REVIEW 3 major objections 4 minor 12 references
Charged-hadron suppression in Pb+Pb and Xe+Xe collisions measured with the ATLAS detector
T0 review · 3 major / 4 minor · reviewed 2026-08-14 · deepseek-v4-flash
Pith's one-line read The ATLAS measurement shows that charged-hadron suppression in Pb+Pb and Xe+Xe collisions is set by the geometric size of the collision system.
desk verdict A clean, modest ATLAS proceedings that shows a first system-size comparison of R_AA in Xe+Xe and Pb+Pb; the conclusion is qualitative and the main soft spot is the model-dependent pp energy extrapolation. 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 key object is the nuclear modification factor, defined as $R_\mathrm{AA} = \frac{1}{\langle T_\mathrm{AA}\rangle}\frac{1/N_\mathrm{evt}\,d^2N_\mathrm{ch}/d\eta dp_T}{d^2\sigma_{pp}/d\eta dp_T}$, where $\langle T_\mathrm{AA}\rangle$ is the nuclear thickness function from a Glauber model that converts the parton flux in a nucleus–nucleus collision to an effective number of binary nucleon–nucleon collisions. This ratio cancels the trivial scaling of particle production with system size and leaves the suppression caused by the quark-gluon plasma. The paper's argument works by comparing $R_\mathrm{AA}$ between two systems in bins chosen to equalise either centrality percentiles, forward-calorimeter energy, $\langle N_\mathrm{part}\rangle$, or $\langle N_\mathrm{coll}\rangle$, and observing which binning makes the two curves coincide.
What would settle it
Measure $R_\mathrm{AA}$ in a third collision system, such as oxygen–oxygen or ruthenium–zirconium, at a different centre-of-mass energy and bin it by the same forward-calorimeter energy intervals used here; if the resulting suppression curve does not overlap the Pb+Pb and Xe+Xe curves within uncertainties, the claimed scaling with system size would be disproved.
Extended reading notes
Core claim
The central discovery is that the nuclear modification factor $R_\mathrm{AA}$ for charged hadrons is consistent between Xe+Xe collisions at $\sqrt{s_{NN}}=5.44$ TeV and Pb+Pb collisions at $\sqrt{s_{NN}}=5.02$ TeV when the comparison is made for centrality intervals that correspond to approximately the same total transverse energy in the forward calorimeter, $E_T^\mathrm{FCal}$. Because $E_T^\mathrm{FCal}$ is a measure of the geometric overlap of the two nuclei, the authors interpret this agreement as evidence that $R_\mathrm{AA}$ scales with the system size. At fixed centrality percentiles, Pb+Pb shows stronger suppression, which is expected from its larger size. When matching instead by similar average number of participants $\langle N_\mathrm{part}\rangle$ or binary collisions $\langle N_\mathrm{coll}\rangle$, the agreement is worse but still compatible within uncertainties at high $p_T$ (26–30 GeV). The paper therefore concludes that the suppression is primarily a function of system size, with collision energy and initial energy density playing a minor role in this comparison.
Load-bearing premise
The load-bearing premise is that the pp reference cross-section at 5.44 TeV is obtained by scaling the measured 5.02 TeV spectrum with a Pythia 8 ratio; a systematic error in that extrapolation would shift the Xe+Xe $R_\mathrm{AA}$ and could break the apparent scaling with system size.
Editorial extensions
If this is right
- If $R_\mathrm{AA}$ scales with system size, then the suppression pattern observed in Pb+Pb collisions at 5.02 TeV should be reproducible in Xe+Xe collisions at 5.44 TeV at the same $E_T^\mathrm{FCal}$, which the measurement confirms within uncertainties.
- The characteristic shape of $R_\mathrm{AA}$ — a maximum near 2 GeV, a minimum near 7 GeV, and a rise up to 60 GeV — appears to be a property of the system size rather than of the collision energy.
- Comparing different collision systems at the LHC using a geometric variable rather than centrality percentiles is a more direct test of energy-loss models.
- The systematic uncertainty from the extrapolation of the pp reference from 5.02 TeV to 5.44 TeV is small enough not to spoil the observed scaling, but it matters for precise quantification.
Reading between the lines
- If the scaling with system size holds at the LHC, it suggests that parton energy loss in the quark-gluon plasma is governed primarily by the path length through the medium, a prediction that could be tested with non-spherical or deformed nuclei such as U+U.
