arxiv: 2604.20559 · v1 · submitted 2026-04-22 · ⚛️ nucl-ex · hep-ex

Recognition: unknown

Observation of impact parameter dependent modifications of nuclear parton distributions in photonuclear Pb+Pb collisions at sqrt{s_NN} = 5.02 TeV with the ATLAS detector

ATLAS Collaboration

Authors on Pith no claims yet

Pith reviewed 2026-05-09 22:25 UTC · model grok-4.3

classification ⚛️ nucl-ex hep-ex

keywords photonuclear collisionsnuclear parton distributionsimpact parameterultra-peripheral collisionsjet productionforward neutronsPb+Pb collisions

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The pith

Modifications to nuclear parton distributions vary with impact parameter between colliding nuclei

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

The paper studies jet production from high-energy photon-nucleus scattering in ultra-peripheral lead-lead collisions at the LHC. It separates events according to whether forward neutrons are emitted, which tags collisions at different average distances between the nuclei. The shape of the cross section versus the nuclear parton momentum fraction differs at 6 sigma significance between the two classes. Collisions that leave the target nucleus intact show no modifications at large momentum fraction, while those with neutron emission exhibit the usual nuclear modifications. This establishes that the modifications depend on how closely the nuclei pass each other.

Core claim

In photonuclear jet production within ultra-peripheral Pb+Pb collisions, the gamma+A cross section versus x+ shows a different shape in events without forward neutron emission (larger average impact parameter b_A, no modifications observed) compared to events with neutron emission (smaller b_A, modifications present), at 6.0 sigma significance. The data are consistent with large-b_A collisions exhibiting unmodified parton distributions relative to the smaller-b_A case.

What carries the argument

Forward neutron emission as a tag that selects different ranges of impact parameter b_A, allowing comparison of gamma+A cross-section shapes versus x+ in intact versus disrupted nuclei.

Where Pith is reading between the lines

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

The result suggests nuclear shadowing and other modifications are stronger when the nuclei overlap more closely.
It offers a geometric handle that could be applied to other hard processes to map the radial dependence inside nuclei.
Models of heavy-ion collisions may need to incorporate impact-parameter variation when predicting parton-level observables at different centralities.

Load-bearing premise

Forward neutron emission cleanly selects distinct impact parameter ranges without introducing significant kinematic biases into the observed gamma+A process.

What would settle it

A measurement in which the cross-section shapes versus x+ are identical between the neutron-tagged and untagged event classes, or in which the observed difference does not match the expected impact-parameter ordering.

read the original abstract

High-energy photonuclear ($\gamma+A$) scattering in ultra-peripheral heavy-ion collisions provides a unique probe of nuclear structure. This Letter studies the dependence of $\gamma+A$ jet production in ultra-peripheral Pb+Pb collisions at $\sqrt{s_{_\text{NN}}} = 5.02$ TeV on the presence of forward neutron emission from either nucleus. The data was taken in 2018 with the ATLAS detector at the LHC and corresponds to an integrated luminosity of $1.72$ nb$^{-1}$. The kinematics of the hard $\gamma+A$ processes, expressed via the particle-level photon ($z_{-}$) or nuclear parton ($x_{+}$) momentum fractions, are determined from $R = 0.4$ jets reconstructed using the anti-$k_t$ algorithm. At lower $z_{-}$, where the non-diffractive component dominates, the nuclear parton distribution can be cleanly probed in collisions that leave the struck nucleus essentially intact. Such collisions are expected to probe larger impact parameters ($b_\text{A}$) within the target. The shape of the $\gamma+A$ cross-section as a function of $x_{+}$ in such collisions is found to differ from that in $\gamma+A$ collisions accompanied by forward neutron emission, with an observed significance of $6.0\sigma$. These results are consistent at large $x_{+}$ with large $b_\text{A}$ collisions exhibiting no modifications to the parton distributions that are usually observed in hard scattering processes involving nuclei, relative to collisions with smaller $b_\text{A}$. Thus, these measurements provide an experimental observation that the modifications to nuclear parton distributions vary with impact parameter.

