arxiv: 2604.18010 · v1 · submitted 2026-04-20 · ✦ hep-ph · nucl-th

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Jet Quenching in the Smallest Hadronic Collision Systems

Coleridge Faraday , Ben Bert , Jack Brand , Werner Vogelsang , W. A. Horowitz

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Pith reviewed 2026-05-10 04:33 UTC · model grok-4.3

classification ✦ hep-ph nucl-th

keywords jet quenchingsmall collision systemsquark-gluon plasmaperturbative QCDnuclear modification factorparton energy losselliptic flow

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The pith

pQCD calculations predict observable jet quenching suppression in ion collisions as small as ^{3}He+^{3}He, following an approximate scaling R_AB ≃ (√(A B))^{1/3}.

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

The paper applies perturbative quantum chromodynamics to compute how high-momentum particle yields change in very light ion collisions including ^{10}B+^{10}B, ^{6}Li+^{6}Li, ^{4}He+^{4}He, and ^{3}He+^{3}He, both with and without medium-induced energy loss. It reports non-trivial suppression that persists from large Pb+Pb systems down to these tiny symmetric and asymmetric collisions, with the suppression factor scaling roughly as the cube root of the geometric mean of the mass numbers. The work identifies ^{3}He and ^{6}Li systems as especially clean for detecting final-state parton energy loss from possible quark-gluon plasma formation. It further shows that standard energy-loss models produce essentially zero elliptic flow v_{2}{SP} in small systems, so the positive v_{2} measured in p+Pb collisions cannot be explained by energy loss alone.

Core claim

The central claim is that medium-induced partonic energy loss produces measurable suppression of high-p_T yields in symmetric A+A collisions from ^{208}Pb+^{208}Pb down to ^{3}He+^{3}He and in asymmetric A+B systems, with the nuclear modification factor obeying the approximate scaling R_AB ≃ (√(A B))^{1/3}. The calculations single out ^{3}He+^{3}He and ^{6}Li+^{6}Li collisions as particularly clean environments in which to observe final-state energy loss associated with quark-gluon plasma formation in extremely small volumes. Energy-loss models generically give v_{2}{SP} ≈ 0 in such small systems, demonstrating that the sizable measured v_{2} > 0 in p+^{208}Pb collisions must originate from

What carries the argument

The central mechanism is the application of pQCD medium-induced parton energy loss to small nuclear collision systems, which generates the reported geometric scaling of the suppression factor R_AB.

If this is right

Suppression of high-p_T particles should be detectable in ^{3}He+^{3}He and ^{6}Li+^{6}Li collisions at the LHC.
The observed scaling permits systematic extrapolation of quenching effects from large to small collision systems.
Elliptic flow v_{2} in small systems must arise from mechanisms other than final-state energy loss.
Light nuclei such as ^{3}He and ^{6}Li provide controlled settings for testing QGP formation in tiny volumes.

Where Pith is reading between the lines

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

Confirmation of the predicted suppression would indicate that quark-gluon plasma effects can appear in collision systems far smaller than traditionally assumed.
The results suggest that future fixed-target or collider runs with light ions could isolate final-state energy loss with minimal initial-state contamination.
The zero v_{2} prediction from energy loss alone sharpens the need to identify the origin of flow signals in proton-nucleus data.

Load-bearing premise

The calculations assume that final-state partonic energy loss dominates the suppression and that pQCD can cleanly isolate those final-state effects from initial-state nuclear modifications or other contributions even in the smallest systems.

What would settle it

A direct measurement of the nuclear modification factor R_AB in ^{3}He+^{3}He collisions that deviates substantially from the predicted (√(A B))^{1/3} scaling would falsify the claim that energy loss follows this simple geometric dependence across system sizes.

