REVIEW 2 major objections 4 minor 60 references
Predictions for $R_{AA}$ in 5.36 TeV C+C, O+O, and Ne+Ne collisions at the LHC
T0 review · 2 major / 4 minor · reviewed 2026-08-04 · deepseek-v4-flash
Pith's one-line read For C+C and O+O at 5.36 TeV, R_AA differs by ~0.1–0.15 depending on whether pp collisions make a mini-QGP.
desk verdict A clear, falsifiable set of predictions for R_AA in light-ion collisions that deserves referee time, but the headline difference rests on an unobservable R_pp computed inside the same model. read the letter →
The pith
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
The reading
What carries the argument
The central object is the identity R_AA = R_st^AA / R_pp, where R_st^AA is the nuclear modification factor computed with ordinary pQCD in the denominator and R_pp is the medium modification factor for jets in pp collisions. The energy-loss side is the LCPI approach to induced gluon emission, extended with a temperature-dependent running coupling α_s(Q,T) with a single fitted parameter κ. For small systems, the N=1 rescattering term dominates the induced gluon spectrum and is a linear functional of the medium density profile, which is what suppresses event-by-event fluctuations in the model.
What would settle it
Measure minimum-bias charged-hadron R_AA in 5.36 TeV C+C and O+O collisions at pT ~ 10–20 GeV: if the data match the no-mini-QGP prediction within ~0.02 while the mini-QGP prediction lies 0.1–0.15 above, the central claim fails; conversely, if the data land in the upper band, the claim is supported.
Extended reading notes
Core claim
The paper's central claim is that the identity R_AA = R_st^AA / R_pp, where R_pp is the medium modification factor for jets in pp collisions, makes light-ion collisions a clean discriminator of mini-QGP formation in pp. Because the energy-loss calculation is refitted to heavy-ion data so that both scenarios give nearly the same R_AA for large nuclei, the difference for C, O, and Ne comes almost entirely from R_pp being less than unity. The magnitude is driven by the paper's Eq. (12): ΔR_AA ≈ R_st^AA(w/o mQGP)(1 − R_pp), giving 0.1–0.15 at pT ~ 10–20 GeV for minimum-bias C+C and O+O. The predictions are parameter-free after fitting a single coupling parameter κ to 5.02 TeV Pb+Pb data, and the
Load-bearing premise
The induced gluon spectrum for small systems is dominated by the single-rescattering term, which is a linear functional of the medium density; if higher-order rescatterings or nonlinear density fluctuations matter, the extrapolation from heavy-ion fits would carry much larger uncertainties than the paper assumes.
Editorial extensions
If this is right
- Minimum-bias C+C and O+O collisions at 5.36 TeV should show ΔR_AA of about 0.1–0.15 at pT ~ 10–20 GeV, with the mini-QGP scenario giving the larger R_AA.
- The scenario gap grows as atomic number decreases, so C+C should show the largest difference and Ne+Ne a smaller but nonzero one.
- Measuring the 0–100% centrality bin avoids the multiplicity–impact parameter decorrelation problem, because the nuclear overlap factor for the full range equals A²/σ_in and is insensitive to decorrelation.
- Nuclear-PDF uncertainties are subdominant: for minimum-bias C+C, the no-quenching nuclear-PDF factor differs from unity by at most 20–30% of the jet-quenching deviation at pT ≤ 30 GeV.
- Model uncertainties (thermalization time, fireball radius, soft coupling form) change the predicted R_AA much less than the 0.1–0.15 scenario difference.
Reading between the lines
- If the predicted gap is observed, R_pp can be extracted point-by-point as R_st^AA/R_AA from light-ion data, turning the measurement into a direct readout of pp energy loss.
- The mechanism suggests even lighter systems, such as He+He or p+Pb at comparable energies, could amplify the sensitivity; the paper does not compute these cases.
- The predictions could be tested with already-collected LHC oxygen-run data, at least for the 0–100% centrality bin, before dedicated C+C runs become available.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper uses the author's LCPI-based jet-quenching framework, previously fitted to Pb+Pb R_AA data, to predict the nuclear modification factor R_AA for 5.36 TeV C+C, O+O, and Ne+Ne collisions. It compares two scenarios: one with and one without mini-QGP formation in pp collisions. The central result is that the with-mini-QGP scenario gives larger R_AA, with a difference of about 0.1-0.15 at pT ~ 10-20 GeV for minimum-bias C+C and O+O, growing as the atomic number decreases. This difference is traced to the model-computed pp medium modification factor R_pp, which divides the standard R_AA in the with-mini-QGP scenario. Parameter scans over thermalization time, fireball geometry, and the soft-Q behavior of alpha_s show that the predictions are stable for the four tested parameter sets.
Significance. If the predictions are correct, they offer a concrete, falsifiable way to address the question of whether jet quenching occurs in pp collisions, using light-ion data that are already being collected at the LHC. The paper's strengths are the use of a mature jet-quenching formalism with resummation of rescatterings and finite-size effects, explicit treatment of Coulomb effects, and a systematic scan over model parameters. The paper also provides the no-quenching nuclear-PDF baseline R_pdf_AA, which helps assess the dominant background. However, the headline signal is strongly tied to the unobservable model-computed R_pp, and the treatment of event-by-event density fluctuations is not quantitatively justified. These issues need to be addressed before the predictions can be considered robust.
major comments (2)
- [Sec. II.A (fluctuation remark) and Eqs. (3), (8)] The assertion that event-by-event QGP density fluctuations are negligible because the N=1 induced-gluon spectrum is a linear functional of the density profile does not control the observable. The medium-modified fragmentation function in Eq. (8) depends on the exponentiated one-gluon spectrum; R_pp and R_AA are averages of exp(-x) over fluctuating densities, not exp(-<x>). With R_pp ~ 0.78 at pT ~ 10 GeV the relevant exponent is ~0.25, while the UE multiplicity density ~12.85 is used only as a mean and pp multiplicity fluctuations are known to be large. A 30-50% relative fluctuation of the line-integrated density would shift R_pp by several percent, comparable to the predicted Delta R_AA ~ 0.1-0.15. A quantitative estimate, e.g., using a fluctuation distribution matched to measured dN_ch/deta fluctuations, is needed to support the neglect of event-by-event fluctuations.
