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The ratio of di-lepton production cross sections on a proton, using the $gamma prightarrow l^+ l^- p$ process, above and below di-muon production threshold allows to extract the effective lepton-proton interaction, which is required to be identical for electrons and muons if lepton universality is exact. To test for a scenario of broken universality at the percent level, of the size which could explain the different proton charge radii extracted from electron scattering and from muonic hydrogen spectroscopy, we evaluate all one-loop QED corrections to this process, including the full lepton mass dependencies. We furthermore show that two-photon exchange processes with both photons attached to the proton line vanish after averaging over di-lepton angles, and estimate the relatively small radiation off the proton. We compare the full one-loop calculation with a soft-photon approximation of the same order, and present estimates for a planned experiment.
We develop the full finite lepton mass formalism for the production of real photons via the Bethe-Heitler reaction of unpolarized leptons off unpolarized nucleons. Genuine lepton mass effects are described, in particular their dependence upon the lep
A calculation of the loop-induced gluon-fusion process gg --> Z(photon)Z(photon) --> l anti-l l anti-l is presented, which provides an important background for Higgs boson searches in the H --> ZZ channel at the LHC. We find that the photon contribut
We present the cross section for $e^{+}e^{-}to hZ$ with arbitrary sets of electron and $Z$ boson polarizations at the full next-to-leading order in various extended Higgs models, such as the Higgs singlet model (HSM), the inert doublet model (IDM) an
We calculate O(alpha_s) two-loop virtual corrections to the differential decay width dGamma(B --> X_s l^+ l^-)/ds, where s is the invariant mass squared of the lepton pair. We also include those contributions from gluon bremsstrahlung which are neede
In this work, the two-photon-exchange (TPE) effects in $eprightarrow enpi^+$ at small $-t$ are discussed within a hadronic model. The TPE contributions to the amplitude and the unpolarized differential cross section are both estimated and we find tha