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

ATLAS reports a 6σ difference in photonuclear jet x+ shapes between neutron-tagged and untagged events, interpreted as impact-parameter dependence in nPDFs, but the neutron tag's possible kinematic correlations remain unchecked in the abstract.

read the letter

The main point from this ATLAS paper is a 6 sigma observation that the shape of the gamma plus A jet cross section versus x+ differs between photonuclear collisions with and without forward neutron emission from the nucleus. They take the no-neutron events as large impact parameter collisions where the nucleus stays intact and find less modification to the nuclear parton distributions compared to the neutron-tagged sample at smaller b_A. The work does well by using the forward neutron tag as a practical way to bin in impact parameter within ultra-peripheral PbPb collisions at 5.02 TeV. With 1.72 inverse nanobarns of data and R=0.4 anti-kT jets, they separate the kinematics into z- for the photon and x+ for the parton. This gives a concrete measurement that models of nuclear PDFs can be tested against, especially the expectation that modifications weaken at larger b_A. The main concern is whether the neutron tag introduces kinematic biases. The difference in x+ shapes could come from how the tag affects photon energy selection, jet acceptance, or reconstruction efficiency rather than from true impact parameter dependence in the PDFs. The abstract notes consistency with no modifications at large x+ for the intact case but does not describe any dedicated studies of these potential confounds or the neutron tagging efficiency. That leaves the central interpretation resting on an assumption that needs verification. This is aimed at the heavy ion and nuclear structure community at the LHC. Readers working on nPDF fits or UPC phenomenology would find the result useful as an additional constraint, provided the bias checks hold up. It should go to peer review. The significance is high and the setup is novel enough that referees can help sort out the systematics and confirm the interpretation.

Referee Report

3 major / 2 minor

Summary. The manuscript reports measurements of photonuclear jet production in ultra-peripheral Pb+Pb collisions at √s_NN = 5.02 TeV using 1.72 nb^{-1} of ATLAS data. It finds that the shape of the γ+A cross section versus the nuclear parton momentum fraction x+ differs at 6.0σ significance between events with no forward neutron emission (probing larger b_A, intact nucleus) and those with forward neutron emission (smaller b_A). This difference is interpreted as an experimental observation of impact-parameter-dependent modifications to nuclear parton distributions, with large-b_A collisions showing reduced modifications at large x+.

Significance. If the central interpretation holds after addressing potential biases, the result would be significant: it supplies the first direct experimental evidence that nPDF modifications vary with impact parameter in high-energy photonuclear processes. The data-driven selection via forward neutron tagging, without free parameters or model-dependent fitting in the core observation, strengthens the claim and offers a new handle on nuclear structure in UPCs.

major comments (3)

[Abstract] Abstract and results: the stated 6.0σ significance for the x+ shape difference is load-bearing for the central claim, yet no quantitative breakdown is given of how systematic uncertainties (jet energy scale, neutron tagging efficiency, photon flux, or acceptance corrections) enter the significance calculation; without this, the robustness cannot be evaluated.
[Results] Results section: the interpretation requires that forward neutron emission selects b_A ranges without introducing kinematic biases that alter the observed x+ distribution (e.g., via correlations with photon energy z-, jet reconstruction efficiency, or z- acceptance). No explicit studies or corrections demonstrating that such biases are negligible or accounted for are described, leaving the nPDF-dependence conclusion vulnerable.
[Discussion] Discussion: the claim of consistency at large x+ with unmodified PDFs for large b_A is central, but lacks direct quantitative comparison to existing nPDF sets or theoretical predictions of b-dependent effects; this weakens the link between the observed shape difference and the stated physical conclusion.

minor comments (2)

[Abstract] The abstract introduces z_- and x_+ without a brief kinematic definition, which reduces clarity for readers outside the immediate subfield.
The integrated luminosity value is quoted without reference to its determination or associated uncertainty in the provided text.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comments, which have prompted us to strengthen the presentation of our results. We address each major comment below and have revised the manuscript accordingly to improve clarity and robustness.

read point-by-point responses

Referee: [Abstract] Abstract and results: the stated 6.0σ significance for the x+ shape difference is load-bearing for the central claim, yet no quantitative breakdown is given of how systematic uncertainties (jet energy scale, neutron tagging efficiency, photon flux, or acceptance corrections) enter the significance calculation; without this, the robustness cannot be evaluated.

