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Abstract Lepton universality (LU) typically refers to the lepton coupling, which is considered to be the same fore,μ, andτleptons, if the interaction is electroweak according to the Standard Model, and it is hence a compelling probe for New Physics. The same principle of universal electroweak lepton interaction leads to the expectation that lepton scattering yields are equal foreandμbeams under the same kinematic condition. The mere mass difference betweeneandμaffects kinematic quantities (such as the relation between scattering angle andQ²), and the lepton mass dependence of elastic cross sections for leptons scattered from structured and pointlike objects are taken into account. By comparinge⁺,e⁻,μ⁺, andμ⁻scattering yields, two-photon exchange (TPE) effects, universal or not, can be separated from the general LU test of thee/μyield ratio. With its separable mixed beams ofe⁺/μ⁺ande⁻/μ⁻, respectively, the MUSE experiment at PSI is not only designed to measure the proton charge radius with four lepton species, but is also uniquely suited to probe TPE and LU, while benefitting from partial cancellations of certain shared systematics. An overview will be given of the MUSE experiment, the sensitivity, and the present status. 

Abstract Radiative corrections are crucial for modern high-precision physics experiments, and are an area of active research in the experimental and theoretical community. Here we provide an overview of the state of the field of radiative corrections with a focus on several topics: lepton–proton scattering, QED corrections in deep-inelastic scattering, and in radiative light-hadron decays. Particular emphasis is placed on the two-photon exchange, believed to be responsible for the proton form-factor discrepancy, and associated Monte-Carlo codes. We encourage the community to continue developing theoretical techniques to treat radiative corrections, and perform experimental tests of these corrections.