We report measurements of the branching fraction ($\mathcal{B}$) and direct
$CP$-violating asymmetry ($A_{CP}$) of the charmless decay $B^{0} \to K^0
\pi^0$ at Belle II. A sample of $e^{+} e^{-}$ ...collisions, corresponding to
$189.8 fb^{-1}$ of integrated luminosity, recorded at the $\Upsilon(4S)$
resonance is used for the first decay-time-dependent analysis of these decays
within the experiment. We reconstruct about 135 signal candidates, and measure
$\mathcal{B}(B^{0} \to K^{0} \pi^{0})= 11.0 \pm 1.2 (stat) \pm 1.0 (syst)
\times 10^{-6}$ and $A_{CP} (B^{0} \to K^{0} \pi^{0})= -0.41_{-0.32}^{+0.30}
(stat) \pm 0.09 (syst)$.
We report a measurement of the branching fraction of $B \to
K^{\ast}(892)\ell^+\ell^-$ decays, where $\ell^+\ell^- = \mu^+\mu^-$ or
$e^+e^-$, using electron-positron collisions recorded at an energy ...at or near
the $\Upsilon(4S)$ mass and corresponding to an integrated luminosity of $189$
fb$^{-1}$. The data was collected during 2019--2021 by the Belle II experiment
at the SuperKEKB $e^{+}e^{-}$ asymmetric-energy collider. We reconstruct
$K^{\ast}(892)$ candidates in the $K^+\pi^-$, $K_{S}^{0}\pi^+$, and $K^+\pi^0$
final states. The signal yields with statistical uncertainties are $22\pm 6$,
$18 \pm 6$, and $38 \pm 9$ for the decays $B \to K^{\ast}(892)\mu^+\mu^-$, $B
\to K^{\ast}(892)e^+e^-$, and $B \to K^{\ast}(892)\ell^+\ell^-$, respectively.
We measure the branching fractions of these decays for the entire range of the
dilepton mass, excluding the very low mass region to suppress the $B \to
K^{\ast}(892)\gamma(\to e^+e^-)$ background and regions compatible with decays
of charmonium resonances, to be \begin{equation} {\cal B}(B \to
K^{\ast}(892)\mu^+\mu^-) = (1.19 \pm 0.31 ^{+0.08}_{-0.07}) \times 10^{-6},
{\cal B}(B \to K^{\ast}(892)e^+e^-) = (1.42 \pm 0.48 \pm 0.09)\times 10^{-6},
{\cal B}(B \to K^{\ast}(892)\ell^+\ell^-) = (1.25 \pm 0.30 ^{+0.08}_{-0.07})
\times 10^{-6}, \end{equation} where the first and second uncertainties are
statistical and systematic, respectively. These results, limited by sample
size, are the first measurements of $B \to K^{\ast}(892)\ell^+\ell^-$ branching
fractions from the Belle II experiment.
We present a reconstruction in early data of the semileptonic decay $B^+ \to
\pi^0 \ell^+ \nu_\ell$, and first results of a reconstruction of the decays
$B^+ \to \rho^0 \ell^+ \nu_\ell$ and $B^0 \to ...\rho^- \ell^+ \nu_\ell$ in a
sample corresponding to 62.8 fb$^{-1}$ of Belle II data using hadronic
$B$-tagging via the full-event-interpretation algorithm. We determine the total
branching fractions via fits to the distribution of the square of the missing
mass, and find $\mathcal{B}(B^+ \to \pi^0 \ell^+ \nu_\ell)$ = (8.29 $\pm$
1.99(stat) $\pm$ 0.46(syst)) $\times 10^{-5}$. We obtain $95\%$ CL upper limits
on the branching fractions with $\mathcal{B}(B^0 \to \rho^- \ell^+ \nu_\ell) <
3.37 \times 10^{-4}$ and $\mathcal{B}(B^+ \to \rho^0 \ell^+ \nu_\ell) < 19.7
\times 10^{-5}$. We also obtain an updated branching fraction for the $B^0 \to
\pi^- \ell^+ \nu_\ell$ decay, $\mathcal{B}(B^0 \to \pi^- \ell^+ \nu_\ell)$ =
(1.47 $\pm$ 0.29(stat) $\pm$ 0.05(syst)) $\times 10^{-4}$, based on the sum of
the partial branching fractions in three bins of the squared momentum transfer
to the leptonic system.
