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arxiv: 2406.08026 · v3 · submitted 2024-06-12 · ✦ hep-ph

Photoproduction in general-purpose event generators

Ilkka Helenius , Peter Meinzinger , Simon Pl\"atzer , Peter Richardson This is my paper

Pith reviewed 2026-05-23 23:51 UTC · model grok-4.3

classification ✦ hep-ph
keywords photoproductionMonte Carlo event generatorsjet productionHERWIGPYTHIASHERPAEICnon-perturbative effects
0
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The pith

Three Monte Carlo generators describe jet photoproduction data from LEP and HERA within uncertainties, with PYTHIA and SHERPA performing particularly well.

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

The paper compares the HERWIG, PYTHIA, and SHERPA generators for jet photoproduction in electron-positron and electron-proton collisions. It isolates how each models beam remnants, parton showers, multiparton interactions, and hadronisation. Comparisons to LEP and HERA data show that all three reproduce the measurements adequately, though the leading-order PYTHIA and next-to-leading-order SHERPA achieve the closest agreement. The study then provides EIC predictions and identifies the updates required for reliable use at that collider.

Core claim

All generators provide a decent description of the data within the uncertainties, with particularly good descriptions by the LO-accurate PYTHIA and the NLO-accurate SHERPA. Predictions for upcoming EIC jet observables and event shapes are given, leading to the conclusion that a modern global refit of the photon parton distributions together with dedicated experimental measurements ported to the RIVET framework are the key prerequisites for precision photoproduction phenomenology at the EIC.

What carries the argument

Systematic breakdown of contributions from beam remnants, parton showers, multiparton interactions, and hadronisation across the three generators, validated against LEP and HERA data.

If this is right

  • A global refit of photon parton distributions is required before precision EIC predictions.
  • Dedicated photoproduction measurements must be added to the RIVET framework to constrain non-perturbative parameters.
  • PYTHIA and SHERPA can serve as the most reliable starting points for initial EIC studies among the three generators.
  • Event-shape observables at the EIC will help further constrain hadronisation modelling beyond jet rates.

Where Pith is reading between the lines

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

  • Without the recommended refit, photon-structure uncertainties will dominate EIC photoproduction predictions.
  • The same disentangling method could be applied to other photon-induced processes to isolate which components drive discrepancies.
  • If EIC data reveal new discrepancies, they may point to energy-scale dependence in multiparton interactions not captured by current models.

Load-bearing premise

Differences between the generators can be cleanly attributed to individual non-perturbative components and that LEP and HERA data are representative enough for EIC kinematics without further tuning.

What would settle it

EIC measurements of jet observables or event shapes that deviate from all three generators even after a global refit of the photon parton distributions and implementation of the new data in RIVET.

Figures

Figures reproduced from arXiv: 2406.08026 by Ilkka Helenius, Peter Meinzinger, Peter Richardson, Simon Pl\"atzer.

