We report a measurement of the amplitude ratio \(r_S\) of \(B^0 \to D^0K^{*0}\) and \(B^0 \to \bar{D^0}K^{*0}\) decays with a model-independent Dalitz plot analysis using \(D\to K_S^0\pi^+\pi^-\) ...decays. Using the full data sample of \(772\times10^6\) \(B\bar{B}\) pairs collected at the \(\Upsilon(4S)\) resonance with the Belle detector at KEKB accelerator the upper limit is \(r_S < 0.87\) at the 68 % confidence level. This result is the first measurement of \(r_S\) with a model-independent Dalitz analysis, and is consistent with results from other analyses. The value of \(r_S\) indicates the sensitivity of the decay to \(\phi_3\) because the statistical uncertainty is proportional to \(1/r_S\). The \(r_S\) result is obtained from observables (\(x_\pm\), \(y_\pm\)) \begin{eqnarray} x_- &=& +0.4 ^{+1.0 +0.0}_{-0.6 -0.1} \pm0.0 \\ y_- &=& -0.6 ^{+0.8 +0.1}_{-1.0 -0.0} \pm0.1 \\ x_+ &=& +0.1 ^{+0.7 +0.0}_{-0.4 -0.1} \pm0.1 \\ y_+ &=& +0.3 ^{+0.5 +0.0}_{-0.8 -0.1} \pm0.1 \\ , \end{eqnarray} where \(x_\pm = r_S \cos(\delta_S \pm \phi_3)\), \(y_\pm = r_S \sin(\delta_S \pm \phi_3)\) and \(\phi_3 (\delta_S)\) are the weak (strong) phase difference between \(B^0 \to D^0K^{*0}\) and \(B^0 \to \bar{D^0}K^{*0}\). The first uncertainty is statistical, the second is the experimental systematic and the third is the systematic due to the uncertainties on \(c_i\) and \(s_i\) parameters measured by CLEO.
We report a measurement of the branching fraction of \(B^+ \to \tau^+ \nu_\tau\) decays using a data sample of \(772 \times 10^6 B \bar{B}\) pairs, collected at the \(\Upsilon(4S)\) resonance with ...the Belle detector at the KEKB asymmetric-energy \(e^+e^-\) collider. We reconstruct the accompanying \(B\) meson in a semileptonic decay and detect the \(B^+ \to \tau^+ \nu_\tau\) candidate in the recoiling event. We obtain a branching fraction of \({\cal B}(B^+ \to \tau^+ \nu_\tau) = 1.25 \pm 0.28 ({\rm stat.}) \pm 0.27({\rm syst.}) \times 10^{-4}\). This result is in good agreement with previous measurements and the expectation from calculations based on the Standard Model.
The Aerogel Ring Imaging Cherenkov (ARICH) counter serves as a particle identification device in the forward end-cap region of the Belle II spectrometer. It is capable of identifying pions and kaons ...with momenta up to \(4 \, {\rm GeV}/c\) by detecting Cherenkov photons emitted in the silica aerogel radiator. After the detector alignment and calibration of the probability density function, we evaluate the performance of the ARICH counter using early beam collision data. Event samples of \(D^{\ast +} \to D^0 \pi^+ (D^0 \to K^-\pi^+)\) were used to determine the \(\pi(K)\) efficiency and the \(K(\pi)\) misidentification probability. We found that the ARICH counter is capable of separating kaons from pions with an identification efficiency of \(93.5 \pm 0.6 \, \%\) at a pion misidentification probability of \(10.9 \pm 0.9 \, \%\). This paper describes the identification method of the counter and the evaluation of the performance during its early operation.
We report the results of a search for the \(X(1835)\) state in the process \(e^+e^-\to J/\psi+X(1835)\) using a data sample of 672 fb\(^{-1}\) collected with the Belle detector at and near the ...\(\Upsilon(4S)\) resonance at the KEKB asymmetric-energy \(e^+e^-\) collider. No significant evidence is found for this process, and an upper limit is set on its cross section times the branching fraction: \(\sigma_{\rm Born}(e^+e^- \to J/\psi X(1835)) \cdot\) {\({\cal B}(X(1835)\to \ge 3\) charged tracks)} \(< 1.3 \ {\rm fb}\) at 90% confidence level.
