Abstract
Direct photon production in hadronic collisions provides a handle on the gluon PDF by means of the QCD Compton scattering process. In this work we revisit the impact of direct photon production on a global PDF analysis, motivated by the recent availability of the next-to-next-to-leading (NNLO) calculation for this process. We demonstrate that the inclusion of NNLO QCD and leading-logarithmic electroweak corrections leads to a good quantitative agreement with the ATLAS measurements at 8 and 13 TeV, except for the most forward rapidity region in the former case. By including the ATLAS 8 TeV direct photon production data in the NNPDF3.1 NNLO global analysis, we assess its impact on the medium-x gluon. We also study the constraining power of the direct photon production measurements on PDF fits based on different datasets, in particular on the NNPDF3.1 no-LHC and collider-only fits. We also present updated NNLO theoretical predictions for direct photon production at 13 TeV that include the constraints from the 8 TeV measurements.
Highlights
Non-perturbative quark-to-photon and gluon-to-photon fragmentation functions (FFs)
We present the main results of this work, namely √the impact of the ATLAS direct photon production data at s = 8 TeV on the NNPDF3.1 global analysis
This is not the case in NNPDF3.1, where the gluon PDF is already reasonably well constrained at medium and small-x from the combination of jet, tt, and Z pT data, and we expect the impact of the direct photon data on the gluon to be moderate
Summary
Non-perturbative quark-to-photon and gluon-to-photon fragmentation functions (FFs). This fragmentation component is only loosely constrained by LEP data [14], inducing a potentially large source of theoretical uncertainty. In order to relate theory calculations with experimental measurements one needs to account for the poorly-understood fragmentation component The first of these objections was removed by the availability of the NNLO QCD calculation [21], which together with the corresponding electroweak corrections [22] was found to provide a good√quantitative description of the ATLAS measurements at s = 8 TeV [23] at central photon rapidities [21]. Recent searches for BSM physics with photons in the final state from ATLAS and CMS include searches for new particles by looking for high-mass resonances [25,26,27], anomalous couplings [28,29,30,31], and by measuring missing ET distributions [32,33,34,35] These searches rely on a good understanding of the QCD background for photon production. It is necessary to account for higher order QCD and electroweak corrections and to use recent global PDF fits that can properly model the background and signal events
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