Measurement of di-muons from 400 GeV/c protons interacting in a thick molybdenum/tungsten target
Pith reviewed 2026-05-13 17:37 UTC · model grok-4.3
The pith
Bμμ σ(J/ψ)/A = (1.18 ± 0.04 ± 0.10) nb/nucleon for 0.3 < y_cm < 0.6 in thick target, consistent with NA50 thin-target result within errors and limiting cascade contributions to <32%.
A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.
Core claim
For 0.3 < y_cm < 0.6, Bμμ σ(J/ψ)/A = (1.18 ± 0.04 ± 0.10) nb/nucleon; within systematic errors no significant enhancement due to secondary production of J/ψ inside the target is observed, with upper limit of possible contributions from cascade collisions of <32%.
Load-bearing premise
That the Pythia v8 Monte Carlo accurately models both primary production and the effects of the thick target geometry, and that differences in target thickness and acceptance between this measurement and NA50 do not introduce unaccounted biases in the comparison.
read the original abstract
Di-muon events emanating from a replica of the SHiP target at the CERN SPS (with a 400 GeV/c proton beam) contain a clear signal of \jpsi production. The target consisted of 13 interaction lengths of molybdenum and tungsten, followed by a 2.4m iron hadron absorber. The di-muon rate is in reasonable agreement with \MC based on Pythia v8. However, the $J/\psi$ rate observed in the data is much lower than predicted by the Monte Carlo simulation, which is advantageous for SHiP since muons with a large $p_T$ are more difficult to deflect by the active muon shield. For the center of mass rapidity ($y_{\rm cm}$) interval with the largest overlap with the NA50 measurement, $0.3 < y_{\rm cm} < 0.6$, we obtain the production cross-section per nucleon $\sigma(J/\psi)/A$ including the branching ratios of $J/\psi$ decays into muons, $B_{\mu^+\mu^-}$: $B_{\mu^+\mu^-}\sigma(J/\psi)/A=(1.18~\pm~0.04~\pm~0.10)~\mathrm{nb}$/nucleon. This is to be compared to the NA50 extrapolated result (using a much thinner target): $B_{\mu^+\mu^-}\sigma(J/\psi)/A=(0.99~\pm~0.04)~\mathrm{nb}$/nucleon. Within the systematic errors, no significant enhancement due to secondary production of \jpsi inside the target is observed. An upper limit at 90\% confidence level of possible contributions from secondary collisions of $<32\%$ is obtained.
Editorial analysis
A structured set of objections, weighed in public.
Referee Report
Summary. The manuscript reports a measurement of di-muon events from 400 GeV/c protons interacting in a thick molybdenum/tungsten target replica of the SHiP experiment at the CERN SPS. A clear J/ψ signal is observed, and the production cross-section per nucleon including branching ratio is extracted as Bμμσ(J/ψ)/A = (1.18 ± 0.04 ± 0.10) nb/nucleon for 0.3 < y_cm < 0.6. This value is compared to Pythia v8 Monte Carlo simulations and to the NA50 thin-target extrapolation, yielding no significant enhancement from secondary production and an upper limit of <32% on contributions from cascade collisions.
Significance. If the central result holds, the measurement supplies a direct experimental constraint on J/ψ production in thick targets relevant to the SHiP experiment. The separation of statistical and systematic uncertainties, together with the numerical comparison to both simulation and prior NA50 data, provides a useful benchmark for primary versus secondary production mechanisms in high-energy proton-nucleus collisions.
major comments (1)
- [Abstract] The <32% upper limit on cascade contributions is derived by comparing the measured Bμμσ(J/ψ)/A to the NA50 thin-target extrapolation after acceptance corrections performed with Pythia v8. No quantitative validation of the Monte Carlo (e.g., χ² per degree of freedom, pull distributions, or bin-by-bin agreement in the relevant y_cm and pT bins) is supplied, so it is unclear whether generator-specific uncertainties in nuclear PDFs or secondary production are fully covered by the quoted 0.10 nb systematic uncertainty.
minor comments (1)
- Define the SHiP acronym and the precise kinematic acceptance of the di-muon spectrometer at first use.
Simulated Author's Rebuttal
We thank the referee for the careful reading of our manuscript and the constructive comment. We address the major point below and will incorporate the requested quantitative validation in the revised version.
read point-by-point responses
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Referee: [Abstract] The <32% upper limit on cascade contributions is derived by comparing the measured Bμμσ(J/ψ)/A to the NA50 thin-target extrapolation after acceptance corrections performed with Pythia v8. No quantitative validation of the Monte Carlo (e.g., χ² per degree of freedom, pull distributions, or bin-by-bin agreement in the relevant y_cm and pT bins) is supplied, so it is unclear whether generator-specific uncertainties in nuclear PDFs or secondary production are fully covered by the quoted 0.10 nb systematic uncertainty.
Authors: We agree that additional quantitative validation of the Pythia v8 Monte Carlo would strengthen the presentation. In the revised manuscript we will add a dedicated subsection (or appendix) that includes: (i) a direct comparison of the measured di-muon invariant-mass spectrum and the extracted J/ψ y_cm and pT distributions with the Pythia v8 prediction after acceptance and efficiency corrections; (ii) the χ² per degree of freedom for the relevant rapidity and transverse-momentum bins; and (iii) an explicit statement of how the 0.10 nb systematic uncertainty was evaluated, including variations over nuclear PDF sets and different modeling options for secondary production inside the thick target. These additions will make clear that the quoted systematic already encompasses the dominant generator-specific effects and thereby supports the <32% upper limit on cascade contributions. revision: yes
Axiom & Free-Parameter Ledger
axioms (2)
- domain assumption Pythia v8 Monte Carlo generator correctly models J/ψ production and propagation in thick targets
- standard math Branching ratio Bμμ for J/ψ → μ+μ− is known from independent prior measurements
discussion (0)
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