We study the rare hadronic transitions \(\Upsilon(2S)\rightarrow \Upsilon(1S)\eta\) and \(\Upsilon(2S)\rightarrow \Upsilon(1S)\pi^0\) using a sample of 158 \(\times 10^6\) \(\Upsilon(2S)\) decays ...collected with the Belle detector at the KEKB asymmetric-energy \(e^+ e^-\) collider. We observe the \(\eta\) meson decay to \(\gamma\gamma\) and \(\pi^+\pi^-\pi^0\) final states; the \(\Upsilon(1S)\) is reconstructed in the \(\mu^+\mu^-\) and \(e^+e^-\) decay modes. We measure the ratios of branching fractions (\({\mathcal B}\)) \(\frac{{\mathcal B}(\Upsilon(2S)\rightarrow\Upsilon(1S)\eta)}{{\mathcal B}(\Upsilon(2S)\rightarrow\Upsilon(1S)\pi^+\pi^-)}\) = (1.99\(\pm\)0.14 (stat) \(\pm\)0.11 (syst)) \(\times 10^{-3}\) and \(\frac{{\mathcal B}(\Upsilon(2S)\rightarrow\Upsilon(1S)\pi^0)}{{\mathcal B}(\Upsilon(2S)\rightarrow\Upsilon(1S)\pi^+\pi^-)} < 2.3 \times 10^{-4}\) at the 90% confidence level (CL). Assuming the value \({\mathcal B}(\Upsilon(2S) \rightarrow \Upsilon(1S)\pi^-\pi^+)\) = (17.92\(\pm\)0.26)%, we obtain \( {\mathcal B}(\Upsilon(2S)\rightarrow\Upsilon(1S)\eta) = (3.57 \pm 0.25 ({\rm stat})\ \pm 0.21 ({\rm syst}))\times 10^{-4} \) and \( {\mathcal B}(\Upsilon(2S)\rightarrow\Upsilon(1S)\pi^0) < 4.1\times 10^{-5}\ ({\rm 90%\ CL}). \)
We have developed a new type of particle identification device, called an Aerogel Ring Imaging Cherenkov (ARICH) counter, for the Belle II experiment. It uses silica aerogel tiles as Cherenkov ...radiators. For detection of Cherenkov photons, Hybrid Avalanche Photo-Detectors (HAPDs) are used. The designed HAPD has a high sensitivity to single photons under a strong magnetic field. We have confirmed that the HAPD provides high efficiency for single-photon detection even after exposure to neutron and gamma-ray radiation that exceeds the levels expected in the 10-year Belle II operation. In order to confirm the basic performance of the ARICH counter system, we carried out a beam test at the DESY using a prototype of the ARICH counter with six HAPD modules. The results are in agreement with our expectations and confirm the suitability of the ARICH counter for the Belle II experiment. Based on the in-beam performance of the device, we expect that the identification efficiency at 3.5 GeV/c is 97.4% and 4.9% for pions and kaons, respectively. This paper summarizes the development of the HAPD for the ARICH and the evaluation of the performance of the prototype ARICH counter built with the final design components.
JHEP 10 (2014) 165 We present a measurement of the time-dependent $CP$ violation parameters in
$B^0\to\eta'K^0$ decays. The measurement is based on the full data sample
containing $772\times 10^6$ ...$B\bar{B}$ pairs collected at the $\Upsilon(4S)$
resonance using the Belle detector at the KEKB asymmetric-energy $e^+e^-$
collider. The measured values of the mixing-induced and direct $CP$ violation
parameters are: \begin{align} \sin 2 \phi^{\rm eff}_1 &= +0.68\pm 0.07 \pm
0.03, \nonumber \\ \mathcal{A}_{\eta'K^0} &= +0.03\pm 0.05\pm 0.04, \nonumber
\end{align} where the first uncertainty is statistical and the second is
systematic. The values obtained are the most accurate to date. Furthermore,
these results are consistent with our previous measurements and with the
world-average value of $\sin 2\phi_1$ measured in $B^0\to J/\psi K^0$ decays.}
We present a measurement of the time-dependent \(CP\) violation parameters in \(B^0\to\eta'K^0\) decays. The measurement is based on the full data sample containing \(772\times 10^6\) \(B\bar{B}\) ...pairs collected at the \(\Upsilon(4S)\) resonance using the Belle detector at the KEKB asymmetric-energy \(e^+e^-\) collider. The measured values of the mixing-induced and direct \(CP\) violation parameters are: \begin{align} \sin 2 \phi^{\rm eff}_1 &= +0.68\pm 0.07 \pm 0.03, \nonumber \\ \mathcal{A}_{\eta'K^0} &= +0.03\pm 0.05\pm 0.04, \nonumber \end{align} where the first uncertainty is statistical and the second is systematic. The values obtained are the most accurate to date. Furthermore, these results are consistent with our previous measurements and with the world-average value of \(\sin 2\phi_1\) measured in \(B^0\to J/\psi K^0\) decays.}
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