Abstract
The impact of isolated-photon data from proton–(anti)proton collisions at RHIC, Spp¯S, Tevatron and LHC energies, on the parton distribution functions of the proton is studied using a recently developed Bayesian reweighting method. The impact on the gluon density of the 35 existing isolated-γ measurements is quantified using next-to-leading order (NLO) perturbative QCD calculations complemented with the NNPDF2.1 parton densities. The NLO predictions are found to describe well most of the datasets from 200 GeV up to 7 TeV centre-of-mass energies. The isolated-photon spectra recently measured at the LHC are precise enough to constrain the gluon distribution and lead to a moderate reduction (up to 20%) of its uncertainties around fractional momenta x≈0.02. As a particular case, we show that the improved gluon density reduces the PDF uncertainty for the Higgs boson production cross section in the gluon-fusion channel by more than 20% at the LHC. We conclude that present and future isolated-photon measurements constitute an interesting addition to coming global PDF analyses.
Highlights
The impact of isolated-photon data from proton-(anti)proton collisions at RHIC, SppS, Tevatron and LHC energies, on the parton distribution functions of the proton is studied using a recently developed Bayesian reweighting method
In view of the various accurate measurements of isolated-photon spectra carried out recently at the LHC [34,35,36,37,38], which have added up about 130 new points to the existing world data, it is a timely moment to revisit first if next-to-leading order (NLO) pQCD provides a good description of all available isolated-γ collider data, and if so, what quantitative constraints these new results impose on the gluon PDF
Computational alternatives exist, though, such as e.g. (i) the ApplGrid framework [39] or the FastNLO software [40], both of which make use of lookup tables with cross-section weights which can be a posteriori fast-convoluted with any parton density set, and (ii) the NNPDF Bayesian reweighting technique [41, 42] which can be applied to any set of Monte Carlo (MC)-based parton densities and which relies on the computation of the χ2 distribution between the data and each MC replica in order to determine the new PDFs
Summary
The world isolated-γ data covers all prompt-photon measurements at collider machines in the last. Of parton The plot fractional momentum x± = shows that the photon LHC data extend the kinematic coverage in particular over the region x ≈ 10−3 − 10−2 at moderately large energy scales (Eγ 2 ≈ 103 − 105 GeV2) not directly covered by the other experimental T datasets. Deep Inelastic Scattering Drell-Yan (FT), W, Z (Tevatron) Inclusive jets (Tevatron) Isolated photons ( s=2.76,7 TeV) Isolated photons ( s=1.8,1.96) Isolated photons ( s=200-630 GeV)
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