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Reweighting Monte Carlo Predictions and Automated Fragmentation Variations in Pythia 8

by Christan Bierlich, Philip Ilten, Tony Menzo, Stephen Mrenna, Manuel Szewc, Michael K. Wilkinson, Ahmed Youssef, Jure Zupan

This Submission thread is now published as

Submission summary

Authors (as registered SciPost users): Christian Bierlich · Philip Ilten · Tony Menzo
Submission information
Preprint Link:  (pdf)
Code repository:
Date accepted: 2024-05-07
Date submitted: 2024-03-14 15:10
Submitted by: Ilten, Philip
Submitted to: SciPost Physics
Ontological classification
Academic field: Physics
  • High-Energy Physics - Phenomenology
Approaches: Computational, Phenomenological


This work reports on a method for uncertainty estimation in simulated collider-event predictions. The method is based on a Monte Carlo-veto algorithm, and extends previous work on uncertainty estimates in parton showers by including uncertainty estimates for the Lund string-fragmentation model. This method is advantageous from the perspective of simulation costs: a single ensemble of generated events can be reinterpreted as though it was obtained using a different set of input parameters, where each event now is accompanied with a corresponding weight. This allows for a robust exploration of the uncertainties arising from the choice of input model parameters, without the need to rerun full simulation pipelines for each input parameter choice. Such explorations are important when determining the sensitivities of precision physics measurements. Accompanying code is available at

Author comments upon resubmission

We thank the reviewers for the detailed reading of our manuscript. We have updated our manuscript with a number of changes based on their suggestions.

List of changes

* Added missing references, as suggested by the referees.
* Clarified how the reweighting method outlined can be used with MPI switched on, as well as how the general methods are applicable to varying MPI parameters.
* Included an extended discussion on using 1 - mu as well as the number of effective events, and why both metrics are necessary when performing variations.
* The effective number of events has now been included in Table 1.
* Fixed a bug in the code where the diquark production was not being handled correctly, which resulted in a small change in the distributions.
* Corrected our discussion about how the final string end is joined into two hadrons. Specifically, the weights of the strings which fail this final joining need to be included in the overall weight of the event.

Published as SciPost Phys. 16, 134 (2024)

Reports on this Submission

Anonymous Report 2 on 2024-4-10 (Invited Report)


I thank the authors for answering my questions from my first report and I am pleased that the code has been further improved and that the production of diquarks is now correctly accounted for.


The authors have responded to all the weaknesses contained in my previous report.


I propose that the article be accepted for publication.


Publish (easily meets expectations and criteria for this Journal; among top 50%)

  • validity: high
  • significance: high
  • originality: high
  • clarity: high
  • formatting: excellent
  • grammar: excellent

Anonymous Report 1 on 2024-3-25 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:2308.13459v3, delivered 2024-03-25, doi: 10.21468/SciPost.Report.8768


This is a follow-up report to my report submitted at 2023-12-5.

The draft has been revised to meet all requested changes, thus removing the identified weaknesses.

I particularly welcome the improved discussion of the quality criteria, i.e. the discussion on $1-\mu$, and the addition of the effective sample size.

When I noted that the "reweighting in the context of matching and merging [...] should perhaps be mentioned", I was rather pointing to the existence of on-the-fly variation methods that even work for matched and merged simulations [arXiv:1606.08753], not so much the non-variational application of reweighting in matching/merging. However, that is a very minor quibble, and I explicitly do not request another change on the basis of that, especially because this is somehow implicit in the preceding formulation "efficient reweighting methods exist for the hard process and the parton shower" (as matching/merging is just a combination of the hard process and the parton shower), and all the proper citations are already given for that.

Hence, I suggest this revision for publication without further changes.

  • validity: top
  • significance: top
  • originality: good
  • clarity: top
  • formatting: perfect
  • grammar: perfect

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