read the original abstract

We present perturbative quantum chromodynamics (pQCD) predictions for high-momentum particle yield modification in very light ion collisions - ${}^{10}\mathrm{B}+{}^{10}\mathrm{B}$, ${}^{6}\mathrm{Li}+{}^{6}\mathrm{Li}$, ${}^{4}\mathrm{He}+{}^{4}\mathrm{He}$, and ${}^{3}\mathrm{He}+{}^{3}\mathrm{He}$ - with and without medium-induced energy loss. We find non-trivial suppression in symmetric systems from ${}^{208}\mathrm{Pb}+{}^{208}\mathrm{Pb}$ to ${}^{3}\mathrm{He}+{}^{3}\mathrm{He}$ and in asymmetric $A+B$ systems, with the suppression scaling approximately as $R_{AB} \simeq (\sqrt{AB})^{1/3}$. Further, we find that ${}^{3}\mathrm{He}$ and ${}^{6}\mathrm{Li}$ offer particularly clean environments for observing final-state partonic energy loss from quark-gluon plasma (QGP) formation in extremely small systems. Finally, we show that energy loss models generically predict $v_2\{\mathrm{SP}\} \approx 0$ in small systems, indicating that the large measured $v_2 > 0$ in $p+{}^{208}\mathrm{Pb}$ is not due to energy loss.

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

The paper extends pQCD jet quenching calculations to the lightest ion systems and extracts a simple suppression scaling, but the isolation of final-state effects rests on a baseline that may miss comparable initial-state contributions in small nuclei.

read the letter

The main thing to know is that this work applies standard pQCD energy loss models to collisions from PbPb down to 3He+3He and 6Li+6Li, reports non-trivial suppression that scales roughly as R_AB ≃ (√AB)^{1/3}, and finds that the same models give v2 near zero in small systems so cannot explain the measured flow in pPb. It also flags 3He and 6Li as relatively clean places to hunt for QGP signals. That scaling relation and the explicit predictions for these tiny symmetric systems are the genuinely new pieces here. The calculations look consistent within the chosen framework and give a clear, usable set of numbers for experimental planning. The soft spot is exactly the one flagged in the stress-test note. In light ions the no-loss baseline can be affected by nuclear PDFs, shadowing, or Cronin-type effects that are relatively larger than in heavy systems, and any mismatch there feeds directly into the reported scaling and the claim that these systems are particularly clean for final-state loss. The paper treats the v2 result as generic, which is honest but also limits how firmly it rules out energy loss as a source of flow. This is for people already working on jet quenching or small-system collectivity who want concrete predictions to compare against upcoming light-ion data. It is grounded enough in established tools to deserve a serious referee, mainly to check the baseline treatment and error propagation in the light systems.

Referee Report

2 major / 2 minor

Summary. The manuscript presents pQCD predictions for high-p_T particle yield modification (R_AB) in very light ion collisions such as ^{10}B+^{10}B, ^{6}Li+^{6}Li, ^{4}He+^{4}He, and ^{3}He+^{3}He, both with and without medium-induced energy loss. It reports non-trivial suppression across symmetric systems from ^{208}Pb+^{208}Pb to ^{3}He+^{3}He and in asymmetric A+B systems, with the suppression scaling approximately as R_AB ≃ (√AB)^{1/3}. The authors identify ^{3}He and ^{6}Li collisions as particularly clean environments for observing final-state partonic energy loss from QGP formation, and show that energy loss models generically predict v_2{SP} ≈ 0 in small systems, indicating that the large measured v_2 > 0 in p+^{208}Pb is not due to energy loss.

Significance. If the predictions hold, the work would be significant for extending jet quenching and QGP studies to the smallest hadronic systems, supplying a simple scaling relation for suppression and identifying optimal light-ion systems for experimental tests. The generic v_2 ≈ 0 result provides a falsifiable distinction between energy loss and other mechanisms for small-system anisotropies. The use of standard pQCD and energy loss frameworks enables direct, reproducible comparisons with upcoming data.

major comments (2)

