The ratio of branching fractions $R(D^{*}) = \mathcal{B}(\overline{B}
\rightarrow D^{*} \tau^{-} \overline{\nu}_{\tau})$/$\mathcal{B} (\overline{B}
\rightarrow D^{*} \ell^{-} \overline{\nu}_{\ell})$, ...where $\ell$ is an electron
or muon, is measured using a Belle~II data sample with an integrated luminosity
of $189~\mathrm{fb}^{-1}$ at the SuperKEKB asymmetric-energy $e^{+} e^{-}$
collider. Data is collected at the $\Upsilon(\mathrm{4S})$ resonance, and one
$B$ meson in the $\Upsilon(\mathrm{4S})\rightarrow B\overline{B}$ decay is
fully reconstructed in hadronic decay modes. The accompanying signal $B$ meson
is reconstructed as $\overline{B}\rightarrow D^{*}
\tau^{-}\overline{\nu}_{\tau}$ using leptonic $\tau$ decays. The normalization
decay, $\overline{B}\rightarrow D^{*} \ell^{-} \overline{\nu}_{\ell}$, where
$\ell$ is an electron or muon, produces the same observable final state
particles. The ratio of branching fractions is extracted in a simultaneous fit
to two signal-discriminating variables in both channels and yields $R(D^{*}) =
0.262~_{-0.039}^{+0.041}(\mathrm{stat})~_{-0.032}^{+0.035}(\mathrm{syst})$.
This result is consistent with the current world average and with standard
model predictions.
We search for the $e^+e^-\to\eta_b(1S)\omega$ and
$e^+e^-\to\chi_{b0}(1P)\omega$ processes at a center-of-mass energy of 10.745
GeV, which is close to the peak of the $\Upsilon(10753)$ state. We use ...data
collected by the Belle II experiment during a special run, corresponding to an
integrated luminosity of $9.8\,\mathrm{fb}^{-1}$. We reconstruct
$\omega\to\pi^+\pi^-\pi^0$ decays and use the $\omega$ meson's recoil mass to
search for the signals. We do not find evidence for either process, and set
upper limits on the corresponding Born-level cross sections of 2.5 pb and 7.8
pb, respectively, at the 90% confidence level. The $\chi_{b0}(1P)\omega$ limit
is the result of a combination of this analysis and a previous search using
full reconstruction.
Phys. Rev. D 109, 112006 (2024) We search for the rare decay $B^{+}\rightarrow K^{+}\nu\bar{\nu}$ in a $362\
\rm{fb}^{-1}$ sample of electron-positron collisions at the $\Upsilon(4S)$
resonance ...collected with the Belle II detector at the SuperKEKB collider. We
use the inclusive properties of the accompanying $B$ meson in $\Upsilon(4S) \to
B\kern 0.18em\overline{\kern -0.18em B}{}$ events to suppress background from
other decays of the signal $B$ candidate and light-quark pair production. We
validate the measurement with an auxiliary analysis based on a conventional
hadronic reconstruction of the accompanying $B$ meson. For background
suppression, we exploit distinct signal features using machine learning methods
tuned with simulated data. The signal-reconstruction efficiency and background
suppression are validated through various control channels. The branching
fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic
analyses yield consistent results for the $B^{+}\rightarrow K^{+}\nu\bar{\nu}$
branching fraction of $\left2.7\pm 0.5(\mathrm{stat})\pm
0.5(\mathrm{syst})\right \times 10^{-5}$ and
$\left1.1^{+0.9}_{-0.8}(\mathrm{stat}){}^{+0.8}_{-0.5}(\mathrm{syst})\right
\times 10^{-5}$, respectively. Combining the results, we determine the
branching fraction of the decay $B^{+}\rightarrow K^{+}\nu\bar{\nu}$ to be
$\left2.3 \pm 0.5(\mathrm{stat})^{+0.5}_{-0.4}(\mathrm{syst})\right\times
10^{-5}$, providing the first evidence for this decay at $3.5$ standard
deviations. The combined result is $2.7$ standard deviations above the standard
model expectation.
Phys. Rev. Lett. 132, 211804 (2024) We measure the tau-to-light-lepton ratio of inclusive $B$-meson branching
fractions $R(X_{\tau/\ell}) \equiv \mathcal{B}(B\to X \tau \nu)/\mathcal{B}(B
\to X \ell ...\nu)$, where $\ell$ indicates an electron or muon, and thereby test
the universality of charged-current weak interactions. We select events that
have one fully reconstructed $B$ meson and a charged lepton candidate from
$189~\mathrm{fb}^{-1}$ of electron-positron collision data collected with the
Belle II detector. We find $R(X_{\tau/\ell}) = 0.228 \pm 0.016~(\mathrm{stat})
\pm 0.036~(\mathrm{syst})$, in agreement with standard-model expectations. This
is the first direct measurement of $R(X_{\tau/\ell})$.
Phys. Rev. D 109, 012001 (2024) We report measurements of the branching fractions and direct $\it{CP}$
asymmetries of the decays $B^0 \to K^+ \pi^-$, $B^+ \to K^+ \pi^0$, $B^+ \to
K^0 \pi^+$, and ...$B^0 \to K^0 \pi^0$, and use these for testing the standard
model through an isospin-based sum rule. In addition, we measure the branching
fraction and direct $\it{CP}$ asymmetry of the decay $B^+ \to \pi^+\pi^0$ and
the branching fraction of the decay $B^0 \to \pi^+\pi^-$. The data are
collected with the Belle II detector from $e^+e^-$ collisions at the
$\Upsilon(4S)$ resonance produced by the SuperKEKB asymmetric-energy collider
and contain $387\times 10^6$ bottom-antibottom meson pairs. Signal yields are
determined in two-dimensional fits to background-discriminating variables, and
range from 500 to 3900 decays, depending on the channel. We obtain $-0.03 \pm
0.13 \pm 0.04$ for the sum rule, in agreement with the standard model
expectation of zero and with a precision comparable to the best existing
determinations.