Figure 1
Figure 1. Figure 1: Comparison of fixed-order parton level events between PYTHIA and SHERPA for a LEP-like setup. Pythia Sherpa 0 20 40 60 80 100 10 2 10 3 10 4 Energy of all particles E [GeV] d σ/d E [GeV −1 ] Pythia Sherpa -4 -3 -2 -1 0 1 2 3 4 0 5000 1.0 ·104 1.5 ·104 2.0 ·104 2.5 ·104 Pseudorapidity of all particles η d σ/d η Pythia Sherpa 5 10 15 20 25 30 1 10 1 10 2 10 3 10 4 Transverse momentum of all particles p⊥ [GeV… view at source ↗
Figure 2
Figure 2. Figure 2: Comparison of fixed-order parton level events including ISR parton shower and beam remnants between PYTHIA and SHERPA for a EIC-like setup. the same settings as before, however, now also modelling the Initial State Radiation (ISR) and the beam remnants, including charm and bottom quarks in the processes and using the NNPDF23 lo as 0130 qed PDF for the proton. The total cross-section in PYTHIA is larger tha… view at source ↗
Figure 3
Figure 3. Figure 3: Comparison of the CJKLLO and SAS2M PDF sets for light quarks and the gluon at µ 2 F = 5 GeV2 . 6 [PITH_FULL_IMAGE:figures/full_fig_p006_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: Distribution of average transverse energy of di-jets, E¯ T , from OPAL [60] for all x ± γ (left), x + γ or x − γ < 0.75 (middle) and x ± γ < 0.75 (right), compared to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO-accurate simulations by SHERPA. 3 Comparisons to LEP For the comparison to LEP data, we used a dijet measurement from the OPAL collaboration [60]. At lepton colliders, the cros… view at source ↗
Figure 5
Figure 5. Figure 5: Distribution of pseudo-rapidity η of di-jets from OPAL [60] for x + γ or x − γ < 0.75 (left) and x ± γ < 0.75 (right), compared to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO￾accurate simulations by SHERPA. b b b b b b b b b OPAL HERWIG-LO PYTHIA-LO SHERPA-LO SHERPA-MC@NLO 5 < E¯T < 7 GeV 10 1 10 d 2 σ/d xγ [pb] b b b b b b b b 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 0.5 0.6 0.7 0.8 0… view at source ↗
Figure 6
Figure 6. Figure 6: Distribution of xγ of di-jets from OPAL [60] for bins of average jet transverse energy E¯ T ∈ [5 GeV, 7 GeV] (left) and E¯ T ∈ [11 GeV, 25 GeV] (right), compared to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO-accurate simulations by SHERPA. 8 [PITH_FULL_IMAGE:figures/full_fig_p008_6.png] view at source ↗
Figure 7
Figure 7. Figure 7: Distributions of leading jet transverse energy E jet1 T for xγ > 0.75 (left) and xγ < 0.75 (right) with jet pseudo-rapdities for the two leading jets in 1 < ηjet1 < 2.4 and 0 < ηjet2 < 1 from ZEUS [61], compared to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO-accurate simulations by SHERPA. 4 Comparisons to HERA For this comparison we used data taken by the ZEUS collaboration [61] duri… view at source ↗
Figure 8
Figure 8. Figure 8: Distributions of sub-leading jet pseudo-rapidity η jet2 for xγ > 0.75 (left) and xγ < 0.75 (right) with leading jet pseudo-rapdity in 0 < ηjet1 < 1 from ZEUS [61], compared to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO-accurate simulations by SHERPA. b b b b b b b b b ZEUS HERWIG-LO PYTHIA-LO SHERPA-LO SHERPA-MC@NLO 14 < E jet1 T < 17 GeV 0 500 1000 1500 2000 d σ/d xobs γ [pb] b b b … view at source ↗
Figure 9
Figure 9. Figure 9: Distributions of xγ for leading jet transverse energies in 14 < Ejet1 T < 17 GeV (left) and 25 < E jet1 T < 90 GeV (right) from ZEUS [61], compared to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO-accurate simulations by SHERPA. 10 [PITH_FULL_IMAGE:figures/full_fig_p010_9.png] view at source ↗
Figure 10
Figure 10. Figure 10: Predictions of transverse jet energy ET (left) and xγ (right) for jet production at the EIC, com￾paring to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO-accurate simulations by SHERPA. which can partially be explained by the different PDF sets, αS value and the other differences as pointed out in Subsec. 2.4. This means that, going towards highest possible precision, the perturbative a… view at source ↗
Figure 11
Figure 11. Figure 11: Predictions of transverse thrust T⊥ (left) and transverse sphericity S⊥ (right), for jet produc￾tion at the EIC, comparing to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO-accurate simulations by SHERPA. HERWIG-LO SHERPA-MC@NLO PYTHIA-LO SHERPA-LO 0 5 10 15 20 25 30 35 40 45 10−1 1 10 1 10 2 10 3 10 4 Ncharged d σ/d Ncharged [pb] [PITH_FULL_IMAGE:figures/full_fig_p012_11.png] view at source ↗
Figure 12
Figure 12. Figure 12: Predictions of charged particle multiplicity Ncharged for events with a jet with ET < 6 GeV at the EIC, comparing to Leading Order simulations by HERWIG, PYTHIA and SHERPA and MC@NLO￾accurate simulations by SHERPA. 12 [PITH_FULL_IMAGE:figures/full_fig_p012_12.png] view at source ↗
read the original abstract

