Measurement of the $tau$ lepton polarization and $R(D^*)$ in the decay $bar{B} rightarrow D^* tau^- bar{ u}_tau$ with one-prong hadronic $tau$ decays at Belle

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.

Measurement of the $tau$ lepton polarization and $R(D^*)$ in the decay $bar{B} to D^* tau^- bar{ u}_tau$

We report the first measurement of the $tau$ lepton polarization $P_tau(D^*)$ 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, and the $tau$ is reconstructed in the modes $tau^- rightarrow pi^- u_tau$ and $tau^- rightarrow rho^- u_tau$. We use the full data sample of $772 times 10^6$ $B{bar B}$ pairs recorded with the Belle detector at the KEKB electron-positron collider. 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.

Measurement 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

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.

Studies of charmed strange baryons in the $Lambda D$ final state at Belle

We report the discovery of $Xi_{c}(3055)^{0}$, observed by its decay into the final state $Lambda D^{0}$, and present the first observation and evidence of the decays of $Xi_c(3055)^{+}$ and $Xi_c(3080)^{+}$ into $Lambda D^{+}$. We also perform a combined analysis of the $Lambda D^{+}$ with the $Sigma_{c}^{++}K^{-}$ and $Sigma_{c}^{ast ++}K^{-}$ decay modes to measure the ratios of branching fractions, masses and widths with improved accuracy. We measure the ratios of branching fractions ${cal B}(Xi_{c}(3055)^{+} to Lambda D^{+})/{cal B}(Xi_{c}(3055)^{+} to Sigma_{c}^{++}K^{-})=5.09pm1.01pm0.76$, ${cal B}(Xi_{c}(3080)^{+} to Lambda D^{+})/{cal B}(Xi_{c}(3080)^{+} to Sigma_{c}^{++}K^{-})=1.29pm0.30pm0.15$, and ${cal B}(Xi_{c}(3080)^{+} to Sigma_{c}^{ast ++}K^{-})/{cal B}(Xi_{c}(3080)^{+} to Sigma_{c}^{++}K^{-})=1.07pm0.27pm0.01$, where the uncertainties are statistical and systematic. The analysis is performed using a 980 fb$^{-1}$ data sample collected with the Belle detector at the KEKB asymmetric-energy $e^{+}e^{-}$ collider.

Search for doubly charmed baryons and study of charmed strange baryons at Belle

We report results of a study of doubly charmed baryons and charmed strange baryons. The analysis is performed using a 980 fb^-1 data sample collected with the Belle detector at the KEKB asymmetric-energy e^+e^- collider. We search for doubly charmed baryons Xi_cc^+(+) with the Lambda_c^+K^-pi^+(pi^+) and Xi_c^0pi^+(pi^+) final states. No significant signal is observed. We also search for two excited charmed strange baryons, Xi_c(3055)^+ and Xi_c(3123)^+ with the Sigma_c^++(2455)K^- and Sigma_c^++(2520)K^- final states. The Xi_c(3055)^+ signal is observed with a significance of 6.6 standard deviations including systematic uncertainty, while no signature of the Xi_c(3123)^+ is seen. We also study properties of the Xi_c(2645)^+ and measure a width of 2.6 +- 0.2 (stat) +- 0.4 (syst) MeV/c^2, which is the first significant determination.

First observation of exclusive $Upsilon(1S)$ and $Upsilon(2S)$ decays into light hadrons

Using samples of 102 million $Upsilon(1S)$ and 158 million $Upsilon(2S)$ events collected with the Belle detector, we study exclusive hadronic decays of these two bottomonium resonances to the three-body final states $phi K^+ K^-$, $omega pi^+ pi^-$ and $K^{ast 0}(892) K^- pi^+ $, and to the two-body Vector-Tensor ($phi f_2(1525)$, $omega f_2(1270)$, $rho a_2(1320)$ and $K^{ast 0}(892) bar{K}_2^{ast 0}(1430) $) and Axial-vector-Pseudoscalar ($K_1(1270)^+ K^-$, $K_1(1400)^+ K^- $ and $b_1(1235)^+ pi^- $) pairs. Signals are observed for the first time in the $Upsilon(1S) to phi K^+ K^-$, $omega pi^+ pi^-$, $K^{ast 0} K^- pi^+$, $K^{ast0} K_2^{ast 0}$ and $Upsilon(2S) to phi K^+ K^-$, $K^{ast 0} K^- pi^+$ decay modes. Branching fractions are determined for all the processes, while 90% confidence level upper limits are established on the branching fractions for the modes with a statistical significance less than $3sigma$. The ratios of the branching fractions of $Upsilon(2S)$ and $Upsilon(1S)$ decays into the same final state are used to test a perturbative QCD prediction for OZI suppressed bottomonium decays.

A novel type of proximity focusing RICH counter with multiple refractive index aerogel radiator

A proximity focusing ring imaging Cherenkov detector, with the radiator consisting of two or more aerogel layers of different refractive indices, has been tested in 1-4 GeV/c pion beams at KEK. Essentially, a multiple refractive index aerogel radiator allows for an increase in Cherenkov photon yield on account of the increase in overall radiator thickness, while avoiding the simultaneous degradation in single photon angular resolution associated with the increased uncertainty of the emission point. With the refractive index of consecutive layers suitably increasing in the downstream direction, one may achieve overlapping of the Cherenkov rings from a single charged particle. In the opposite case of decreasing refractive index, one may obtain well separated rings. In the former combination an approximately 40% increase in photon yield is accompanied with just a minor degradation in single photon angular resolution. The impact of this improvement on the pion/kaon separation at the upgraded Belle detector is discussed.