatz is chosen for the preliminary study as a minimal, physically motivated link between the string dynamics and the interaction rate; it is not claimed to be perturbatively derived from first principles. We acknowledge that alternative forms (e.g., quadratic) could alter the quantitative difference relative to EPOS4HQ. In the revised manuscript we will (i) state the proportionality explicitly in the model section, (ii) add a short paragraph motivating the choice from the color-string picture, and (iii) include a brief discussion of the sensitivity to the functional form, noting that a full perturbative matching or calibration lies beyond the scope of this exploratory work but is planned for follow-up studies. These additions will make the assumption transparent without changing the underlying string dynamics or the reported qualitative result of lower energy loss. revision: partial

Circularity Check

0 steps flagged

No significant circularity; derivation self-contained against external benchmark

full rationale

The paper models HF quark propagation through a fluctuating color-string medium initialized by longitudinal oscillations and overlaps, with elastic scattering rates governed by the resulting color-field energy density on an event-by-event basis. The headline quantitative result is a direct comparison of computed momentum loss to the independent expanding hydrodynamic scenario of the external EPOS4HQ model. No load-bearing step reduces by construction to a fitted parameter, self-citation chain, or internal redefinition of the target observable; the simulation inputs (string dynamics, overlaps) are distinct from the external hydrodynamic reference, satisfying the criterion for a self-contained derivation against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The model rests on standard high-energy QCD assumptions about color strings and multi-pomeron exchanges; the hybrid simulation and dynamic initialization constitute the main additions. No new particles or forces are introduced.

axioms (1)
  • domain assumption Color strings originate from multi-pomeron exchanges in minimum-bias p+p collisions.
    This is invoked to model the fluctuating medium in which HF quarks propagate.

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