We compare the three general-purpose Monte Carlo event generators, HERWIG, PYTHIA, and SHEPRA for jet photoproduction processes in $e^+e^-$ and $ep$ collisions. Due to the lower energy scales probed, photoproduction is particularly sensitive to non-perturbative corrections. In a systematic analysis we disentangle and quantify the differences between the generators in these processes, i.e. contributions from beam remnants, parton showers, multiparton interactions (MPIs), and hadronisation modelling. We outline the default inputs and implementation differences and compare the computations with experimental data from LEP and HERA. We find that all generators provide a decent description of the data within the uncertainties, with particularly good descriptions by the LO-accurate PYTHIA and the NLO-accurate SHERPA. Finally, we also present predictions for the upcoming EIC for jet observables and event shapes and conclude that a modern global refit of the photon parton distributions and dedicated experimental measurements ported to the RIVET framework to constrain non-perturbative parameters are the key prerequisites for precision photoproduction phenomenology at the EIC.

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.

Referee Report

0 major / 3 minor

Summary. The paper compares the three general-purpose Monte Carlo generators HERWIG, PYTHIA and SHERPA for jet photoproduction in e+e− and ep collisions. It systematically disentangles contributions from beam remnants, parton showers, multiparton interactions and hadronisation, validates the default settings against LEP and HERA data, reports that all three generators describe the data within uncertainties (with PYTHIA and SHERPA performing particularly well), and supplies EIC predictions while calling for a global photon-PDF refit and new RIVET analyses.

Significance. If the reported generator comparisons and data agreements hold under scrutiny, the work supplies a timely benchmark for non-perturbative modelling in photoproduction and identifies concrete prerequisites (updated photon PDFs, RIVET-portable measurements) for precision EIC phenomenology. The explicit quantification of generator differences and the cautious stance on kinematic extrapolation are strengths.

minor comments (3)
  1. [§3] §3 (implementation differences): the description of how each generator treats the photon remnant and the transition between resolved and direct photoproduction would benefit from an explicit table listing the relevant switches and default parameters.
  2. [Figure 5] Figure 5 (EIC predictions): the uncertainty bands shown for the three generators are not explained in the caption; it is unclear whether they reflect scale variations, PDF uncertainties, or generator-specific tuning variations.
  3. [Abstract / §4] The abstract states that differences are 'disentangled and quantified'; the text should make clear in which section the quantitative decomposition (e.g., percentage contributions from MPIs vs. hadronisation) is presented.

Simulated Author's Rebuttal

0 responses · 0 unresolved

We thank the referee for the positive and accurate summary of our manuscript, which correctly identifies the scope of our systematic comparison of HERWIG, PYTHIA and SHERPA for jet photoproduction, the validation against LEP and HERA data, and the forward-looking conclusions regarding EIC requirements. We appreciate the recommendation for minor revision and the recognition of the work's timeliness.

Circularity Check

0 steps flagged

No significant circularity; comparisons rest on external data

full rationale

The paper compares three event generators (HERWIG, PYTHIA, SHERPA) against external LEP/HERA jet photoproduction data and presents generator-based predictions for EIC observables. No load-bearing step reduces by construction to a fit performed inside this analysis, nor does any central claim rest on a self-citation chain whose content is unverified outside the present work. The explicit call for future photon-PDF refits and new RIVET measurements further indicates the derivation chain is not closed on internal inputs.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Only the abstract is available, so free parameters, axioms and invented entities cannot be extracted; the work relies on standard modeling assumptions inside public event generators.

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