SciPost Submission Page
disorder: Deep inelastic scattering at high orders
by Alexander Karlberg
Submission summary
| Authors (as registered SciPost users): | Alexander Karlberg |
| Submission information | |
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| Preprint Link: | https://arxiv.org/abs/2401.16964v2 (pdf) |
| Code repository: | https://github.com/alexanderkarlberg/disorder |
| Date submitted: | 2024-02-09 09:38 |
| Submitted by: | Karlberg, Alexander |
| Submitted to: | SciPost Physics Codebases |
| Ontological classification | |
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| Academic field: | Physics |
| Specialties: |
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Abstract
We present a Fortran 77/95 code capable of computing QCD corrections in deep inelastic scattering (DIS). The code uses the Projection-to-Born method to combine an exclusive DIS + 1 jet calculation with the inclusive DIS structure functions, thereby obtaining fully differential DIS predictions at $\mathcal{O}(\alpha_{\mathrm{s}}^2)$. The code is lightweight and fast, and yet includes the most common functionalities found in typical perturbative QCD programs, like automatic renormalisation and factorisation scale uncertainties, options to run and combine multiple seeds, and interfaces to FastJet and LHAPDF. Due to the underlying exclusive DIS + 1 jet code, the program also provides stable results in the infrared, relevant for extracting logarithmic coefficients for analytic resummations. As a by-product the code provides access to the DIS structure functions and (reduced) cross sections up to $\mathcal{O}(\alpha_{\mathrm{s}}^3)$
Current status:
Reports on this Submission
Strengths
1. easy-to-use code
Weaknesses
1. limited scope of applicability
2. limited originality (code is essentially a front-end to existing HOPPET and DISENT codes)
3. code inherits limitations of DISENT
4. manuscript is highly misleading
Report
The manuscript and code can not be recommended for publication in their present form.
The DISORDER code provides an easy-to-use front-end to the HOPPET code for the evaluation of fully inclusive deeply inelastic structure functions (inclusive mode of DISORDER) and combines these results with the DISENT code to compute exclusive jet final states at order alphas^2 using the projection-to-Born method (P2B mode of DISORDER).
The description of exclusive final states and the application of the P2B method is highly misleading in the manuscript. The author distinguishes DIS and DIS+1jet calculations, with the latter being an ill-defined and frame-dependent notion. The description of laboratory-frame and Breit-frame kinematics in the manuscript is welcome, but does not lead to the necessary clarifications. It is in particular not spelled out that the P2B method as implemented in DISORDER will only work in the laboratory frame.
In the laboratory frame, DIS will always produce one final-state jet at Born level, while this jet is boosted to zero transverse momentum in the Breit frame. At NLO QCD in DIS (alphas^1), 2-jet final states are produced due to real radiation in both the laboratory and the Breit frame. The DISENT code computes the NLO corrections (alpha_s^2) to 2-jet production both in the Breit frame or in the laboratory frame, which amount to an NNLO contribution to inclusive DIS. By using the P2B method (i.e. reconstructing the Born-level momentum of the single jet at LO), this can be used to compute the NNLO corrections to single-jet observables in the laboratory frame (and only there, since this jet is inexistent in the Breit frame, where P2B just nullifies the jet kinematics to zero transverse momentum). It should be noted that the vast majority of neutral-current DIS jet production results from HERA are in the Breit frame, where the DISORDER code is not applicable.
Whlie the HOPPET code allows to compute the full set of neutral-current (NC) and charged-current structure functions, DISENT only provides predictions for neutral-current photon-exchange. This limitation is inherited by DISORDER, which is consequently unable to provide predictions for Z-boson exchange contributions to NC and for jet production in CC (where they would actually matter, since CC jet production can only be measured reliably in the laboratory frame).
Requested changes
1. Extend DISENT code to include CC jet production (Z-boson contributions in NC would be good-to-have, but are certainly less relevant).
2. Classify the kinematics of DIS jet production both in laboratory and Breit frame, and explain the detailed application of the P2B method and its limitations.
3. Use correct terminology for the classification of the different jet production processes: '2-jet production in DIS' instead of 'DIS + jet', etc.
Recommendation
Ask for major revision
Strengths
1. speedy and well documented code that generates fast and reliable high-order differential cross section predictions for DIS (inclusive cross section up to N3LO and exclusive cross sections up to NNLO)
2. highly relevant given renewed interest in DIS
3. options for extending range of applicability are discussed
Weaknesses
1. Not fully state of the art (N3LO exclusive cross sections available) - although mitigated by having a relatively speedy package
2. limited to the photon exchange for NC exclusive cross sections
Report
This is a speedy and well documented code that generates fast and reliable high-order differential cross section predictions for DIS. It uses the projection-to-born technique to leverage existing codes DISENT, HOPPET and links LHAPDF and fastjet. The author notes that the code provides little theoretical advance on its own, but that there is renewed interest in DIS due to the upcoming Electron-Ion- Collider (EIC). This referee agrees that well-maintained public code will be extremely valuable for both the experimental and theoretical communities, and that the documentation provided will enable the wide use of the code.