This paper reports a study of $B \to K^{*}\gamma$ decays using $62.8\pm 0.6$
fb$^{-1}$ of data collected during 2019--2020 by the Belle II experiment at the
SuperKEKB $e^{+}e^{-}$ asymmetric-energy ...collider, corresponding to $(68.2 \pm
0.8) \times 10^6$ $B\overline{B}$ events. We find $454 \pm 28$, $50 \pm 10$,
$169 \pm 18$, and $160 \pm 17$ signal events in the decay modes $B^{0} \to
K^{*0}K^{+}\pi^{-}\gamma$, $B^{0} \to K^{*0}K^0_{\rm S}\pi^{0}\gamma$,
$B^{+} \to K^{*+}K^{+}\pi^{0}\gamma$, and $B^{+} \to
K^{*+}K^{+}\pi^{0}\gamma$, respectively. The uncertainties quoted for the
signal yield are statistical only. We report the branching fractions of these
decays: $$\mathcal{B} B^{0} \to K^{*0}K^{+}\pi^{-}\gamma = (4.5 \pm 0.3 \pm
0.2) \times 10^{-5}, $$ $$\mathcal{B} B^{0} \to K^{*0}K^0_{\rm
S}\pi^{0}\gamma = (4.4 \pm 0.9 \pm 0.6) \times 10^{-5},$$ $$\mathcal{B}
B^{+} \to K^{*+}K^{+}\pi^{0}\gamma = (5.0 \pm 0.5 \pm 0.4)\times
10^{-5},\text{ and}$$ $$\mathcal{B} B^{+} \to K^{*+}K^0_{\rm
S}\pi^{+}\gamma = (5.4 \pm 0.6 \pm 0.4) \times 10^{-5},$$ where the first
uncertainty is statistical, and the second is systematic. The results are
consistent with world-average values.
We report on the first Belle II measurement of the branching fraction
($\mathcal{B}$) and longitudinal polarization fraction ($f_L$) of $B^+\to
\rho^+\rho^0$ decays. We reconstruct $B^+\to \rho^+(\to ...\pi^+\pi^0(\to
\gamma\gamma))\rho^0(\to \pi^+\pi^-)$ decays in a sample of SuperKEKB
electron-positron collisions collected by the Belle II experiment in 2019 and
2020 at the $\Upsilon$(4S) resonance and corresponding to $62.8$ fb$^{-1}$ of
integrated luminosity. We fit the distributions of the difference between
expected and observed $B$ candidate energy, continuum-suppression variable,
dipion masses, and angular distributions of the resulting samples, to determine
a signal yield of $104\pm16$ events. The signal yields are corrected for
efficiencies determined from simulation and control data samples to obtain
$\mathcal{B}(B^+ \to \rho^+\rho^0) = 20.6 \pm 3.2(\rm stat) \pm 4.0(\rm
syst)\times 10^{-6}$, and $f_L(B^+ \to \rho^+\rho^0) = 0.936
^{+0.049}_{-0.041}(\rm stat)\pm 0.021(\rm syst)$. This first Belle II $B^+ \to
\rho^+\rho^0$ angular analysis yields results compatible with previous
determinations, and indicates Belle II performance superior to early Belle
results.
