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Register a new userMeasurement of the branching ratio of $bar{B}^0 rightarrow D^{*+} tau^- bar{ u}_{tau}$ relative to $bar{B}^0 rightarrow D^{*+} ell^- bar{ u}_{ell}$ decays with a semileptonic tagging method
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We report a measurement of ratio ${cal R}(D^*) = {cal B}(bar{B}^0 rightarrow D^{*+} tau^- bar{ u}_{tau})/{cal B}(bar{B}^0 rightarrow D^{*+} ell^- bar{ u}_{ell})$, where $ell$ denotes an electron or a muon. The results are based on a data sample containing $772times10^6$ $Bbar{B}$ pairs recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. We select a sample of $B^0 bar{B}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*mp} ell^{pm}$. We measure ${cal R}(D^*)= 0.302 pm 0.030({rm stat)} pm 0.011({rm syst)}$, which is within $1.6 sigma$ of the Standard Model theoretical expectation, where $sigma$ is the standard deviation including systematic uncertainties.
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We report a measurement of the ratio ${cal R}(D^*) = {cal B}(bar{B}^0 rightarrow D^{*+} tau^- bar{ u}_{tau})/{cal B}(bar{B}^0 rightarrow D^{*+} ell^- bar{ u}_{ell})$, where $ell$ denotes an electron or a muon. The results are based on a data sample containing $772times10^6$ $Bbar{B}$ pairs recorded at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB $e^+ e^-$ collider. We select a sample of $B^0 bar{B}^0$ pairs by reconstructing both $B$ mesons in semileptonic decays to $D^{*mp} ell^{pm}$. We measure ${cal R}(D^*)= 0.302 pm 0.030({rm stat)} pm 0.011({rm syst)}$, which is within $1.6 sigma$ of the Standard Model theoretical expectation, where the standard deviation $sigma$ includes systematic uncertainties. We use this measurement to constrain several scenarios of new physics in a model-independent approach.
We report a measurement of the branching fraction ratios R(D(*)) of Bbar -> D(*) tau- nubar_tau relative to Bbar -> D()* l- nubar_l (where l = e or mu) using the full Belle data sample of 772 x 10^6 BBbar pairs collected at the Y(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. The measured values are R(D)= 0.375 +- 0.064(stat.) +- 0.026(syst.) and R(D*) = 0.293 +- 0.038(stat.) +- 0.015(syst.). The analysis uses hadronic reconstruction of the tag-side B meson and purely leptonic tau decays. The results are consistent with earlier measurements and do not show a significant deviation from the standard model prediction.
We evaluate long-distance electromagnetic (QED) contributions to $bar{B}{}^0 to D^+ tau^{-} bar{ u}_{tau}$ and $B^- to D^0 tau^{-} bar{ u}_{tau}$ relative to $bar{B}{}^0 to D^+ mu^{-} bar{ u}_{mu}$ and $B^- to D^0 mu^{-} bar{ u}_{mu}$, respectively, in the standard model. We point out that the QED corrections to the ratios $R(D^{+})$ and $R(D^{0})$ are not negligible, contrary to the expectation that radiative corrections are almost canceled out in the ratio of the two branching fractions. The reason is that long-distance QED corrections depend on the masses and relative velocities of the daughter particles. We find that theoretical predictions for $R(D^{+})^{tau/mu}$ and $R(D^{0})^{tau/mu}$ can be amplified by $sim4%$ and $sim3%$, respectively, for the soft-photon energy cut in range $20$-$40$ MeV.
We report the first measurement of the $tau$ lepton polarization in the decay ${bar B} rightarrow D^* tau^- {bar u_{tau}}$ as well as a new measurement of the ratio of the branching fractions $R(D^{*}) = mathcal{B}({bar B} rightarrow D^* tau^- {bar u_{tau}}) / mathcal{B}({bar B} rightarrow D^* ell^- {bar u_{ell}})$, where $ell^-$ denotes an electron or a muon, with the decays $tau^- rightarrow pi^- u_{tau}$ and $tau^- rightarrow rho^- u_{tau}$. We use the full data sample of $772 times 10^6$ $B{bar B}$ pairs accumulated with the Belle detector at the KEKB electron-positron collider. Our preliminary results, $R(D^*) = 0.276 pm 0.034{rm (stat.)} ^{+0.029} _{-0.026}{rm (syst.)}$ and $P_{tau} = -0.44 pm 0.47 {rm (stat.)} ^{+0.20} _{-0.17} {rm (syst.)}$, are consistent with the theoretical predictions of the Standard Model within $0.6$ standard deviation.
With the full data sample of $772 times 10^6$ $B{bar B}$ pairs recorded by the Belle detector at the KEKB electron-positron collider, the decay $bar{B} rightarrow D^* tau^- bar{ u}_tau$ is studied with the hadronic $tau$ decays $tau^- rightarrow pi^- u_tau$ and $tau^- rightarrow rho^- u_tau$. The $tau$ polarization $P_tau(D^*)$ in two-body hadronic $tau$ decays is measured, as well as the ratio of the branching fractions $R(D^{*}) = mathcal{B}(bar {B} rightarrow D^* tau^- bar{ u}_tau) / mathcal{B}(bar{B} rightarrow D^* ell^- bar{ u}_ell)$, where $ell^-$ denotes an electron or a muon. Our results, $P_tau(D^*) = -0.38 pm 0.51 {rm (stat)} ^{+0.21}_{-0.16} {rm (syst)}$ and $R(D^*) = 0.270 pm 0.035{rm (stat)} ^{+0.028}_{-0.025}{rm (syst)}$, are consistent with the theoretical predictions of the Standard Model. The polarization values of $P_tau(D^*) > +0.5$ are excluded at the 90% confidence level.
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