Sophie Kollatzsch

We present disperon QED, a method to deal with data input in loop processes in Monte Carlo codes. It relies on dispersion relations, automated tools such as OpenLoops, effective field theory methods and a threshold subtraction. We motivate this method and apply it to the process ee\to\pi\pi e e → π π in McMule to deal with hadronic vacuum polarisation insertions in two-loop contributions as well as the vector form factor of the pion within the form-factor scalar QED approximation. The generality of this method for more complicated processes is emphasised.

We include electroweak effects in Møller scattering at low energies in an effective field theory approach and compute the left-right parity-violating asymmetry. The calculation using low-energy effective field theory provides a solid framework to integrate out heavy particles with masses of the order of the electroweak scale, allowing the resummation of all large logarithms between the electroweak scale and the scale, where QCD perturbation theory breaks down. The NLO electroweak corrections with leading logarithmic resummation, combined with QED corrections at NNLO and hadronic effects are implemented into the Monte Carlo framework McMule. Thus, we obtain a fully differential description and present results adapted to the MOLLER experiment. The potential impact of large logarithms at the next-to-leading logarithmic level is investigated.

We present the results of Phase I of an ongoing review of Monte Carlo tools relevant for low-energy hadronic cross sections. This includes a detailed comparison of Monte Carlo codes for electron–positron scattering into a muon pair, pion pair, and electron pair, for scan and radiative-return experiments. After discussing the various approaches that are used and effects that are included, we show differential cross sections obtained with AfkQed, BabaYaga@NLO, KKMC, MCGPJ, McMule, Phokhara, and Sherpa, for scenarios that are inspired by experiments providing input for the dispersive evaluation of the hadronic vacuum polarisation.

We present a fully differential calculation of lepton pair production, taking into account the dominant next-to-next-to-leading order QED corrections as well as next-to-leading order electroweak and polarisation effects. We include all lepton masses, hard photon emission, as well as non-perturbative hadronic corrections. The corresponding matrix elements are implemented in the Monte Carlo framework McMule. In order to obtain a numerically stable implementation, we extend next-to-soft stabilisation, a universal technique based on a next-to-leading-power expansion, to calculations with polarised leptons. As an example, we show results tailored to the Belle II detector with the current setup as well as a potential future configuration that includes polarised beams.