- [Sec. III, Eqs. (2), (3), (12)] The headline difference is essentially driven by the model-computed, unobservable R_pp rather than by a new light-nucleus effect. The paper correctly states R_pp is unobservable, but the numerical size of the prediction is fixed by R_pp ~ 0.78 at 10 GeV through Eq. (12). Since kappa is separately fitted to Pb+Pb data in each scenario, the two scenarios are both tuned to describe heavy-ion data, and R_pp itself is not constrained by that fit. The phrase 'without free parameters' should be softened to 'without additional free parameters'. The sensitivity of R_pp to the pp fireball parameters (R_f, entropy density, profile) is asserted to be small via a compensation argument, but no numerical evidence is shown. A quantitative propagation of these uncertainties should be included.
minor comments (4)
- [Table I] The caption says 'w/ mQCD and w/o mQCD scenarios'; this should be 'mQGP' rather than 'mQCD'.
- [Figs. 1-3] The horizontal axes in the figures appear to start at pT = 20 GeV, while the abstract and text emphasize the difference at pT ~ 10-20 GeV. If the plotted range indeed starts at 20 GeV, the largest signal region is not displayed; the figures should be extended to 10 GeV or the claims adjusted to the plotted range.
- [Eq. (12)] The approximation Delta R_AA ~ R_AA^st(1 - R_pp) omits the 1/R_pp factor that follows from Eq. (3); with R_pp = 0.78 the exact factor is (1/R_pp - 1) = 0.282 versus 0.22. The '~' hides this, but it would be clearer to define the approximation precisely.
- [Sec. II.B] The description of the pp geometry in Eq. (6) is brief: the MIT bag model distribution for hard partons and the averaging procedure for central pp collisions are mentioned but not specified. A short definition or reference to Eq. numbers in [16] would help reproducibility.
Circularity Check
No circular reduction; light-ion R_AA are parameter-free extrapolations from a heavy-ion fit.
full rationale
The central light-ion predictions are genuine extrapolations, not refitted outputs. The single free parameter κ is fitted to external 5.02 TeV Pb+Pb R_AA data, and the same κ is then used without further adjustment to compute R_AA for C+C, O+O, and Ne+Ne in both scenarios. R_pp is a computed quantity of the model, not a parameter fitted to the predicted light-ion data, and ΔR_AA follows from Eqs. (3), (10)–(12) as an extrapolation of the fitted heavy-ion behavior. The heavy use of self-citations defines the LCPI jet-quenching framework, but the framework is anchored by external heavy-ion data and lattice-motivated α_s; no self-citation is invoked as a uniqueness theorem or as a substitute for the calculation. The paper's explicit admission that 'Rpp is an unobservable quantity' (after Eq. (2)) is a model-dependence limitation, not a circular step. Similarly, the event-by-event fluctuation argument may be too weak because R_pp is an exponential functional of the medium, but that is a correctness/robustness concern, not an identity or fitted-vs-predicted circularity. The predictions are falsifiable against future light-ion data.
Assumptions & free parameters
free parameters (6)
- kappa (w/o mQGP) =
3.2 (set A)
- kappa (w/ mQGP) =
2.35 (set A)
- tau_0 (thermalization time) =
0.5 fm (0.8 fm variant)
- k (overlap radius factor) =
2 (3 variant)
- c (low-Q shape of alpha_s) =
0.8 (0 variant)
- R_f (effective pp fireball radius) =
1.493 fm
assumptions (6)
- domain assumption LCPI approach to induced gluon emission is valid, with N=1 rescattering dominating for small systems.
- domain assumption Bjorken 1+1D expansion with s(tau) proportional to 1/tau.
- domain assumption Optical Glauber wounded-nucleon plus binary-collision model predicts dN_ch/deta for light nuclei.
- domain assumption Hard parton production in pp occurs in central head-on collisions with the MIT bag model spatial distribution.
- domain assumption Transverse flow corrections to R_AA are small.
- domain assumption EPS09 nuclear PDFs describe nuclear modifications for light ions.
Cite this review
Pith. "Pith review of Predictions for $R_{AA}$ in 5.36 TeV C+C, O+O, and Ne+Ne collisions at the LHC." pith.science (2026). https://pith.science/paper/DLFTUFSD
@misc{pith2026250907741,
author = {Pith},
title = {Pith review of: Predictions for $R_AA$ in 5.36 TeV C+C, O+O, and Ne+Ne collisions at the LHC},
year = {2026},
howpublished = {\url{https://pith.science/paper/DLFTUFSD}},
note = {Machine review of arXiv:2509.07741}
}
abstract
Experiments on collisions of light nuclei at $\sqrt{s}=5.36$ TeV have recently begun at the LHC. In this regard we make predictions for nuclear modification factor $R_{AA}$ in 5.36 TeV C+C, O+O, and Ne+Ne collisions for scenarios with and without quark-gluon plasma formation in $pp$ collisions. We find a sizeable difference in $R_{AA}$ for these two scenarios, which grows with decreasing atomic number. This says that data on $R_{AA}$ for light nuclei could potentially give information on the presence of jet quenching in $pp$ collisions.
Figures
Reference graph
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Reviewed August 4, 2026 · model on record in the stance chip above.
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