Phys.Rev.D 108 (2023) 9, 092013 We determine the CKM matrix-element magnitude $|V_{cb}|$ using
$\overline{B}^0\to D^{*+}\ell^-\bar\nu_\ell$ decays reconstructed in $189 \,
\mathrm{fb}^{-1}$ of ...collision data collected by the Belle II experiment,
located at the SuperKEKB $e^+e^-$ collider. Partial decay rates are reported as
functions of the recoil parameter $w$ and three decay angles separately for
electron and muon final states. We obtain $|V_{cb}|$ using the
Boyd-Grinstein-Lebed and Caprini-Lellouch-Neubert parametrizations, and find
$|V_{cb}|_\mathrm{BGL}=(40.57\pm 0.31 \pm 0.95\pm 0.58)\times 10^{-3}$ and
$|V_{cb}|_\mathrm{CLN}=(40.13 \pm 0.27 \pm 0.93\pm 0.58 )\times 10^{-3}$ with
the uncertainties denoting statistical components, systematic components, and
components from the lattice QCD input, respectively. The branching fraction is
measured to be ${\cal B}(\overline{B}^0\to D^{*+}\ell^-\bar\nu_\ell)=(4.922 \pm
0.023 \pm 0.220)\%$. The ratio of branching fractions for electron and muon
final states is found to be $0.998 \pm 0.009 \pm 0.020$. In addition, we
determine the forward-backward angular asymmetry and the $D^{*+}$ longitudinal
polarization fractions. All results are compatible with lepton-flavor
universality in the Standard Model.
We report results from a study of $B^\pm \rightarrow DK^\pm$ decays followed
by $D$ decaying to $CP$~eigenstates, where $D$ indicates a $D^0$ or
$\bar{D}^{0}$ meson. These decays are sensitive to the
...Cabibbo-Kobayashi-Maskawa unitarity-triangle angle $\phi_{3}$. The results are
based on a combined analysis of the final data set of $772 \times
10^6~B\bar{B}$ pairs collected by the Belle experiment and a data set of $198
\times 10^6~B\bar{B}$ pairs collected by the Belle~II experiment, both in
electron-positron collisions at the $\Upsilon(4S)$ resonance. We measure the
$CP$ asymmetries to be $\mathcal{ A}_{CP +} =~(+12.5 \pm 5.8 \pm 1.4)\% $ and
$\mathcal{ A}_{CP -} =~(-16.7 \pm 5.7 \pm 0.6)\%$, and the ratios of branching
fractions to be $\mathcal{ R}_{CP+}=~1.164 \pm 0.081 \pm 0.036 $ and $\mathcal{
R}_{CP-} =~1.151 \pm 0.074 \pm 0.019$. The first contribution to the
uncertainties is statistical, and the second is systematic. The asymmetries
$\mathcal{A}_{CP +}$ and $\mathcal{A}_{CP -}$ have similar magnitudes and
opposite signs; their difference corresponds to 3.5~standard deviations. From
these values we calculate 68.3\% confidence intervals of
($8.5^{\circ}<\phi_{3}<16.5^{\circ}$) or ($84.5^{\circ}<\phi_{3}<95.5^{\circ}$)
or ($163.3^{\circ}<\phi_{3}<171.5^{\circ}$) and $0.321<r_{B}<0.465$.
Phys. Rev. Lett. 131, 181801 (Published 31 October 2023) We present the first comprehensive tests of light-lepton universality in the
angular distributions of semileptonic $B^0$-meson decays to ...charged spin-1
charmed mesons. We measure five angular-asymmetry observables as functions of
the decay recoil that are sensitive to lepton-universality-violating
contributions. We use events where one neutral $B$ is fully reconstructed in
$\Upsilon\left(4S\right)\to{}B \overline{B}$ decays in data corresponding to
$189~\mathrm{fb}^{-1}$ integrated luminosity from electron-positron collisions
collected with the Belle II detector. We find no significant deviation from the
standard model expectations.
Phys. Rev. D 108, 072012 (2023) We present a measurement of time-dependent rate asymmetries in $B^0\to \phi
K^0_S$ decays to search for non-standard-model physics in $b\to q
\overline{q}s$ ...transitions. The data sample is collected with the Belle II
detector at the SuperKEKB asymmetric-energy $e^{+}e^{-}$ collider in 2019-2022
and contains $(387\pm 6)\times 10^6$ bottom-antibottom mesons from
$\Upsilon(4S)$ resonance decays. We reconstruct $162\pm17$ signal events and
extract the charge-parity ($CP$) violating parameters from a fit to the
distribution of the proper-decay-time difference of the two $B$ mesons. The
measured direct and mixing-induced $CP$ asymmetries are $A=0.31\pm0.20\pm0.05$
and $S=0.54\pm0.26^{+0.06}_{-0.08}$, respectively, where the first
uncertainties are statistical and the second are systematic. The results are
compatible with the $CP$ asymmetries observed in $b\to c\overline{c} s$
transitions.