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GRIFFIN: A C++ library for electroweak radiative corrections in fermion scattering and decay processes
by Lisong Chen, Ayres Freitas
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Submission summary
Authors (as registered SciPost users):  Lisong Chen · Ayres Freitas 
Submission information  

Preprint Link:  https://arxiv.org/abs/2211.16272v1 (pdf) 
Code repository:  https://github.com/lisongc/GRIFFIN/releases 
Date submitted:  20230104 19:36 
Submitted by:  Chen, Lisong 
Submitted to:  SciPost Physics Codebases 
Ontological classification  

Academic field:  Physics 
Specialties: 

Approaches:  Theoretical, Computational, Phenomenological 
Abstract
This paper describes a modular framework for the description of electroweak scattering and decay processes, including but not limited to Zresonance physics. The framework consistently combines a complexpole expansion near a schannel resonance with a regular fixedorder perturbative description away from the resonance, in a manifestly gaugeinvariant scheme. Leading vertex correction contributions are encapsulated in form factors that can be predicted or treated as numerical fit parameters. This framework has been implemented in the publicly available objectoriented C++ library GRIFFIN. Version 1.0 of this library provides Standard Model predictions for the process $f\bar{f} \to f'\bar{f}'$ with full NNLO and leading higherorder contributions on the Zresonance, and with NLO corrections off resonance. The library can straightforwardly be extended to include higherorder corrections, should they become available, or predictions for new physics models. It can be interfaced with MonteCarlo programs to account for QED and QCD initialstate and finalstate radiation.
Current status:
Reports on this Submission
Anonymous Report 3 on 2023213 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2211.16272v1, delivered 20230213, doi: 10.21468/SciPost.Report.6733
Strengths
 describes a valuable software package for precision calculations
 provides enough theory overview to inform the content of the package without having to resort to the original literature
 generally wellwritten
Weaknesses
 some choices made in the implementation are not ideal for modern calculations, e.g. the choice of infrared subtraction scheme, missing complex mass schemes, etc
 differences to Dizet not sufficiently explained
Report
This manuscript describes a new software library that has the potential to be very useful for precision calculations for some precision calculations at future e+e colliders.
Requested changes
1) p.1, first paragraph: Measurements of DY at the LHC and the Tevatron also include initial state bquarks, whose contributions play a role a the precision provided by the LHC experiments. Of course, their contribution at higherorder EW differs from the other light quarks due to the top, but they should at least be mentioned here.
2) p.2, first paragraph: Along the examples for QED corrections in MC for e+e a few more recent advances should be mentioned, in particular 1911.12040 and 2203.10948, possibly refering to 2203.12557 for an overview.
3) p.3, eq. (1): the meaning of the $\otimes$ symbol should be introduced.
4) p.3, eq. (3): $c_\theta$ is undefined.
5) p.3, eq. (3): $s$ is undefined.
6) p.3, eq. (5): $s_w$ and $c_w$ are undefined.
7) p.3, eq. (6): Possibly connect eq. (6) to the otherwise wellknown conversion between pole and onshell scheme, which I think are the same just using different terminology.
8) It is not entirely clear how logarithms of $1s/s_0$ are handled in the expansion of eq. (35) and following, please elaborate.
9) In the matching of the onshell resummation to the complete offshell fixedorder calculation, it is not obvious that no artifacts appear away from the onshell limit as the resummation is never switched off. Please comment.
10) Further, also in the limit that the offshell calculation is in the vicinity of the onshell production, since the pole is never reached by the offshell calculation due to the finite width, it is not clear either that no artifacts are introduced as in this limit the expanded resummation and the fixedorder do not (logarithmically) coincide (at least when a modern universal scheme like the complex mass scheme is used).
11) The details on the implementation of the IR subtraction are insuffient, in particular if the code is to be used and interfaced to other tools and event generators. In particular, with the limited information provided a conversion to modern subtraction schemes using dimensional regularisation is not straightforward.
12) In the comparison to Dizet, the observed differences are not sufficiently explained in text, in particular in processes with bottom quarks. Please elaborate. In particular since the deviations of up to 2% naively seem too large to be attributed to generic NNLO effects. If they are enhanced by some mechanism, please discuss these.
Anonymous Report 2 on 2023211 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2211.16272v1, delivered 20230210, doi: 10.21468/SciPost.Report.6718
