We present a study of rare decay modes $B^{+} \to D_{s}^{+}h^{0}$, $B^{+} \to
D_{s}^{\ast+}h^{0}$, and $B^{+} \to D^{+}h^{0}$, where $h^{0}$ denotes the
neutral mesons $\eta$ or $K^{0}$, using a data ...sample of $(772 \pm 10 ) \times
10^{6}$ $B\bar{B}$ events produced at the $\Upsilon(4S)$ resonance. The data
were collected by the Belle detector operating at the asymmetric-energy KEKB
collider. We observe no evidence for these decays, so we provide upper limits
at the 90$\%$ confidence level on the branching fractions of $B^{+} \to
D_{s}^{+}h^{0}$, $D_{s}^{\ast+}h^{0}$, and $D^{+}h^{0}$ decay modes. Along with
rare decay modes, we report improved measurements of the color-suppressed decay
branching fractions $\mathcal{B}(\bar{B}^{0} \to D^{0}\eta)$ = (26.6 $\pm$ 1.2
$\pm$ 2.1) $\times$ $10^{-5}$ and $\mathcal{B}(\bar{B}^{0} \to
D^{0}\bar{K}^{0})$ = (5.6 $\pm$ 0.5 $\pm$ 0.2) $\times$ $10^{-5}$. The first
and second uncertainties are statistical and systematic, respectively.
Using $(771.6 \pm 10.6) \times 10^6$ $B\bar{B}$ meson pairs recorded by the
Belle experiment at the KEKB $e^+ e^-$ collider, we report the branching
fractions $\mathcal{B}(\bar{B}^0 \to D^{*+} \pi^-) ...= (2.62 \pm 0.02 \pm
0.09)\times 10^{-3}$ and $\mathcal{B}(\bar{B}^0 \to D^{*+} K^-) = (2.22 \pm
0.06 \pm 0.08)\times 10^{-4}$; the quoted uncertainties are statistical and
systematic, respectively. A measurement of the ratio of these branching
fractions is also presented, $\mathcal{R}_{K/\pi}=\mathcal{B}(\bar{B} \to
D^{*+}K^-) /\mathcal{B}( \bar{B} \to D^{*+}\pi^-) = (8.41 \pm 0.24 \pm
0.13)\times 10^{-2}$, where systematic uncertainties due to the $D^{*+}$ meson
reconstruction cancel out. Furthermore, we report a new QCD factorization test
based on the measured ratios for $\bar{B} \to D^{*+}h^-$ and $\bar{B} \to
D^{*+}\ell^-\nu$ decays at squared momentum transfer values equivalent to the
mass of the $h = \pi$ or $K$ hadron. The parameters $|a_1(h)|$ are measured to
be $|a_1(\pi)|= 0.884 \pm 0.004 \pm 0.003 \pm 0.016$ and $|a_1(K)|=0.913 \pm
0.019 \pm 0.008 \pm 0.013$, where the last uncertainties account for all
external inputs. These values are approximately $15\%$ lower than those
expected from theoretical predictions. Subsequently, flavor $SU(3)$ symmetry is
tested by measuring the ratios for pions and kaons, $|a_1(K)|^2/
|a_1(\pi)|^2=1.066 \pm 0.042 \pm 0.018 \pm 0.023$, as well as for different
particle species. The ratio is consistent with unity and therefore no evidence
for $SU(3)$ symmetry breaking effects is found at the $5\%$ precision level.
We report the measurement of the two-photon decay width of $\chi_{c2}(1P)$ in
two-photon processes at the Belle experiment. We analyze the process $ \gamma
\gamma \rightarrow \chi_{c2}(1P) ...\rightarrow J/\psi\gamma$, $J/\psi \rightarrow
\ell^{+} \ell^{-}$ $(\ell = e \ {\rm or} \ \mu)$ using a data sample of 971
fb$^{-1}$ collected with the Belle detector at the KEKB $e^{+} e^{-}$ collider.