- The same comparison could be extended to RHIC energies to see whether the scaling breaks down as the initial energy density drops; if it does, that would identify where energy density becomes a relevant control variable.
- A direct measurement of the pp cross-section at 5.44 TeV would remove the largest systematic in the Xe+Xe $R_\mathrm{AA}$ and would sharpen the scaling test.
- The authors' choice of $E_T^\mathrm{FCal}$ as a size proxy could be cross-checked by using the measured charged-particle multiplicity at midrapidity as the binning variable, which would test whether the scaling is robust to the geometric definition.
Signed reviews
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. This proceedings contribution reports the ATLAS measurement of charged-hadron nuclear modification factors R_AA in Xe+Xe collisions at sqrt(s_NN)=5.44 TeV and compares them with R_AA in Pb+Pb collisions at 5.02 TeV. The paper describes the reconstruction, corrections, and systematic uncertainties, then presents R_AA as a function of p_T for several centrality intervals, as well as comparisons in bins chosen to match approximately the same FCal transverse energy, the same average number of participating nucleons, and the same average number of binary collisions. The central physics claim is that the suppression is consistent between the two systems when they are compared at similar system size, suggesting that R_AA scales with the geometric size of the collision system. The pp reference cross-section for the Xe+Xe measurement is obtained by extrapolating the measured 5.02 TeV pp cross-section to 5.44 TeV using a Pythia 8 ratio, with a systematic uncertainty assigned as half the difference between the two energies.
Significance. If the claimed system-size scaling holds, the result is a valuable constraint on models of jet quenching and on the system-size dependence of parton energy loss in heavy-ion collisions. The comparison of Xe+Xe and Pb+Pb at the LHC is a novel and direct way to isolate geometric effects from other collision properties. The paper has the strength of relying on a direct ratio of measured spectra to an external pp reference; no parameters are fitted to the Xe+Xe/Pb+Pb comparison, and the Glauber and Pythia inputs are clearly identified as external model assumptions. The main limitations are the model dependence of the pp energy extrapolation and the absence of a quantitative significance statement for the scaling conclusion.
major comments (3)
- [Section 2 and Fig. 3] The Xe+Xe R_AA relies on a pp reference obtained by extrapolating the ATLAS 5.02 TeV pp measurement to 5.44 TeV using a Pythia 8 ratio, while the Pb+Pb R_AA uses the unsmoothed 5.02 TeV reference. The extrapolation correction rises from about 4% at p_T around 1 GeV to about 26% at the highest p_T and |eta|, and the p_T interval 26-30 GeV in Fig. 3 is one of the intervals used to support the scaling claim. The systematic uncertainty assigned as half the difference between the 5.02 and 5.44 TeV cross-sections is a reasonable bracketing prescription, but it does not validate the Pythia 8 energy dependence for this specific 8% step. I request a sensitivity test that shows how the Xe+Xe R_AA and the Xe+Xe/Pb+Pb comparison change when an alternative pp reference is used (for example, the 5.02 TeV pp data directly, or an NLO pQCD-based reference), together with a discussion of whether the scaling conclusion survives that change.
- [Section 3, Fig. 1 (right) and Fig. 3] The central claim that the suppressions are 'consistent between the two systems within the systematics uncertainties' is based on visual inspection; no quantitative compatibility test is reported. I ask for a numerical measure of agreement, such as a chi-square or p-value for the ratio R_AA(Xe+Xe)/R_AA(Pb+Pb) in the selected p_T and centrality bins, together with a statement of which systematic components are treated as correlated between the two systems. Without such a test, the strength of the scaling conclusion cannot be assessed.
- [Section 3, Fig. 2 and Fig. 3] The paper notes that the agreement between the two systems is worse when comparing at similar N_part than when comparing at similar FCal ET, and that at p_T near 7 GeV the central Xe+Xe events show slightly stronger suppression while peripheral events show slightly milder suppression. This pattern is relevant to the scaling claim, but the text does not quantify how large these differences are relative to the systematic uncertainties. A quantitative comparison in the low-p_T region, where the difference appears, is needed to determine whether the scaling statement should be restricted to high p_T or to matched FCal ET.
minor comments (4)
- [Section 2] There is a typo: 'an increase of the cross-section section' should read 'an increase of the cross-section'; and in the same section 'the later ones' should be 'the latter ones'.