Strengths
 Stateofthe art precision calculations valid in the resonance region are matched to offshell calculations in a systematic way
 Thanks to the modular C++ implementation, this code can be extended in various directions and interfaced to a variety of Monte Carlo tools.
Weaknesses
 The implemented factorization scheme for IR singularities does not corresponds to the conventional subtraction schemes (e.g. CataniSeymour or FKS) that are used in modern Monte Carlo tools for highenergy colliders.
Report
I believe that this code meets the requirements for publication in SciPost Physics Codebases. Its content is well documented, and its modular and flexible structure opens the door to a variety of applications at highenergy colliders.
Requested changes
1) The title, abstract and introduction give the impression that GRIFFIN may be applicable to cross sections, while it only computes IRsubtracted $2\to 2$ matrix elements. This should be stated in a more explicit way, starting from the abstract.
2) The treatment of logarithms of $1s/s_0$ in GRIFFIN should be described in some details.
3) The implemented IRfactorization prescription should be documented in full detail, i.e. with explicit formulas for all factorized singularities. This information is crucial for interfacing the code to Monte Carlo generators.
4) The IR factorisation scheme should also be adapted to dimensional regularisation (with massless photons and massive or massless fermions)
5) The matching prescription (35) involves offshell matrix elements with real Zboson masses. The authors should comment on its applicability to modern offshell calculations based on the complexmass scheme.
6) The terminology "cross section matrix elements" at the beginning of Sect. 4 is confusing and should be clarified.
7) Table 1: the authors should clearly indicate which parameters play the role of userprovided input parameters and which ones are derived from other input parameters. For instance, gaugeboson widths are not independent input parameters: how are they treated in GRIFFIN?
8) The origin of the percentlevel differences in Fig. 1 should be clarfied in some detail.
Anonymous Report 1 on 202327 (Invited Report)
 Cite as: Anonymous, Report on arXiv:2211.16272v1, delivered 20230207, doi: 10.21468/SciPost.Report.6546
Strengths
• The code can be systematically extended to include higherorder corrections and newphysics models.
• The code will be relevant for future lepton colliders and supersedes software packages used at LEP 1.
• The implementation has been verified against the DIZET library and includes all available higherorder corrections for the leading pole term.
Weaknesses
• The differences between GRIFFIN and DIZET in the differential cross section are not understood.
• The paper contains several typos.
Report
The paper describes a modular framework for the calculation of electroweak scattering and decay processes including perturbative corrections. It is specifically realised for processes with 4 external fermions and combines a pole expansion near a vectorboson resonance with a fixedorder calculation in the nonresonant region. Owing to its modular structure, the library can be easily extended to include further higherorder corrections or predictions in newphysics models. In its present state, the code includes all known corrections to fermionpair production on and off the Z resonance. This is relevant for the investigation of fermionpair production at future electronpositron colliders.
Requested changes
• The formulation in Section 2 relies on the literature for the corresponding calculations for LEP 1. The due credit to the original papers should be given.
• It should be made clear, how logarithms of the form ln(1 − s/s 0 ) are treated in the pole expansion.
• The code contains only the IRfinite part of the NLO corrections. The precise definition of this part should be given, for instance for the vertex form factors.
• The cancellation of IFI between γZ boxes and the corresponding real radiation mentioned on page 5 is only valid for inclusive quantities. This should be stressed.
• While it is obvious that Eq. (35) does not lead to double counting close to the resonance, this is not so clear away from the resonance. The authors should comment on this point.
• The “etc.” in the first list of items on page 9 should be explained or omitted.
• In the comparison between GRIFFIN and DIZET the largest differences show up for quantities related to bottom quarks. This needs to be commented.
• The differences between GRIFFIN and DIZET of up to 2% in the differential cross section away from the resonance region appear to be quite large. Can these really be explained by NNLO effects? Which enhancements of NNLO corrections can cause differences of this size?
Author: Ayres Freitas on 20230411 [id 3574]
(in reply to Report 1 on 20230207)Please see attached file for our response to the referee report
Author: Ayres Freitas on 20230411 [id 3575]
(in reply to Report 2 on 20230211)Please see attached file for our response to the referee report
Attachment:
response_letter_griffin_FmHTGAH.pdf