Abstract
We detail a calculation of W gamma production in hadronic collision, at Next-to-Leading Order (NLO) QCD interfaced to a shower generator according to the POWHEG prescription supplemented with the MiNLO procedure. The fixed order result is matched to an interleaved QCD+QED parton shower, in such a way that the contribution arising from hadron fragmentation into photons is fully modeled. In general, our calculation illustrates a new approach to the fully exclusive simulation of prompt photon production processes accurate at the NLO level in QCD. We compare our predictions to those of the NLO program MCFM, which treats the fragmentation contribution in terms of photon fragmentation functions. We also perform comparisons to available LHC data at 7 TeV, for which we observe good agreement, and provide phenomenological results for physics studies of the W gamma production process at the Run II of the LHC. The new tool, which includes W leptonic decays and the contribution of anomalous gauge couplings, allows a fully exclusive, hadron-level description of the W gamma process, and is publicly available at the repository of the POWHEG BOX. Our approach can be easily adapted to deal with other relevant isolated photon production processes in hadronic collisions.
Highlights
At hadron colliders, as backgrounds to W γ and Zγ production can be significantly reduced through the identification of the W and Z bosons via their leptonic decay modes
We detail a calculation of W γ production in hadronic collision, at Next-toLeading Order (NLO) QCD interfaced to a shower generator according to the POWHEG prescription supplemented with the MiNLO procedure
Afterwards, the cross section measurements are compared to the Next-to-Leading Order (NLO) QCD predictions of the parton-level Monte Carlo (MC) program MCFM [18], that includes the full set of LO diagrams and NLO QCD corrections contributing to V γ production, and takes care of the contribution coming from the fragmentation of secondary quarks and gluons into isolated photons via the formalism of photon fragmentation functions [19]
Summary
At LO, the production of a W boson and a photon in hadronic collisions, with leptonic decays of the vector boson, is an EW process which proceeds via quark-antiquark annihilation qq → ±νγ,. A prominent feature of the W γ LO matrix element is the appearance of a so called radiation zero, which corresponds to the existence of some kinematic configurations for which the amplitude vanishes [43, 44] This can appear in some observables as dip in the rapidity distributions and can provide a handle to extract information on the anomalous couplings, since the latter partially fill the dip. The effect of ATGCs is expressed in terms of their deviation from the SM values, leading to the two parameter set (λγ, ∆kγ), with ∆kγ ≡ kγ − 1.5 The full amplitude resulting from the calculation of the diagrams shown in figure 1 with the modifications introduced by the Lagrangian of eq (2.2) has been computed using FORM. As a cross-check, we compared our LO predictions, both without and with ATGCs, with those of MCFM, finding perfect agreement
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