- [Fig. 3 caption] The caption contains incomplete inequalities: 'Pb+Pb, 6.7< < 30 GeV' and 'Xe+Xe, 26 < < 30 GeV' should be written as '6.7 < p_T < 30 GeV' and '26 < p_T < 30 GeV'.
- [Section 3, Fig. 1 (right)] The FCal-ET matching is only approximate: for example, the Xe+Xe 20-30% interval (0.88-1.30 TeV) is compared with the Pb+Pb 30-40% interval (0.88-1.37 TeV). A sentence explaining the matching tolerance and its effect on the comparison would help the reader interpret the right panel of Fig. 1.
- [Section 3] The phrase 'They have a characteristic curvature' is vague; it would be clearer to state explicitly that both data sets show a maximum near p_T of about 2 GeV, a minimum near 7 GeV, and a rise toward higher p_T.
Circularity Check
No circularity: R_AA is a direct measured ratio, and the Pythia 8 pp energy extrapolation is an external model input, not fitted to the Xe+Xe/Pb+Pb comparison.
full rationale
The paper's derivation chain is a standard, self-contained nuclear modification factor measurement. R_AA is defined by Eq. (1) as the ratio of the measured per-event charged-hadron yield in heavy-ion collisions to the measured pp cross-section, normalized by the Glauber thickness function. No parameter is fitted to the Xe+Xe/Pb+Pb comparison, and no quantity in the target result is used to define an input. The pp reference for Xe+Xe is obtained by extrapolating the measured 5.02 TeV pp cross-section to 5.44 TeV using a Pythia 8 ratio; this is an external Monte Carlo model assumption with an explicitly assigned systematic uncertainty of half the difference between the 5.02 and 5.44 TeV cross-sections. Even though this extrapolation is a legitimate weak point that could shift the conclusion if biased, it is not circular: the Pythia 8 ratio is not derived from the Xe+Xe or Pb+Pb data, nor is it equivalent to the claimed system-size scaling. The Glauber model, track-correction simulations, and lepton-subtraction procedures are likewise external inputs with stated uncertainties. The central empirical observation—that R_AA values agree between the two systems when compared at similar FCal ET, <Npart>, or <Ncoll>—is a direct comparison of measured quantities, not a reconstruction of the inputs. Self-citations to prior ATLAS measurements (e.g., the 5.02 TeV pp cross-section) are used as data references, not as unverified load-bearing theorems. Therefore the paper exhibits no circularity by construction; concerns about the Pythia extrapolation belong to systematic uncertainty or model dependence, not circular reasoning.
Assumptions & free parameters
assumptions (4)
- domain assumption Glauber model provides Npart, Ncoll, and T_AA for centrality intervals.
- domain assumption Pythia 8 extrapolates the pp cross-section from 5.02 to 5.44 TeV.
- domain assumption Forward calorimeter transverse energy (FCal ET) is a proxy for system size.
- domain assumption Hijing and Pythia 8 simulations describe detector response and track reconstruction.
Cite this review
Pith. "Pith review of Charged-hadron suppression in Pb+Pb and Xe+Xe collisions measured with the ATLAS detector." pith.science (2026). https://pith.science/paper/STZDUNLS
@misc{pith2026190811636,
author = {Pith},
title = {Pith review of: Charged-hadron suppression in Pb+Pb and Xe+Xe collisions measured with the ATLAS detector},
year = {2026},
howpublished = {\url{https://pith.science/paper/STZDUNLS}},
note = {Machine review of arXiv:1908.11636}
}
abstract
The ATLAS detector at the LHC recorded 0.49nb$^{-1}$ of Pb+Pb collisions and 25pb$^{-1}$ of $pp$ collisions, both at the center-of-mass energy 5.02TeV per nucleon pair. Recently, ATLAS also recorded 3$\mu$b$^{-1}$ of Xe+Xe collisions at the center-of-mass energy 5.44TeV, which provides a new opportunity to study the system-size dependence of the charged-hadron production in heavy-ion collisions. The large acceptance of the ATLAS detector allows to measure the spectra of charged hadrons in a wide range of pseudorapidity and transverse momentum. The nuclear modification factors $R_\mathrm{AA}$ are constructed as a ratio of the spectra measured in Pb+Pb or Xe+Xe collisions to that measured in $pp$ collisions. The $R_\mathrm{AA}$ obtained in the two systems are presented for different centrality intervals and the results are discussed.
Reference graph
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Reviewed August 14, 2026 · model on record in the stance chip above.
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