It has become apparent in recent years that it is important, notably for a range of physics studies at the Large Hadron Collider, to have accurate knowledge on the distribution of photons in the proton. We show how the photon parton distribution function (PDF) can be determined in a model-independent manner, using electron-proton (ep) scattering data, in effect viewing the ep→e+X process as an electron scattering off the photon field of the proton. To this end, we consider an imaginary, beyond the Standard Model process with a flavor changing photon-lepton vertex. We write its cross section in two ways: one in terms of proton structure functions, the other in terms of a photon distribution. Requiring their equivalence yields the photon distribution as an integral over proton structure functions. As a result of the good precision of ep data, we constrain the photon PDF at the level of 1%-2% over a wide range of momentum fractions.

/pdf/how-bright-is-the-proton-a-precise-determination-of-the-18vzss51gv.pdf

How Bright is the Proton? A Precise Determination of the Photon Parton Distribution Function.

We compute the contribution of order α
 2
 α
 2 to the cross section of a top-antitop pair in association with at least one heavy Standard Model boson — Z, W ±, and Higgs — by including all effects of QCD, QED, and weak origin and by working in the automated MadGraph5_aMC@NLO framework. This next-to-leading order contribution is then combined with that of order α
 3
 α, and with the two dominant lowest-order ones, α
 2
 α and α
 S
 α
 2, to obtain phenomenological results relevant to a 8, 13, and 100 TeV pp collider.

/pdf/electroweak-and-qcd-corrections-to-top-pair-hadroproduction-wxahzfphs8.pdf

Electroweak and QCD corrections to top-pair hadroproduction in association with heavy bosons

This Report summarizes the proceedings of the 2013 Les Houches workshop on Physics at TeV Colliders. Session 1 dealt primarily with (1) the techniques for calculating standard model multi-leg NLO and NNLO QCD and NLO EW cross sections and (2) the comparison of those cross sections with LHC data from Run 1, and projections for future measurements in Run 2.

Les Houches 2015: Physics at TeV Colliders Standard Model Working Group Report

Electroweak radiative corrections for collider physics

The photon PDF of the proton is needed for precision comparisons of LHC cross sections with theoretical predictions. In a recent paper, we showed how the photon PDF could be determined in terms of the electromagnetic proton structure functions $F_2$ and $F_L$ measured in electron-proton scattering experiments, and gave an explicit formula for the PDF including all terms up to next-to-leading order. In this paper we give details of the derivation. We obtain the photon PDF using the factorisation theorem and applying it to suitable BSM hard scattering processes. We also obtain the same PDF in a process-independent manner using the usual definition of PDFs in terms of light-cone Fourier transforms of products of operators. We show how our method gives an exact representation for the photon PDF in terms of $F_2$ and $F_L$, valid to all orders in QED and QCD, and including all non-perturbative corrections. This representation is then used to give an explicit formula for the photon PDF to one order higher than our previous result. We also generalise our results to obtain formulae for the polarised photon PDF, as well as the photon TMDPDF. Using our formula, we derive the $P_{\gamma i}$ subset of DGLAP splitting functions to order $\alpha \alpha_s$ and $\alpha^2$, which agree with known results. We give a detailed explanation of the approach that we follow to determine a photon PDF and its uncertainty within the above framework.

/pdf/the-photon-content-of-the-proton-sj09lpbvco.pdf

The Photon Content of the Proton

We present a calculation of the next-to-leading-order electroweak corrections to W+\gamma\ production, including the leptonic decay of the W boson and taking into account all off-shell effects of the W boson, where the finite width of the W boson is implemented using the complex-mass scheme. Corrections induced by incoming photons are fully included and find particular emphasis in the discussion of phenomenological predictions for the LHC. The corresponding next-to-leading-order QCD corrections are reproduced as well. In order to separate hard photons from jets, a quark-to-photon fragmentation function a la Glover and Morgan is employed. Our results are implemented into Monte Carlo programs allowing for the evaluation of arbitrary differential cross sections. We present integrated cross sections for the LHC at 7TeV, 8TeV, and 14TeV as well as differential distributions at 14TeV for bare muons and dressed leptons. Finally, we discuss the impact of anomalous WW\gamma\ couplings.

/pdf/nlo-qcd-and-electroweak-corrections-to-z-gamma-production-39qknf1t2z.pdf

NLO QCD and electroweak corrections to $Z+\gamma$ production with leptonic Z-boson decays

We present strongly improved electroweak precision constraints on the split-UED (universal extra dimension) model. We find that the dominating effect arises from contributions to the muon decay rate by the exchange of even-numbered $W$ boson Kaluza-Klein modes at tree level, which so far have not been discussed in the context of UED models. The constraints on the split-UED parameter space are translated into bounds on the mass difference of the first Kaluza-Klein mode of fermions and the lightest Kaluza-Klein mode, which will be tested at the LHC.

Constraints on split universal extra dimensions from electroweak precision tests

We present a calculation of the next-to-leading-order electroweak corrections to W+ production, including the leptonic decay of the W boson and taking into account all off-shell effects of the W boson, where the finite width of the W boson is implemented using the complex-mass scheme. Corrections induced by incoming photons are fully included and find particular emphasis in the discussion of phenomenological predictions for the LHC. The corresponding next-to-leading-order QCD corrections are reproduced as well. In order to separate hard photons from jets, a quark-to-photon fragmentation functionla Glover and Morgan is employed. Our results are implemented into Monte Carlo programs allowing for the evaluation of arbitrary differential cross sections. We present integrated cross sections for the LHC at 7TeV, 8TeV, and 14TeV as well as differential distributions at 14TeV for bare muons and dressed leptons. Finally, we discuss the impact of anomalous WW couplings.

/pdf/nlo-qcd-and-electroweak-corrections-to-z-g-production-with-usxgwszn9o.pdf

NLO QCD and electroweak corrections to Z + γ production with leptonic Z-boson decays

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NLO QCD and electroweak corrections to $Z+\gamma$ production with leptonic Z-boson decays

Constraints on split universal extra dimensions from electroweak precision tests

NLO QCD and electroweak corrections to Z + γ production with leptonic Z-boson decays