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.
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ItemInvestigation of B+ mesons decay to K+K−π+ at the Belle experiment(2017)Charmless decays of B mesons to three charged hadrons are suppressed in the Standard Model, and thus provide an opportunity to search for physics beyond the Standard Model. An unexpected excess and a large CP asymmetry in the low invariant mass spectrum of the K+K− system for the decay B+ → K+K−π+ were observed by BaBar and LHCb in recent years. We present the measurements of branching fraction and direct CP asymmetry of the charmless decay B+ → K+K−π+. This analysis is performed on a data sample of 772 × 10^6BB pairs produced at the Υ(4S) resonance by the KEKB asymmetric-energy e+e− collider and collected by the Belle detector. We perform a blind analysis, examining signal reconstruc- tion and background suppression with Monte Carlo simulated samples, and extract signal yield and direct CP asymmetry with a 2D extended maximum likelihood fit to the data. The measured branching fraction and direct CP asymmetry are B(B+ → K+K−π+) = (5.38 ± 0.40 ± 0.35) × 10^−6 and ACP = −0.170 ± 0.073 ± 0.017, respectively, where the first uncertainties are statistical and the second are systematic. These results are in agreement with the current world average. We extract the branching fraction and direct CP asymmetry as a function of the K+K− invariant mass. The K+K− invariant mass distribution of the signal candidates shows an excess in the region below 1.5 GeV/c^2, which is consistent with the previous studies from BaBar and LHCb. Strong evidence of a large direct CP asymmetry of −0.90 ± 0.17 ± 0.03 with 4.8σ significance is found in the K+K− low-invariant-mass region.