Abstract
Achieving the highest precision for theoretical predictions at the LHC requires the calculation of hard-scattering cross sections that include perturbative QCD corrections up to (N)NNLO and electroweak (EW) corrections up to NLO. Parton distribution functions (PDFs) need to be provided with matching accuracy, which in the case of QED effects involves introducing the photon parton distribution of the proton, xgamma (x,Q^2). In this work a determination of the photon PDF from fits to recent ATLAS measurements of high-mass Drell–Yan dilepton production at sqrt{s}=8 TeV is presented. This analysis is based on the xFitter framework, and has required improvements both in the APFEL program, to account for NLO QED effects, and in the aMCfast interface to account for the photon-initiated contributions in the EW calculations within MadGraph5_aMC@NLO. The results are compared with other recent QED fits and determinations of the photon PDF, consistent results are found.
Highlights
IntroductionAn important ingredient of these electroweak corrections is the photon parton distribution function (PDF) of the proton, xγ (x, Q2), which must be introduced to absorb the collinear divergences arising in initial-state QED emissions
The first Parton distribution functions (PDFs) fit to include both QED corrections and a photon PDF was MRST2004QED [1], where the photon PDF was taken from a model and tested on HERA data for direct photon production
A new determination of the photon PDF from a fit of HERA inclusive deep-inelastic scattering (DIS) structure functions supplemented by ATLAS data on high-mass Drell–Yan cross sections has been presented, based on the xFitter framework
Summary
An important ingredient of these electroweak corrections is the photon parton distribution function (PDF) of the proton, xγ (x, Q2), which must be introduced to absorb the collinear divergences arising in initial-state QED emissions. The first PDF fit to include both QED corrections and a photon PDF was MRST2004QED [1], where the photon PDF was taken from a model and tested on HERA data for direct photon production. Almost 10 years later, the NNPDF2.3QED analysis [2,3] provided a first model-independent determination of the photon PDF based on Drell–Yan (DY) data from the LHC. The determination of xγ (x, Q2) from NNPDF2.3QED was later combined with the state-of-the-art quark and gluon PDFs from NNPDF3.0, together with an improved QED evolution, to construct the NNPDF3.0QED set [4,5]. The CT group has released a QED fit using a similar strategy as the MRST2004QED one, named the CT14QED set [6]
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