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Coherent QCD Radiation in Vector-Boson-Fusion Higgs Production
by Stefan Höche, Stephen Mrenna, Shay Payne, Christian T Preuss, Peter Skands
This is not the latest submitted version.
This Submission thread is now published as SciPost Phys. 12, 010 (2022)
|As Contributors:||Christian Tobias Preuss|
|Date submitted:||2021-06-22 02:08|
|Submitted by:||Preuss, Christian Tobias|
|Submitted to:||SciPost Physics|
We discuss the QCD coherence properties of several parton-shower models available in Pythia and Vincia, in the context of Higgs production via vector boson fusion (VBF). The distinctive colour topology of VBF processes allows for the definition of observables that are especially sensitive to the coherent radiation pattern of additional jets. We study a set of such observables, using the Vincia sector-antenna shower as our main reference, and contrast it to Pythia's transverse-momentum-ordered DGLAP shower as well as Pythia's dipole-improved shower. We then investigate the robustness of these predictions as successive levels of higher-order perturbative matrix elements are incorporated, including next-to-leading-order matched and tree-level merged calculations, using Powheg-Box and Sherpa respectively to generate the hard events.
Submission & Refereeing History
Published as SciPost Phys. 12, 010 (2022)
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Reports on this Submission
Anonymous Report 1 on 2021-7-30 (Invited Report)
- Cite as: Anonymous, Report on arXiv:scipost_202106_00039v1, delivered 2021-07-30, doi: 10.21468/SciPost.Report.3319
The present manuscript describes predictions for Vector Boson Fusion (VBF) processes at hadron colliders, with an emphasis on parton shower effects and the matching to fixed order, and some first studies of the impact of soft QCD effects. All of these aspects have extensively been discussed in the literature or in talks at recent conferences. The originally new content of the manuscript is the inclusion of (leading order) multijet merging, and the use of the Vincia parton shower algorithm, however the title is not indicative of this fact.
Effects due to additional hard jet radiation are in fact worth to consider in VBF processes; this has so far been done at fixed, well separated jet multiplicities, and merging different multiplicities is certainly a needed addition to these studies. As such the manuscript is a welcome addition to the literature, despite the limited insight it provides into how merging algorithms can be applied to the non-trivial structure of jets present at the Born level, and how uncertainties can be estimated reliably.
Before I can recommend this manuscript for publication I need to raise several points of criticism which I like the authors to address:
The present paper makes excessive use of the notion of 'coherent QCD radiation', including in its title. Nowhere could I find any analysis or proof of coherence properties of the shower algorithms the authors consider, nor are aspects discussed, which would be specific to QCD coherence in VBF.
In fact, at several places the paper contains unproven claims about the Vincia shower exhibiting coherence, supported by statements about how the soft radiation pattern is incorporated in the shower kernerls (which, by itself, is at most a necessary condition for a coherent evolution). The statement that the standard Pythia shower would not include the soft radiation pattern in its kernels in this respect needs to be clarified (since the prove or disprove needs to involve kinematic details, on top of the structure of the kernels and possibly the entire evolution algorithm).
The authors need to remove their claims aboutr coherence or provide detailed explanations and proofs of their statements. An appropriate discussion would then also need to account for a significant amount of recent literature focusing on the accuracy of parton shower algorithms and properties of QCD coherence as encoded in dipole-type parton shower algorithms: nothing along these lines is mentioned or cited. On the same note should the title of the paper be changed to reflect its true content.
The paper highlights several comparisons to the Pythia shower without the dipole recoil option, a comparison which has been performed, and understood, extensively in the literature since several years (the study presented in arXiv:1803.07943 should also be cited in this context). These comparisons do not provide any additional insight and should be removed in favour of the more interesting question of how and why the more physical Pythia evolution agrees or differs with the Vincia algorithm. This understanding is needed to properly discuss how and why the matching and merging algorithms differ. The outcome that merging improves the behaviour of the default Pythia shower is not surprising, but the non-dipole option will still stay unphysical -- I do not understand how this should be a significant result of the study worth highlighting in the conclusions.
The discussion of the VBF process itself lacks several details: In what approximations is the process calculated? Are interferences neglected, are Higgs-Strahlungs-type topologies neglected? If not, how are colour flows assigned? What generating cuts are applied to the hard jets -- none, technical ones at very low thresholds (how large?), or significant ones (of which the impact should then be discussed). What happens to the scale definition inside Pythia of the hard quarks are scattered forwardly?
Besides these general comments, which address a multitude of places in the text, a few more minor comments are:
General: Parton showers should generally not be referred to as models, but algorithms. Coherence is not a floating point quantity -- it is a property of a parton shower algorithm, which is either present, or not. Please clarify 'amount of coherent radiation' and similar terms.
Introduction: Authors talk about 'unique' colour flow in VBF. Please confront with general remarks on VBF approximation above. References [8,9] do not include  which is later referred to one of the previously cited ones.
'DGLAP shower' should be replaced by a precise definition including: partitioning of soft radiation, recoil scheme, and evolution variable.
Footnote 2: Consider to remove this comment or quantify the term 'unphysical'.
Section 2, discussion on truncated showers: More details are needed here. Are the transverse momentum definitions all equivalent? If they differ, how can the authors be confident that there is no need for truncated showering? References to other work on truncated showers and matching/merging in the literature, e.g. arXiv:0905.3072, are missing.
Caption of Figure 2 (as well as later): be more specific about what 'default shower scale' means. Similarly, the argument of the strong coupling in the shower is a property of its (possible) resummation properties but not directly a renormalization scale in the usual sense. Please clarify the term 'shower renormalization scale' in this respect.
Conclusions: Reproducing the soft radiation pattern is not at all impossible or hard to achieve for an evolution which separates initial and final state radiation, as, for instance, done in a coherent branching algorithm. The remark made in that respect should be clarified: is it referring to the first or all emissions? In what phase space region/hierarchy of emissions?