Characterization of nuclear breakup as a function of hard-scattering kinematics using dijets measured by ATLAS in p+Pb collisions
Pith reviewed 2026-05-22 21:37 UTC · model grok-4.3
The pith
Dijets show that nuclear geometry estimators in p+Pb collisions depend on proton Bjorken-x
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
In p+Pb collisions at a nucleon-nucleon center-of-mass energy of 8.16 TeV, dijets serve as a proxy for parton-parton scattering and are correlated with calorimeter-based event geometry estimators. Both the energy deposited in the Zero-Degree Calorimeter and the transverse energy in the Forward Calorimeter depend on xp, with the former about six times less sensitive to xp than the latter.
What carries the argument
Dijets within -2.8 < η < 4.5 used as proxy for parton-parton scattering configuration to correlate with ZDC and FCal energies
If this is right
- Centrality selection in p+Pb collisions must account for xp dependence in geometry estimators.
- The reduced xp sensitivity of ZDC energy relative to FCal transverse energy offers a more stable estimator for nuclear breakup.
- Interpretations of hard-process observables in nuclear collisions require correction for initial-state kinematic variations.
Where Pith is reading between the lines
- The observed sensitivities could influence models that combine hard scattering with soft nuclear geometry.
- ZDC-based centrality might reduce kinematic bias in studies of jet modification compared with FCal-based selection.
Load-bearing premise
Dijets accurately proxy the parton-parton scattering configuration without large nuclear modification biases in the selected kinematics.
What would settle it
A dataset in the same or extended kinematic range showing no dependence of either estimator on xp would falsify the reported sensitivity.
read the original abstract
This Letter analyzes the sensitivity of event geometry estimators to the initial-state kinematics of hard scattering in proton-lead collisions. This analysis uses dijets as a proxy for the parton-parton scattering configuration, correlating it with event geometry estimators, namely the energy deposited in the Zero-Degree Calorimeter and the transverse energy recorded in the Forward Calorimeter in the Pb-going direction. The analysis uses data recorded by the ATLAS detector at the Large Hadron Collider with a nucleon-nucleon center-of-mass energy of 8.16 TeV, corresponding to an integrated luminosity of 56 nb$^{-1}$. The jets are measured within the pseudorapidity interval $-$2.8 $<$ $\eta$ $<$ 4.5, where positive $\eta$ values correspond to the direction of the proton beam. Results are presented as a function of the Bjorken-$x$ of the parton originating from the proton, $x_{p}$. Both event geometry estimators are found to be dependent on $x_{p}$, with the energy deposited in the Zero-Degree Calorimeter about six times less sensitive to $x_{p}$ compared with the transverse energy deposited in the Forward Calorimeter.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The paper analyzes the sensitivity of two event geometry estimators—the energy deposited in the Zero-Degree Calorimeter (ZDC) and the transverse energy in the Forward Calorimeter (FCal) in the Pb-going direction—to the Bjorken-x of the proton parton (xp) in p+Pb collisions. Using 56 nb^{-1} of 8.16 TeV ATLAS data, dijets measured in -2.8 < η < 4.5 are employed as a proxy for the parton-parton scattering configuration. The central result is that both estimators depend on xp, with the ZDC energy approximately six times less sensitive to xp than the FCal ET.
Significance. If the dijet proxy assumption holds without significant xp-dependent biases, the result provides a quantitative measure of how hard-scattering kinematics correlate with soft nuclear breakup observables. The factor-of-six differential sensitivity between ZDC and FCal is a clear, falsifiable experimental finding that could inform centrality estimators in pA and AA collisions. The use of real collision data with stated luminosity and kinematics is a strength.
major comments (1)
- [Abstract, paragraph describing the proxy choice] Abstract, paragraph describing the proxy choice: the headline claim that ZDC is ~6× less sensitive to xp than FCal treats the selected dijets in -2.8 < η < 4.5 as an unbiased proxy for the parton-parton scattering configuration. No quantitative bound is provided on possible xp-dependent nuclear modification biases in jet production, fragmentation, or acceptance that could differ between forward and central regions or between the two calorimeters; such biases would induce an apparent xp dependence that is not purely geometric.
Simulated Author's Rebuttal
We thank the referee for their detailed review and for identifying a key assumption in our analysis. We respond to the single major comment below. A partial revision will be made to clarify the proxy assumptions and note potential biases.
read point-by-point responses
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Referee: [Abstract, paragraph describing the proxy choice] Abstract, paragraph describing the proxy choice: the headline claim that ZDC is ~6× less sensitive to xp than FCal treats the selected dijets in -2.8 < η < 4.5 as an unbiased proxy for the parton-parton scattering configuration. No quantitative bound is provided on possible xp-dependent nuclear modification biases in jet production, fragmentation, or acceptance that could differ between forward and central regions or between the two calorimeters; such biases would induce an apparent xp dependence that is not purely geometric.
Authors: We agree that the headline result relies on the dijet selection serving as a proxy for the underlying parton-parton kinematics without large xp-dependent biases from nuclear effects. The analysis reconstructs xp from the dijet four-momenta under a leading-order 2→2 assumption and uses a broad pseudorapidity interval (-2.8 < η < 4.5) to sample a range of scattering configurations. While the manuscript does not supply a quantitative bound on possible nuclear modification of jet production or fragmentation (which would require dedicated modeling beyond the scope of the Letter), the observed factor-of-six difference is presented as an experimental correlation within this proxy. To address the concern, we will revise the abstract to qualify the proxy language and add a short paragraph in the discussion section outlining the main sources of potential bias and why they are not expected to dominate the reported sensitivity difference. This is a partial revision. revision: partial
Circularity Check
Direct experimental measurement; no derivation reduces to inputs by construction
full rationale
The paper reports measured correlations between dijet kinematics (used as xp proxy) and calorimeter-based geometry estimators in p+Pb data. No equations, fits, or self-citations are presented that define the reported xp sensitivities in terms of themselves or reduce the factor-of-six ratio to a fitted parameter. The proxy choice is an analysis assumption, not a self-definitional step. The result is data-driven and self-contained against external benchmarks.
Axiom & Free-Parameter Ledger
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