We report the result of a search for the rare decay $B^{0} \to \gamma \gamma$
using a combined dataset of $753\times10^{6}$ $B\bar{B}$ pairs collected by the
Belle experiment and $387\times10^{6}$ ...$B\bar{B}$ pairs collected by the Belle
II experiment from decays of the $\rm \Upsilon(4S)$ resonance produced in
$e^{+}e^{-}$ collisions. A simultaneous fit to the Belle and Belle II data sets
yields $11.0^{+6.5}_{-5.5}$ signal events, corresponding to a 2.5$\sigma$
significance. We determine the branching fraction $\mathcal{B}(B^{0} \to
\gamma\gamma) = (3.7^{+2.2}_{-1.8}(\rm stat)\pm0.5(\rm syst))\times10^{-8}$ and
set a 90% credibility level upper limit of $\mathcal{B}(B^{0} \to \gamma\gamma)
< 6.4\times10^{-8}$.
We report measurements of the $e^+e^- \to B\bar{B}$, $B\bar{B}{}^*$, and
$B^*\bar{B}{}^*$ cross sections at four energies, 10653, 10701, 10746 and 10805
MeV, using data collected by the Belle~II ...experiment. We reconstruct one $B$
meson in a large number of hadronic final states and use its momentum to
identify the production process. In the first $2-5$ MeV above $B^*\bar{B}{}^*$
threshold, the $e^+e^- \to B^*\bar{B}{}^*$ cross section increases rapidly.
This may indicate the presence of a pole close to the threshold.
We present a measurement of the ratio $R_\mu = \mathcal{B}(\tau^-\to
\mu^-\bar\nu_\mu\nu_\tau) / \mathcal{B}(\tau^-\to e^-\bar\nu_e\nu_\tau)$ of
branching fractions $\mathcal{B}$ of the $\tau$ lepton ...decaying to muons or
electrons using data collected with the Belle II detector at the SuperKEKB
$e^+e^-$ collider. The sample has an integrated luminosity of 362 fb$^{-1}$ at
a centre-of-mass energy of 10.58 GeV. Using an optimised event selection, a
binned maximum likelihood fit is performed using the momentum spectra of the
electron and muon candidates. The result, $R_\mu = 0.9675 \pm 0.0007 \pm
0.0036$, where the first uncertainty is statistical and the second is
systematic, is the most precise to date. It provides a stringent test of the
light-lepton universality, translating to a ratio of the couplings of the muon
and electron to the $W$ boson in $\tau$ decays of $0.9974 \pm 0.0019$, in
agreement with the standard model expectation of unity.
We present the result of a search for the charged-lepton-flavor violating
decay $\tau^- \to \mu^-\mu^+\mu^-$ using a $424fb^{-1}$ sample of data recorded
by the Belle II experiment at the SuperKEKB ...$e^{-}e^{+}$ collider. The
selection of $e^{-}e^{+}\to\tau^+\tau^-$ events is based on an inclusive
reconstruction of the non-signal tau decay, and on a boosted decision tree to
suppress background. We observe one signal candidate, which is compatible with
the expectation from background processes. We set a $90\%$ confidence level
upper limit of $1.9 \times 10^{-8}$ on the branching fraction of the \taumu
decay, which is the most stringent bound to date.
We report a determination of the CKM angle $\phi_{3}$, also known as
$\gamma$, from a combination of measurements using samples of up to
711~fb$^{-1}$ from the Belle experiment and up to ...362~fb$^{-1}$ from the Belle
II experiment. We combine results from analyses of $B^+\to DK^+, B^+\to
D\pi^+$, and $B^+ \to D^{*}K^+$ decays, where $D$ is an admixture of $D^0$ and
$\overline{D}{}^{0}$ mesons, in a likelihood fit to obtain $\phi_{3} =
(78.6^{+7.2}_{-7.3})^{\circ}$. We also briefly discuss the interpretation of
this result.
PRD 109, 111103 (2024) We measure the branching fraction of the decay $B^- \to D^0 \rho(770)^-$
using data collected with the Belle II detector. The data contain 387 million
$B\overline{B}$ pairs ...produced in $e^+e^-$ collisions at the $\Upsilon(4S)$
resonance. We reconstruct $8360\pm 180$ decays from an analysis of the
distributions of the $B^-$ energy and the $\rho(770)^-$ helicity angle. We
determine the branching fraction to be $(0.939 \pm 0.021\mathrm{(stat)} \pm
0.050\mathrm{(syst)})\%$, in agreement with previous results. Our measurement
improves the relative precision of the world average by more than a factor of
two.
We present the results of a search for the $b \to d\ell^+\ell^-$
flavor-changing neutral-current rare decays $B^{+, 0} \to (\eta, \omega,
\pi^{+,0}, \rho^{+, 0}) e^+e^-$ and $B^{+, 0} \to (\eta, ...\omega, \pi^{0},
\rho^{+}) \mu^+\mu^-$ using a $711$ fb$^{-1}$ data sample that contains $772
\times 10^{6}$ $B\overline{B}$ events. The data were collected at the
$\Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy
$e^+e^-$ collider. We find no evidence for signal and set upper limits on
branching fractions at the $90\%$ confidence level in the range $(3.8 - 47)
\times 10^{-8}$ depending on the decay channel. The obtained limits are the
world's best results. This is the first search for the channels $B^{+, 0} \to
(\omega, \rho^{+,0}) e^+e^-$ and $B^{+, 0} \to (\omega, \rho^{+})\mu^+\mu^-$.
Phys. Rev. D 109, 112015 (2024) We report on a search for a resonance $X$ decaying to a pair of muons in
$e^{+}e^{-}\rightarrow \mu^+ \mu^- X$ events in the 0.212-9.000 GeV/$c^{2}$
mass range, using ...178 fb$^{-1}$ of data collected by the BelleII experiment at
the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis
probes two different models of $X$ beyond the standard model: a $Z^{\prime}$
vector boson in the $L_{\mu}-L_{\tau}$ model and a muonphilic scalar. We
observe no evidence for a signal and set exclusion limits at the 90$\%$
confidence level on the products of cross section and branching fraction for
these processes, ranging from 0.046 fb to 0.97 fb for the $L_{\mu}-L_{\tau}$
model and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses
below 6 GeV/$c^{2}$, the corresponding constraints on the couplings of these
processes to the standard model range from 0.0008 to 0.039 for the
$L_{\mu}-L_{\tau}$ model and from 0.0018 to 0.040 for the muonphilic scalar
model. These are the first constraints on the muonphilic scalar from a
dedicated search.
We report a measurement of decay-time dependent charge-parity ($CP$)
asymmetries in $B^0 \rightarrow K^0_S K^0_S K^0_S$ decays. We use $387 \times
10^6 B\bar{B}$ pairs collected at the $\Upsilon(4S)$ ...resonance with the Belle
II detector at the SuperKEKB asymmetric-energy electron-positron collider. We
reconstruct 220 signal events and extract the $CP$-violating parameters $S$ and
$C$ from a fit to the distribution of the decay-time difference between the two
$B$ mesons. The resulting confidence region is consistent with previous
measurements in $B^0 \rightarrow K^0_S K^0_S K^0_S$ and $B^0 \rightarrow
(c\bar{c})K^0$ decays, and with predictions based on the standard model.