[§3 (pQCD baseline calculations)] §3 (pQCD baseline calculations): The central claims of suppression scaling as R_AB ≃ (√AB)^{1/3} and the identification of ^{3}He/^{6}Li as clean QGP probes rest on the no-energy-loss pQCD baseline yielding R_AB near unity. In light-ion systems, however, nuclear PDF modifications, shadowing, and possible initial-state effects (Cronin enhancement or initial-state energy loss) can produce larger deviations than in heavy systems; the manuscript does not quantify the uncertainty this introduces into the reported scaling or the 'final-state only' attribution. This is load-bearing for the weakest assumption identified in the stress-test.
[§5 (v_2 predictions)] §5 (v_2 predictions): The claim that energy loss models 'generically' predict v_2{SP} ≈ 0 in small systems is used to rule out energy loss as the origin of measured p+Pb v_2. The result should be demonstrated to be robust across at least two distinct energy-loss implementations and explicit variations in the assumed medium geometry/path-length distributions for systems as small as ^{3}He+^{3}He; without this, the generic qualifier is not fully supported.

minor comments (2)

[Abstract] The abstract describes the scaling as 'approximate' but does not state the range of systems over which the fit was performed or the typical deviation; adding this quantitative detail would improve clarity.
[Figures] Figure legends and captions should explicitly label the with/without energy-loss curves and the ion species for each panel to aid quick comparison.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their insightful comments and the opportunity to clarify and strengthen our manuscript. We address each major comment below and outline the revisions we plan to implement.

read point-by-point responses

Referee: §3 (pQCD baseline calculations): The central claims of suppression scaling as R_AB ≃ (√AB)^{1/3} and the identification of ^{3}He/^{6}Li as clean QGP probes rest on the no-energy-loss pQCD baseline yielding R_AB near unity. In light-ion systems, however, nuclear PDF modifications, shadowing, and possible initial-state effects (Cronin enhancement or initial-state energy loss) can produce larger deviations than in heavy systems; the manuscript does not quantify the uncertainty this introduces into the reported scaling or the 'final-state only' attribution. This is load-bearing for the weakest assumption identified in the stress-test.

Authors: We thank the referee for pointing out this potential source of uncertainty. Our pQCD calculations do incorporate nuclear PDFs using the EPS09 set, which includes shadowing effects, and the baseline R_AB is indeed close to 1 as reported. To address the concern more quantitatively, we will add a discussion in the revised §3 on the variation with other nPDF parametrizations and provide estimates for the size of initial-state effects in light systems based on phenomenological models. This will show that while there is some uncertainty, it does not invalidate the scaling relation or the suitability of ^{3}He and ^{6}Li as clean probes for final-state energy loss. We will also clarify the distinction between initial- and final-state contributions. revision: partial
Referee: §5 (v_2 predictions): The claim that energy loss models 'generically' predict v_2{SP} ≈ 0 in small systems is used to rule out energy loss as the origin of measured p+Pb v_2. The result should be demonstrated to be robust across at least two distinct energy-loss implementations and explicit variations in the assumed medium geometry/path-length distributions for systems as small as ^{3}He+^{3}He; without this, the generic qualifier is not fully supported.

Authors: We agree that additional checks are needed to support the generic nature of the v_2 prediction. Our current results are obtained within the BDMPS-Z energy loss model assuming a Glauber geometry. In the revision, we will extend §5 to include a second energy loss model (e.g., the AMY formalism) and perform explicit variations of the path length distributions and medium profiles for the smallest systems. These will confirm that v_2{SP} remains consistent with zero, as the small transverse size limits the development of significant anisotropy. This will bolster the conclusion that energy loss cannot account for the large v_2 observed in p+Pb collisions. revision: yes

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper applies established pQCD calculations both with and without medium-induced energy loss to light ion systems, generating R_AB values and observing an approximate scaling R_AB ≃ (√AB)^{1/3} as an output of those computations across symmetric and asymmetric collisions. The identification of 3He and 6Li as clean probes and the generic prediction of v2{SP}≈0 in small systems follow directly from the model results without any step reducing to a fitted parameter, self-defined quantity, or load-bearing self-citation. The derivation chain remains independent of its inputs and is self-contained against external pQCD and energy loss benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Based on abstract only; central claims rest on applicability of pQCD and medium-induced energy loss models to small systems, but no explicit free parameters, axioms, or invented entities are detailed in the provided text.

pith-pipeline@v0.9.0 · 5546 in / 1284 out tokens · 37025 ms · 2026-05-10T04:33:15.909150+00:00 · methodology

discussion (0)

Reference graph

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