We report on measurements of branching fractions ($\mathcal{B}$) and
CP-violating charge asymmetries ($\mathcal{A}_{\rm CP}$) of multibody charmless
$B$ decays reconstructed by the Belle II ...experiment at the SuperKEKB
electron-positron collider. We use a sample of collisions collected in 2019 and
2020 at the $\Upsilon(4S)$ resonance and corresponding to $62.8$ fb$^{-1}$ of
integrated luminosity. We use simulation to determine optimized event
selections. The $\Delta E$ and $M_{\rm bc}$ distributions of the resulting
samples are fit to determine signal yields of approximately 690, 840, and 380
decays for the channels $B^+ \to K^+K^-K^+$, $B^+ \to K^+\pi^-\pi^+$, and $B^0
\to K^+\pi^-\pi^0$, respectively. These yields are corrected for efficiencies
determined from simulation and control data samples to obtain $\mathcal{B}(B^+
\to K^+K^-K^+) = 35.8 \pm 1.6(\rm stat) \pm 1.4 (\rm syst)\times 10^{-6}$,
$\mathcal{B}(B^+ \to K^+\pi^-\pi^+) = 67.0 \pm 3.3 (\rm stat)\pm 2.3 (\rm
syst)\times 10^{-6}$, $\mathcal{B}(B^0 \to K^+\pi^-\pi^0) = 38.1 \pm 3.5 (\rm
stat)\pm 3.9 (\rm syst)\times 10^{-6}$, $\mathcal{A}_{\rm CP}(B^+ \to
K^+K^-K^+) = -0.103 \pm 0.042(\rm stat) \pm 0.020 (\rm syst)$,
$\mathcal{A}_{\rm CP}(B^+ \to K^+\pi^-\pi^+) = -0.010 \pm 0.050 (\rm stat)\pm
0.021(\rm syst)$, and $\mathcal{A}_{\rm CP}(B^0 \to K^+\pi^-\pi^0) = 0.207 \pm
0.088 (\rm stat)\pm 0.011(\rm syst)$. Results are consistent with previous
measurements and demonstrate detector performance comparable with the best
Belle results.
We report the first reconstruction of the $B^{0} \to \pi^{0} \pi^{0}$ decay
mode at Belle II using samples of 2019 and 2020 data that correspond to 62.8
fb$^{-1}$ of integrated luminosity. We find ...$14.0^{+6.8}_{-5.6}$ signal decays,
corresponding to a significance of 3.4 standard deviations and determine a
branching ratio of $\mathcal{B}(B^{0} \rightarrow \pi^{0} \pi^{0}) =
0.98^{+0.48}_{-0.39} \pm 0.27 \times 10^{-6}$. The results agree with
previous determinations and contribute important information to an early
assessment of detector performance and Belle II's potential for future
determinations of $\alpha/\phi_2$ using $B \rightarrow \pi \pi$ modes.
We present preliminary results on the reconstruction of the $B^0\to J\mskip
1mu / \psi\mskip 2mu K^0_{\scriptscriptstyle L}$ decay, where $J\mskip 1mu /
\psi\mskip 2mu\to\mu^+\mu^-$ or $e^+e^-$. ...Using a dataset corresponding to a
luminosity of $62.8\pm0.6\mbox{fb}^{-1}$ collected by the Belle II experiment
at the SuperKEKB asymmetric energy $e^+e^-$ collider, we measure a total of
$267\pm21$ candidates with $J\mskip 1mu / \psi\mskip 2mu\to\mu^+\mu^-$ and
$226\pm20$ with with $J\mskip 1mu / \psi\mskip 2mu\to e^+e^-$. The quoted
errors are statistical only.
We report updated measurements of branching fractions ($\mathcal{B}$) and
CP-violating charge asymmetries ($\mathcal{A_{\rm CP}}$) for charmless $B$
decays at Belle II, which operates on or near the ...$\Upsilon$(4S) resonance at
the SuperKEKB asymmetric energy $e^{+}e^{-}$ collider. We use samples of 2019
and 2020 data corresponding to 62.8 fb$^{-1}$ of integrated luminosity. The
samples are analysed using two-dimensional fits in $\Delta E$ and $M_{\it bc}$
to determine signal yields of approximately 568, 103, and 115 decays for the
channels $B^0 \to K^+\pi^-$, $B^+ \to K_{\rm S}^0\pi^+$, and $B^0 \to
\pi^+\pi^-$, respectively. Signal yields are corrected for efficiencies
determined from simulation and control data samples to obtain branching
fractions and CP-violating asymmetries for flavour-specific channels. The
results are compatible with known determinations and contribute important
information to an early assessment of Belle II detector performance.
We present the first measurement of the time-integrated mixing probability
$\chi_d$ using Belle II data collected at a center-of-mass (CM) energy of 10.58
GeV, corresponding to the mass of the ...$\Upsilon$(4S) resonance, with an
integrated luminosity of $34.6 {\rm fb}^{-1}$ at the SuperKEKB $e^+ e^-$
collider. We reconstruct pairs of B mesons both of which decay to semileptonic
final states. Using a novel methodology, we measure $\chi_d = 0.187 \pm 0.010
\text{ (stat.)} \pm 0.019 \text{ (syst.)}$, which is compatible with existing
indirect and direct determinations.
