We describe a measurement of charge-parity ($CP$) violation asymmetries in
$B^0\to\eta'K^0_S$ decays using Belle II data. We consider
$\eta'\to\eta(\to\gamma\gamma)\pi^+\pi^-$ and
...$\eta'\to\rho(\to\pi^+\pi^-)\gamma$ decays. The data were collected at the
SuperKEKB asymmetric-energy $e^+e^-$ collider between the years 2019 and 2022,
and contain $(387\pm 6) \times 10^6$ bottom-antibottom meson pairs. We
reconstruct $829\pm35$ signal decays and extract the $CP$ violating parameters
from a fit to the distribution of the proper-decay-time difference between the
two $B$ mesons. The measured direct and mixing-induced $CP$ asymmetries are
$\text{C}_{\eta'K^0_S} = -0.19 \pm 0.08 \pm 0.03 $ and $\text{S}_{\eta'K^0_S} =
+0.67 \pm 0.10 \pm 0.04 $, respectively, where the first uncertainties are
statistical and the second are systematic. These results are in agreement with
current world averages and standard model predictions.
We present an analysis of the process
$e^{+}e^{-}\to\pi^{+}\pi^{-}\Upsilon(nS)$ (where $n$ = 1, 2, or 3)
reconstructed in $19.6\rm$ $\rm fb^{-1}$ of Belle II data during a special run
of the ...SuperKEKB collider at four energy points near the peak of the
$\Upsilon(10753)$ resonance. By analyzing the mass distribution of the
$\pi^+\pi^-\Upsilon(nS)$ system and the Born cross sections of the
$e^{+}e^{-}\to\pi^{+}\pi^{-}\Upsilon(nS)$ process, we report the first
observation of $\Upsilon(10753)$ decays to the $\pi^{+}\pi^{-}\Upsilon(1S)$ and
$\pi^{+}\pi^{-}\Upsilon(2S)$ final states, and find no evidence for decays to
$\pi^{+}\pi^{-}\Upsilon(3S)$. Possible intermediate states in the
$\pi^+\pi^-\Upsilon(1S,2S)$ transitions are also investigated, and no evidence
for decays proceeding via the $\pi^\mp Z_b^\pm$ or $f_0(980)\Upsilon(nS)$
intermediate states is found. We measure Born cross sections for the
$e^{+}e^{-}\to\pi^{+}\pi^{-}\Upsilon(nS)$ process that, combined with results
from Belle, improve the precision of measurements of the $\Upsilon(10753)$ mass
and width by nearly a factor of two to $(10756.3\pm2.7\pm0.6)$ MeV/$c^2$ and
$(29.7\pm8.5\pm1.1)$ MeV, respectively. The relative ratios of the Born cross
sections at the $\Upsilon(10753)$ resonance peak are also reported for the
first time.
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 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.
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. 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.
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 $0.321<r_{B}<0.465$.
