PoS - Proceedings of Science
Volume 340 - The 39th International Conference on High Energy Physics (ICHEP2018) - Parallel: Beyond the Standard Model
New results on R(D) and R(D*) from Belle
K. Matsuoka* and  On behalf of the BELLE Collaboration
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Published on: August 02, 2019
Indications of lepton flavor universality violation in the decays \(B \to D^{(*)}\tau\nu\) have triggered substantial interest and could be a hint for new physics. Useful observables to probe new physics in these decays are the ratios of the branching fractions, \(R(D^{(*)}) \equiv \mathcal{B}(\bar{B} \to D^{(*)}\tau\bar{\nu}_{\tau})/\mathcal{B}(\bar{B} \to D^{(*)} \ell \bar{\nu}_{\ell})\), where \(\ell = e\) or \(\mu\), and the polarizations of the \(\tau\) lepton and \(D^*\) meson. This article summarizes analyses on those observables in the Belle experiment at the KEKB \(e^+ e^-\) collider based on the full data set recorded by the Belle detector at the \(\Upsilon(4S)\) resonance containing 772 million \(B\bar{B}\) pairs. The analyses utilize different methods to reconstruct or tag the accompanying \(B\) meson other than the \(B\) of the semitauonic decay: semileptonic tagging for the simultaneous measurement of \(R(D)\) and \(R(D^*)\), and hadronic and inclusive ones for the measurement of the polarizations. In terms of the lepton flavor universality test, the ratio of the branching fractions of the purely leptonic decays, \(\mathcal{B}(B^- \to \tau^- \bar{\nu}_{\tau})/\mathcal{B}(B^- \to \mu^- \bar{\nu}_{\mu})\), is also interesting because its theoretical uncertainty is small, though the \(B^- \to \mu^- \bar{\nu}_{\mu}\) decay has not been discovered yet. Belle searched for this decay, and the result is also overviewed in this article.
DOI: https://doi.org/10.22323/1.340.0567
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