Search for New Physics and the precision determination of the CKM matrix element |Vub| with the Belle II experiment

• Applicant: Professor Dr. Florian Bernlochner
• Subject Area: Nuclear and Elementary Particle Physics, Quantum Mechanics, Relativity, Fields
• Term: from 2016 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 316996433

Matter and interactions of elementary particles in nature are so far successfully described by the Standard Model of particle physics. Albeit this success of the Standard Model, a range of physical phenomena observed in nature cannot be explained and a prominent example is the overabundance of matter over anti-matter in the universe. At the end of the year 2018 the super flavour factory experiment Belle II will start to record large samples of B-meson decays. The precision study of B-mesons is interesting as their decay allows for measuring the size of Charge-Parity violation and thus probe the matter and anti-matter asymmetry in particle decays. The size of Charge-Parity violation cannot be determined by a single measurement, but by the combination of several precision observables and any disagreement would be a strong indication for new physics processes beyond the Standard Model. The goal of this proposal is to carry out a precision measurement of the CKM matrix element Vub, whose absolute value is one of the important constraints in the extraction of the size of Charge-Parity violation. Experimentally Vub can be measured by studying semileptonic B-meson decays obtained by studying exclusive or inclusive hadronic final states. There is a long-standing tension between both approaches with a statistical significance of 3.4 standard deviations. One possible reason for this tension is the use of model functions to describe the b-quark motion inside the B-meson in state-of-the-art determinations using inclusive decays. To eliminate this possibility, this application proposes a coherent strategy how this functional form can be extracted simultaneously with Vub in a global analysis of measured kinematic distributions of radiative and semileptonic B-meson decays measured with the Belle II experiment. As part of the preparatory work and to establish the necessary experimental techniques a first analysis using Belle data will be carried out. The direct experimental determination of the functional form of the b-quark motion inside the B-meson will remove the model assumptions made in the state-of-the-art inclusive Vub measurements which in turn will help to shed light on the observed differences between inclusive and exclusive Vub measurements. In addition the shape function can be used to search for new physics contributions in radiative decays in a model independent way.

• DFG Programme: Independent Junior Research Groups