Determination of -Vcb- from exclusive B decays at Belle II

The Standard Model of Particle Physics (SM) contains the known, fundamental particles the quarks and the leptons and describes the interactions between them. It has existed in its main features since the 1960ies and has been confirmed with high precision in any ground based experiment up to date. However, we also know from astronomy, that the particles of the SM make up only about 5% of the energy content of the universe. The search for the physics beyond the SM is thus the prime task of modern particle physics. A promising approach in that direction are precision experiments that look for minute deviations from the SM and then interpret them in terms of new physics. For this, however, also the SM prediction must be known with high precision. One problem in that regard is the magnitude of the Cabibbo-Kobayabashi-Maskawa matrix element Vcb, a fundamental parameter of the SM, that can currently be extracted from experimental data using two different methods, each yielding about 1-2% precision. Unfortunately, these methods, referred to as exclusive and inclusive in literature, give values of Vcb that differ by about three times their experimental uncertainty. The aim of this project, is to revisit the exclusive determination of Vcb using new ideas and also a new, independent data set. Our analysis will be based on the data collected by the Belle II experiment, located at the Japanese center for particle research KEK in Tsukuba. Belle II has been taking physics data since the year 2019. Our group in Vienna made major contributions to the construction of the experiment. In particular we developed the design and led the construction of the silicon strip detector SVD.

The main theme in particle physics today is the search for new physics beyond the Standard Model. For this purpose, either particle collisions at the highest energies or rare processes or small deviations from standard theory at lower energies can be examined. The Belle II experiment takes the second approach. However, there is a problem if the parameters of the standard model are not precisely known, as deviations cannot then be measured. One such problem concerns the parameter Vcb for which different measurements have given different values. In this project, Vcb was measured once with the decay B --> D*lnu and the Belle II data and the experimental discrepancy was actually reduced. A second measurement with the decay B --> Dlnu is still being evaluated internally and will also help to further clarify the situation. The Belle II experiment is located at the Japanese particle research center KEK and is operated by a collaboration of around 1,000 scientists. It records the collisions of electrons and positrons at an energy of 10.58 GeV (which is relatively low by particle physics standards) and examines the decays of the B and D mesons and the tau leptons that are created. Through precision measurements of these decays, it is hoped to find deviations from the standard model of particle physics and thus conclusions about physics at higher energy scales. To date, around 500 million collisions with B mesons have been recorded. By the end of the program in the mid-2030s, this number is expected to increase by around 2 orders of magnitude.