In this analysis, the product of the two-photon decay width of $\chi_{c2}(1P)$
and the branching fraction is determined to be $\Gamma_{\gamma
\gamma}(\chi_{c2}(1P)) \mathcal{B}( \chi_{c2}(1P) \rightarrow J/\psi \, \gamma
)\mathcal{B}( J/\psi \rightarrow \ell^+\ell^- ) = {\rm 14.8}$ $\pm$ ${\rm
0.3}({\rm stat.})$ $\pm$ ${\rm 0.7}({\rm syst.})$ eV, which corresponds to $
\Gamma_{\gamma \gamma}(\chi_{c2}(1P)) $ = 653 $\pm$ 13(stat.) $\pm$ 31(syst.)
$\pm$ 17(B.R.) eV, where the third uncertainty is from $\mathcal{B}(
\chi_{c2}(1P) \rightarrow J/\psi \, \gamma )$ and $\mathcal{B}( J/\psi
\rightarrow \ell^+\ell^- )$.
Phys. Rev. Lett. 131, 051804 (2023) We present the first measurement of the ratio of branching fractions of
inclusive semileptonic $B$-meson decays, $R(X_{e/\mu}) = \mathcal{B}(B\to X \,
e \, \nu) / ...\mathcal{B}(B\to X \, \mu \, \nu)$, a precision test of
electron-muon universality, using data corresponding to $189\,\mathrm{fb}^{-1}$
from electron-positron collisions collected with the Belle II detector. In
events where the partner $B$ meson is fully reconstructed, we use fits to the
lepton momentum spectra above $1.3\,\mathrm{GeV}/c$ to obtain $R(X_{e/\mu}) =
1.007 \pm 0.009~(\mathrm{stat}) \pm 0.019~(\mathrm{syst})$, which is the most
precise lepton-universality test of its kind and agrees with the standard-model
expectation.
We report a search for lepton-flavor-violating decays $\tau^- \to \ell^-
\phi$ ($\ell^- =e^-,\mu^-$) at the Belle II experiment, using a sample of
electron-positron data produced at the SuperKEKB ...collider in 2019-2021 and
corresponding to an integrated luminosity of 190 fb$^{-1}$. We use a new
untagged selection for $e^+e^- \to \tau^+\tau^-$ events, where the signal
$\tau$ is searched for as a neutrinoless final state of a single charged lepton
and a $\phi$ meson and the other $\tau$ is not reconstructed in any specific
decay mode, in contrast to previous measurements by the BaBar and Belle
experiments. We find no evidence for $\tau^- \to \ell^- \phi$ decays and obtain
upper limits on the branching fractions at 90% confidence level of 23 $\times
10^{-8}$ and 9.7$\times 10^{-8}$ for $\tau^- \rightarrow e^-\phi$ and $\tau^-
\rightarrow \mu^-\phi$, respectively
We measure the ratio of branching fractions for the $\Upsilon (4S)$ decays to
$B^+B^-$ and $B^0\bar{B}{}^0$ using $B^+ \rightarrow J/\psi(\ell\ell) K^+$ and
$B^0 \rightarrow J/\psi(\ell\ell) K^0$ ...samples, where $J/\psi(\ell\ell)$ stands
for $J/\psi \to \ell^+\ell^-$ ($\ell = e$ or $\mu$), with $711$ fb$^{-1}$ of
data collected at the $\Upsilon(4S)$ resonance with the Belle detector. We find
the decay rate ratio of $\Upsilon(4S) \rightarrow B^+B^-$ over $\Upsilon(4S)
\rightarrow B^0\bar{B}{}^0$ to be $1.065\pm0.012\pm 0.019 \pm 0.047$, which is
the most precise measurement to date. The first and second uncertainties are
statistical and systematic, respectively, and the third uncertainty is
systematic due to the assumption of isospin symmetry in $B \to J/\psi(\ell\ell)
K$.
