School of Physics - Theses
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ItemMeasurement of R(D) and R(D*) with a semileptonic tag at the Belle experiment( 2019)In recent years, several measurement of the ratios R(D) = B(B ̄ → Dτ−ν ̄τ )/B(B ̄ → Dl−ν ̄l )andR(D∗)=B(B ̄→D∗τ−ν ̄τ )/B(B ̄→D∗l−ν ̄l ),whereldenotesanelectron or a muon, have obtained results in disagreement with the theoretical predictions of the Standard Model (SM) of particle physics. When combining all experimental results together, the statistical significance of the discrepancy with the SM accounts to 4σ. If confirmed, this discrepancy would directly require an extension of the SM with new physics theories, to accommodate the experimental results. This thesis reports a new independent measurement of the ratios R(D) and R(D∗). The results are based on a data sample containing 772 × 106 BB ̄ pairs recorded at the Υ (4S ) resonance with the Belle detector at the KEKB e+ e− collider in Japan. The analysis uses a semileptonic reconstruction of the tag-side B-meson and leptonic τ decays. The measured values are R(D) = 0.307±0.037 (stat.) ± 0.016 (syst.) and R(D∗) = 0.283 ± 0.018 (stat.) ± 0.014 (syst.), which are in agreement with the SM predictions within 0.2σ and 1.1σ respectively. The R(D)-R(D∗) combined result is in agreement with the SM predictions within 1.2σ. The discrepancy of the experimental world average with the SM expectations de- creases from 4 to 3σ when including these latest results, which represent to most precise measurement of R(D) and R(D∗) ever performed.