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 $B^{+, 0} \to (\omega, \rho^{+})\mu^+\mu^-$.
We report a measurement of the $e^+e^- \to \pi^+\pi^-\pi^0$ cross section in
the energy range from 0.62 to 3.50 GeV using an initial-state radiation
technique. We use an $e^+e^-$ data sample ...corresponding to 191 $\text{fb}^{-1}$
of integrated luminosity, collected at a center-of-mass energy at or near the
$\Upsilon{(4S)}$ resonance with the Belle II detector at the SuperKEKB
collider. Signal yields are extracted by fitting the two-photon mass
distribution in $e^+e^- \to \pi^+\pi^-\pi^0\gamma$ events, which involve a
$\pi^0 \to \gamma\gamma$ decay and an energetic photon radiated from the
initial state. Signal efficiency corrections with an accuracy of 1.6% are
obtained from several control data samples. The uncertainty on the cross
section at the $\omega$ and $\phi$ resonances is dominated by the systematic
uncertainty of 2.2%. The resulting cross sections in the 0.62-1.80 GeV energy
range yield $ a_\mu^{3\pi} = 48.91 \pm 0.23~(\mathrm{stat}) \pm
1.07~(\mathrm{syst}) \times 10^{-10} $ for the leading-order hadronic vacuum
polarization contribution to the muon anomalous magnetic moment. This result
differs by $2.5$ standard deviations from the most precise current
determination.
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.
Phys. Rev. D 110, 012001, 2024 We present GFlaT, a new algorithm that uses a graph-neural-network to
determine the flavor of neutral $B$ mesons produced in $\Upsilon(4S)$ decays.
It improves previous ...algorithms by using the information from all charged
final-state particles and the relations between them. We evaluate its
performance using $B$ decays to flavor-specific hadronic final states
reconstructed in a 362 $\text{fb}^{-1}$ sample of electron-positron collisions
collected at the $\Upsilon(4S)$ resonance with the Belle II detector at the
SuperKEKB collider. We achieve an effective tagging efficiency of $(37.40 \pm
0.43 \pm 0.36) \%$, where the first uncertainty is statistical and the second
systematic, which is $18\%$ better than the previous Belle II algorithm.
Demonstrating the algorithm, we use $B^{0}\to J/\psi K^0_\text{S}$ decays to
measure the mixing-induced and direct $CP$ violation parameters, $S = (0.724
\pm 0.035 \pm 0.009)$ and $C = (-0.035 \pm 0.026 \pm 0.029)$.
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.