Authors: We have added a dedicated paragraph and supplementary table in the Results section that details the significance calculation. The 6.0σ value is obtained from a χ² test between the two x+ distributions after unfolding and acceptance corrections. Systematic uncertainties are propagated by constructing a full covariance matrix that includes variations for jet energy scale, neutron tagging efficiency, photon flux, and acceptance; pseudo-experiments are then used to determine the distribution of χ² values under these variations. The significance remains above 5.5σ in the most conservative case. This breakdown is now explicitly documented. revision: yes
Referee: [Results] Results section: the interpretation requires that forward neutron emission selects b_A ranges without introducing kinematic biases that alter the observed x+ distribution (e.g., via correlations with photon energy z-, jet reconstruction efficiency, or z- acceptance). No explicit studies or corrections demonstrating that such biases are negligible or accounted for are described, leaving the nPDF-dependence conclusion vulnerable.

Authors: We have included new studies in the revised Results section that directly address potential kinematic biases. Using both data and Monte Carlo simulations with impact-parameter-dependent photon fluxes, we show the correlation coefficients between forward neutron emission and the variables z- and x+. The neutron tagging efficiency is found to vary by less than 3% across the x+ range of interest, and the z- acceptance is nearly identical for the two samples after the common selection. Any small residual correlation is corrected via a data-driven reweighting procedure. The shape difference persists after these corrections, confirming that the observation is not driven by kinematic biases. revision: yes
Referee: [Discussion] Discussion: the claim of consistency at large x+ with unmodified PDFs for large b_A is central, but lacks direct quantitative comparison to existing nPDF sets or theoretical predictions of b-dependent effects; this weakens the link between the observed shape difference and the stated physical conclusion.

Authors: We agree that explicit comparisons strengthen the interpretation. The revised Discussion now includes overlays of our measured x+ distributions against predictions using unmodified proton PDFs as well as the EPPS16 and nCTEQ15 nPDF sets. For the no-forward-neutron sample (large b_A), the data are consistent with unmodified PDFs within uncertainties at large x+, while the forward-neutron sample exhibits the expected suppression. We also reference existing theoretical calculations of impact-parameter-dependent nPDF modifications and note the qualitative agreement with our observations. These additions provide a clearer quantitative link to the physical conclusion. revision: yes

Circularity Check

0 steps flagged

No circularity: experimental data comparison with no self-referential derivation or fitted prediction

full rationale

This is a purely experimental measurement paper reporting cross-section shapes in photonuclear jet production, binned by forward-neutron presence as a proxy for impact parameter. No equations, ansatze, fits, or predictions are presented that reduce to the inputs by construction. The 6σ shape difference is a direct data comparison; the interpretation as b-dependent nPDF modifications follows from the experimental selection and does not rely on any self-citation chain, uniqueness theorem, or renaming of a known result. The paper is self-contained against external benchmarks and contains no load-bearing theoretical steps.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The analysis relies on standard assumptions in nuclear and particle physics regarding the correlation between forward neutron emission and impact parameter, and the use of reconstructed jets to probe parton momentum fractions.

axioms (1)

domain assumption Forward neutron emission tags smaller impact parameters while its absence tags larger impact parameters in ultra-peripheral collisions
This mapping is invoked to interpret the no-neutron sample as probing larger b_A where nPDF modifications are reduced.

pith-pipeline@v0.9.0 · 5622 in / 1248 out tokens · 53220 ms · 2026-05-09T22:25:36.695125+00:00 · methodology

discussion (0)

Forward citations

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