We measure the $B^0$ lifetime and flavor-oscillation frequency using $B^0\to
D^{(*)-}\pi^+$ decays collected by the Belle II experiment in asymmetric-energy
$e^+e^-$ collisions produced by the ...SuperKEKB collider operating at the
$\Upsilon(4S)$ resonance. We fit the decay-time distribution of signal decays,
where the initial flavor is determined by identifying the flavor of the other
$B$ meson in the event. The results, based on $33000$ signal decays
reconstructed in a data sample corresponding to ${190}\text{fb}^{-1}$, are
$\tau_{B^0} = {1.499}\pm{0.013}\pm{0.008}\;\text{ps}$, $\Delta m_d =
{0.516}\pm{0.008}\pm{0.005}\;\text{ps}^{-1}$, where the first uncertainties are
statistical and the second are systematic. These results are consistent with
the world-average values.
We report a measurement of the mixing-induced and direct $\textit{CP}$
violation parameters $S_{\text{CP}}$ and $A_{\text{CP}}$ from $B^0 \to J/\psi
K^0_S$ decays reconstructed by the Belle II ...experiment at the SuperKEKB
asymmetric-energy electron-positron collider. The data, collected at the
center-of-mass energy of the $\Upsilon(4S)$ resonance, correspond to
$190\text{fb}^{-1}$ of integrated luminosity. We measure ${S_{\text{CP}} =
0.720\pm0.062\pm0.016}$ and $A_{\text{CP}} = 0.094\pm0.044^{+0.042}_{-0.017}$,
where the first uncertainties are statistical and the second systematic. In the
Standard Model, $S_{\text{CP}}$ equals $\sin(2\phi_1)$ to a good approximation.
Phys. Rev. Lett. 130, 181804 (2023) We study $B^{+}\to \pi^{+}\pi^{0}\pi^{0}$ using 711 $\rm{fb}^{-1}$ of data
collected at the $\Upsilon(4S)$ resonance with the Belle detector at the KEKB
...asymmetric-energy $e^{+}e^{-}$ collider. We measure an inclusive branching
fraction of $(19.0\pm 1.5\pm 1.4)\times 10^{-6}$ and an inclusive \textit{CP}
asymmetry of $(9.2 \pm 6.8 \pm 0.7)\%$, where the first uncertainties are
statistical and the second are systematic; and a $B^{+}\to
\rho(770)^{+}\pi^{0}$ branching fraction of $(11.2\pm 1.1\pm 0.9
^{+0.8}_{-1.6})\times 10^{-6}$, where the third uncertainty is due to possible
interference with $B^{+}\to \rho(1450)^{+}\pi^{0}$. We present the first
observation of a structure around 1 GeV/$c^{2}$ in the $\pi^{0}\pi^{0}$ mass
spectrum, with a significance of 6.4$\sigma$, and measure a branching fraction
to be $(6.9\pm 0.9\pm 0.6)\times 10^{-6}$. We also report a measurement of
local \textit{CP} asymmetry in this structure.
We have made a precise measurement of the absolute branching fractions of Bs->Ds(*)+ Ds(*)- decays using 121.4 fb-1 of data recorded by the Belle experiment running on the Upsilon(5S) resonance. The ...results are B(Bs0->Ds+ Ds-) = (0.58 +0.11 -0.09 +-0.13)%, B(Bs0->Ds*+- Ds-+) = (1.8 +-0.2 +-0.4)%, and B(Bs0->Ds*+ Ds*-) = (2.0 +-0.3 +-0.5)%; the sum is B(Bs0->Ds(*)+ Ds(*)- = (4.3 +-0.4 +-1.0)%. Assuming Bs->Ds(*)+ Ds(*)- saturates decays to CP-even final states, the branching fraction constrains the ratio Delta(Gamma_s)/cos(phi_s), where Delta(Gamma_s) is the difference in widths between the two Bs0-Bs0bar mass eigenstates, and phi_s is the CP-violating phase in Bs0-Bs0bar mixing. For phi_s=0 we obtain Delta(Gamma_s)/Gamma_s = 0.090 +-0.009 (stat.) +-0.023 (syst.), where Gamma_s is the mean decay width. We also measure for the first time the longitudinal polarization fraction of Bs->Ds*+ Ds*-; the result is 0.12 +0.30 -0.28 +0.04 -0.05.
