In a recent study published in the journal Physical Review Letters, an international group of scientists from Belle II Collaboration made the first direct measurement of the ratio of tau-outleptons to light leptons in inclusive decays of B-Mesons. This experiment is aimed at checking the universality of weak interactions of charged leptons, a key position of the standard model of physics of elementary particles.
The standard model claims that all charged leptons (electron, muon, and tau-reton) interact the same with electromagnetic and weak forces. However, any deviations from this may indicate the existence of new physics outside the standard model. Of particular interest is the comparison of tau-knobs with lighter muons and electrons, as previous dimensions showed disagreements between experimental data and theoretical forecasts.
Unlike previous studies that focused on decays with one charm, the study at BELLE II includes all types of adrones and their number. The latest such measurement was carried out more than 20 years ago on a large electron-positron collider (LP) at CERN.
The Belle II experiment uses B-Meson pairs formed in the collisions of electrons and positrons. Some of the mesons are completely reconstructed, and then light leptons are examined, which can occur both directly from the decay of the B-Meson and through Tau-Reton. To distinguish the leptons that occur during the decay of tau-heptons, the distribution of their impulse and energy is used. These indicators help to distinguish them from leptons arising directly from the decay of B-Mesons.
BELLE II analysis made it possible to determine this ratio, despite the presence of systematic uncertainty that will decrease as data accumulation. The current results are consistent with both predictions of the standard model and with anomalies observed in exclusive decays. This study paves the way for further inspections of the universality of leptons and the search for new physics.
The BELLE II project plans to continue collecting data and updating the measurement results in the coming years, which will determine with high accuracy whether the universality of leptons in these decays is violated.