Abstract
We present the MSHT20qed set of parton distribution functions (PDFs). These are obtained from the MSHT20 global analysis via a refit including QED corrections to the DGLAP evolution at {{{mathcal {O}}}}(alpha ),{{{mathcal {O}}}}(alpha alpha _S) and mathcal{O}(alpha ^2), and containing the photon PDF of the proton. As in the previous MMHT15qed study we use an input distribution for the photon that is derived from the LUXqed formulation, and find good consistency for the photon PDF with that of MMHT15qed, as well as with other recent sets. We also present a set of QED corrected neutron PDFs and accompanying photon distribution, and provide the photon PDF of the nucleons separated into elastic and inelastic contributions. We assess the general expectations for the impact of photon-initiated (PI) corrections to processes entering PDF fits, and review the effect of QED corrections on the other partons and on the fit quality, where electroweak corrections (including PI production) are appropriately added to the cross sections wherever possible. We explore the phenomenological implications of this set by comparing to a variety of benchmark cross sections, finding small but significant corrections due to the inclusion of QED effects in the PDFs.
Highlights
Entering as corrections to the leading QCD cross section for processes such as Drell–Yan [6], EW boson–boson scattering [7] and Higgs production with an associated EW boson [8]
Page 5 of 15 90 adopted in the fit, where the PI cross section is recalculated using the QCD partons from each iteration of the fit, until convergence is reached. We have performed such a procedure here, in reality we find this converges after one iteration, and we have confirmed that calculating the PI cross section with the MSHT20 QCD partons, without such an iteration, gives an almost identical result in terms of the fit quality and the extracted parton distribution functions (PDFs) set
We have presented a detailed overview of the expectations for the relevance of photon-initiated (PI) production in processes that enter current global PDF fits
Summary
The inclusion of QED corrections in DGLAP, and the corresponding photon PDF, has a long history. Subsequent sets either used similar phenomenological models [12], or constrained the photon by utilising the rather limited sensitivity of DIS and Drell–Yan data via PI final states [13,14] Both approaches lead to photon PDF uncertainties of at least 10% and often more. [17], the contributions from elastic and inelastic emission to the photon PDF are directly related to the corresponding structure functions, F1e,l2, F1i,n2el ; this idea has been revisited over the years in [18,19,20,21] It has recently been placed within a rigorous and precise theoretical framework by the LUXqed group [22,23], who provided a publicly available photon PDF with uncertainties determined by those on the structure functions used as input, i.e. at the level of a few percent. The first global PDF set including a photon distribution based
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