QED corrections and new physics effects in the determination of V_cb and V_ub for Belle II and LHCb

• Applicant: Dr. Christoph Bobeth
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2018 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 411263322

The research proposal addresses the calculation of QED corrections and the study of new physics effects to semileptonic charged-current exclusive and inclusive decays of B mesons governed by b -> (u,c) lepton (l) neutrino (v) at the parton level. Considering that in the standard model of particle physics b -> (u,c) lv decays provide the most important way to determine the elements V_cb and V_ub of the quark mixing matrix, these corrections and studies will be crucial to interpret correctly the high precision measurements that will be available in the next years from the Belle II and LHCb experiments. The results of the proposal will allow to test the mechanism of quark flavor mixing in the SM at a higher level of accuracy and thereby putting stronger constraints on beyond-the-SM effects.In particular I will calculate QED corrections to exclusive b -> (u,c) lv decays in the SM adapting a novel method that I co-developed for the decay B_s -> mu mu. This method provides a systematic approach to QED corrections starting with short-distance scales of order of the b-quark mass and subsequently decoupling contributions at lower scales with the help of effective theories. I will also calculate QED corrections to the inclusive B -> X_c lv decay. The consistent implementation of these process-specific QED corrections in experimental analyses is non-trivial because currently QED Bremsstrahlung corrections are simulated using software tools like "PHOTOS" that however do not account for process-specific QED corrections. Part of this research proposal is devoted to resolve the issue.New Physics can affect the V_cb and V_ub determination from semileptonic charged-current processes b -> (u,c) lv. The SM effective theory (SMEFT) is a convenient framework to parametrize such effects. I will study a global fit of V_cb and V_ub in the framework of SMEFT, thereby taking into account the correlations that arise with other processes due to the additionally involved parameters as well as renormalization group constraints. The results will be important for explicit new physics models that fall into the class of SMEFT and currently must rely on values of V_cb and V_ub determined in the framework of the SM only.As an important part of my research project, all physics results will be implemented in the "EOS" software tool, of which I am co-developer, to provide them to the experimental and theorists community.

• DFG Programme: Research Grants