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A standard convention for particle-level Monte Carlo event-variation weights

by Enrico Bothmann, Andy Buckley, Christian Gütschow, Stefan Prestel, Marek Schönherr, Peter Skands, Jeppe Andersen, Saptaparna Bhattacharya, Jonathan Butterworth, Gurpreet Singh Chahal, Louie Corpe, Leif Gellersen, Matthew Gignac, Deepak Kar, Frank Krauss, Jan Kretzschmar, Leif Lönnblad, Josh McFayden, Andreas Papaefstathiou, Simon Plätzer, Steffen Schumann, Michael Seymour, Frank Siegert, Andrzej Siódmok

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Submission summary

Authors (as registered SciPost users): Enrico Bothmann · Andy Buckley · Jonathan Butterworth · Louie Corpe · Leif Gellersen · Deepak Kar · Frank Krauss · Leif Lönnblad · Steffen Schumann · Frank Siegert
Submission information
Preprint Link:  (pdf)
Date submitted: 2022-09-02 17:34
Submitted by: Buckley, Andy
Submitted to: SciPost Physics Core
Ontological classification
Academic field: Physics
  • High-Energy Physics - Phenomenology
Approaches: Computational, Phenomenological


Streams of event weights in particle-level Monte Carlo event generators are a convenient and immensely CPU-efficient approach to express systematic uncertainties in phenomenology calculations, providing systematic variations on the nominal prediction within a single event sample. But the lack of a common standard for labelling these variation streams across different tools has proven to be a major limitation for event-processing tools and analysers alike. Here we propose a well-defined, extensible community standard for the naming, ordering, and interpretation of weight streams that will serve as the basis for semantically correct parsing and combination of such variations in both theoretical and experimental studies.

Author comments upon resubmission

Dear editor and referees,

Thank you for the helpful comments which have assisted us in alleviating ambiguities in the specification. And our apologies for the long delay in response time, due to some very heavy university commitments for both corresponding authors.

We have uploaded the improved version 3 to the arXiv, which we hope addresses the detailed points made by referee 1. As a general point we would like to highlight to both referees that there is no such object as a "MCnet public note" and that the journal review process has already added value in the intended way; by analogy with the very successful Les Houches Event Format paper, which is similar in many respects including a lack of detailed physics case or impact study, journal publication is established and appropriate for community standards such as this. Our understanding is that SciPost Physics Core is the appropriate venue for such papers which are not "codes or algorithms" cf. SciPost Physics Codebases, but we would be happy to be referred there if the relatively HEP-community aspect of this paper would fit its definition and processes better than Physics Core.

Best wishes,
Andy Buckley, for the editors

List of changes

All referee points have been considered and most accepted. The full set of responses will be posted to the referee comment, as the most efficient way to document the many detail changes.

Current status:
Has been resubmitted

Reports on this Submission

Anonymous Report 1 on 2022-9-29 (Invited Report)

  • Cite as: Anonymous, Report on arXiv:2203.08230v3, delivered 2022-09-29, doi: 10.21468/SciPost.Report.5797


1 - the paper provides a solution of an important standardization problem
2 - the solution is well thought out
3 - the solution is presented clearly (with minor exceptions)
4 - the solution is a consensus of the most important stake holders


1 - there is a confusing phrase on page 6.


As the authors describe very clearly, a practical and well defined standard for communicating the event level weight variations with input parameters from Monte Carlo event generators to analysis software packages is of crucial importance for making the most out of the experiments at the LHC. It is neither practical nor ecologically responsible to regenerate events for performing the many parameter scans that are performed in the analysis of LHC event samples.

By its nature, the paper does not present new physics results, but it should be published as an important reference for the community.

The proposed standard is practical and reflects the experience of the authors. The rationale for the design choices is explained well. The description of the standard is very clear (with one minor exception, see below) and can be adapted easily by authors and users of analysis software packages and by authors of other Monto Carlo event generators.

Nevertheless, I have suggest minor improvements:

In the last paragraph on page 6, the authors refer to "Non-standard weights or weights otherwise unknown to the parton shower". It is not explained why the parton shower component of event generators is singled out here. It is not clear to me whether this is an editorial oversight from an earlier draft or whether there is a reason for excluding other components (PDFs, ME, hadronization) here. The authors should consider dropping "or weights otherwise unknown to the parton shower" altogether or clarify their intentions.

Indeed, the final two paragraphs of section 2.3 are largely redundant with the preceding listing. The authors might want to consider merging the two.

Also in section 2.3, the authors might want to consider making the "USER" or "AUX" class marker obligatory for unblessed physical events.

Finally, reference [29] could be replaced by the publically available ISO/IEC 14977 standard.

Requested changes

1 - the authors should clarify why they single out the parton shower in the phrase "weights otherwise unknown to the parton shower" in the last paragraph on page 6.

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

Author:  Andy Buckley  on 2022-10-03  [id 2870]

(in reply to Report 1 on 2022-09-29)
answer to question

Our thanks for the positive feedback and for identifying the issue of duplication around the prefix-code options!

The duplication and confusing sentence in Section 2.3 have been resolved in a new version now submitted to arXiv to appear tomorrow. We have further explained the distinction between USER and no-prefix modes, and made it explicit that streams not for physical reweighting interpretation *must* be marked as such with a suitable prefix and vector position.

On the EBNF, while searching for the ISO reference we encountered the critique at and took the advice there to use the more compact, familiar, and accessible syntax variant by W3C. The new version cites the relevant W3C document.

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