The paper "Test of lepton flavor universality with a measurement of R(D*) using hadronic B tagging at the Belle II experiment" to which Kazuki Kojima has contributed as the main analyst, was published in the Physical Review D.
In the standard model of elementary particles, lepton flavor universality postulates that the coupling between leptons and gauge bosons is identical for all lepton types. However, in the semileptonic decays of B mesons, previous experiments have reported evidence for violation of the lepton flavor universality in ratios of their branching fractions as an excess from the Standard Model prediction by 3.2 σ. This discrepancy could indicate new physics, such as leptoquarks.
In this paper, the ratio R(D*) was measured using 189 fb⁻¹ of new data collected by the Belle II experiment up to 2021 to search for violations of lepton flavor universality. As the lead analyst, Kazuki Kojima developed the data analysis framework for this study, including optimizing event selections, developing a signal extraction methods, estimating background events based on the real data, and establishing systematic uncertainty evaluation techniques. By introducing new B meson reconstruction methods using multivariate analysis and optimizing event selection criteria to improve signal reconstruction efficiency, we reduced the statistical uncertainty by approximately 40% compared to the previous Belle experiment when scaled to 189 fb⁻¹.
The measured R(D*) value was consistent with the Standard Model prediction, and no significant violation of lepton flavor universality was observed. Combining this measurement with the past measurements, the deviation has enhanced slightly from 3.2 σ to 3.3 σ. The combined experimental results are still suggesting an excess and possibilities of new physics.
Comment from Dr. Kojima:
Paper Information:
I. Adachi et al. "Test of lepton flavor universality with a measurement of R(D*) using hadronic B tagging at the Belle II experiment"
[Phys. Rev. D 110, 072020 (Oct. 2024). DOI: https://doi.org/10.1103/PhysRevD.110.072020]
