We present a search for the rare decays B --> h(*)nunuover , where h(*) stands for a light meson. A data sample of 535 x 10{6} BBover pairs collected with the Belle detector at the KEKB e{+}e{-} ...collider is used. Signal candidates are required to have an accompanying B meson fully reconstructed in a hadronic mode and signal side particles consistent with a single h(*) meson. No significant signal is observed and we set upper limits on the branching fractions at 90% confidence level. The limits on B{0} --> K{*0}nunuover and B{+} --> K{+}nunuover decays are more stringent than the previous constraints, while the first searches for B{0} --> K{0}nunuover , pi{0}nunuover , rho{0}nunuover , varphinunuover and B{+} --> K{*+}nunuover , rho{+}nunuover are reported.
The silicon vertex detector (SVD) is installed at the heart of the Belle II experiment, taking data at the high-luminosity \(B\)-Factory SuperKEKB since 2019. The detector has shown a stable and ...above-99\% hit efficiency, with a large signal-to-noise in all sensors since the beginning of data taking. Cluster position and time resolution have been measured with 2020 and 2022 data and show excellent performance and stability. The effect of radiation damage is visible, but not affecting the performance. As the luminosity increases, higher machine backgrounds are expected and the excellent hit-time information in SVD can be exploited for background rejection. In particular, we have recently developed a novel procedure to select hits by grouping them event-by-event based on their time. This new procedure allows a significant reduction of the fake rate, while preserving the tracking efficiency, and it has therefore replaced the previous cut-based procedure. We have developed a method that uses the SVD hits to estimate the track time (previously unavailable) and the collision time. It has a similar precision to the estimate based on the drift chamber but its execution time is three orders of magnitude smaller, allowing a faster online reconstruction that is crucial in a high luminosity regime. The track time is a powerful information provided to analysis that allows, together with the above-mention grouping selection, to raise the occupancy limit above that expected at nominal luminosity, leaving room for a safety factor. Finally, in June 2022 the data taking of the Belle II experiment was stopped to install a new two-layer DEPFET detector (PXD) and upgrade components of the accelerator. The whole silicon tracker (PXD+SVD) has been extracted from Belle II, the new PXD installed, the detector closed and commissioned. We briefly describe the SVD results of this upgrade.
We present a measurement of $B^{0} \rightarrow \omega \omega$, a charmless
decay into two vector mesons, using 772 $\times 10^6$ $B\overline{B}$ pairs
collected with the Belle detector at the KEKB ...$e^+e^-$ collider. The decay is
observed with a significance of 7.9 standard deviations. We measure a branching
fraction $\mathcal{B} = (1.53 \pm 0.29 \pm 0.17) \times 10^{-6}$, a fraction of
longitudinal polarization $f_L = 0.87 \pm 0.13 \pm 0.13$, and a time-integrated
$CP$ asymmetry $A_{CP}$ = $-0.44 \pm 0.43 \pm 0.11$, where the first
uncertainties listed are statistical and the second are systematic. This is the
first observation of $B^{0} \rightarrow \omega \omega$, and the first
measurements of $f_L$ and $A_{CP}$ for this decay.
We report an observation of the decay B{0}-->D{*-}tau{+}nu{tau} in a data sample containing 535x10{6} BB pairs collected with the Belle detector at the KEKB asymmetric-energy e{+}e{-} collider. We ...find a signal with a significance of 5.2sigma and measure the branching fraction B(B{0}-->D{*-}tau{+}nu{tau})=(2.02{-0.37}{+0.40}(stat)+/-0.37(syst))%. This is the first observation of an exclusive B decay with a b-->ctaunu{tau} transition.
Phys. Rev. D 109, 052003 (2024) Using the full data sample of 980 $\mathrm{fb}^{-1}$ collected with the Belle
detector at the KEKB asymmetric energy electron-positron collider, we report
the results ...of the first search for the rare semileptonic decays $\Xi_c^0 \to
\Xi^0\ell^+\ell^-$ ($\ell=e$ or $\mu)$. No significant signals are observed in
the $\Xi^0\ell^+\ell^-$ invariant-mass distributions. Taking the decay $\Xi_c^0
\to \Xi^- \pi^+$ as the normalization mode, we report 90\% credibility upper
limits on the branching fraction ratios ${\cal{B}} (\Xi_c^0 \to \Xi^0 e^+ e^-)
/ {\cal{B}}(\Xi_c^0\to \Xi^-\pi^+) < 6.7 \times 10^{-3}$ and ${\cal{B}}
(\Xi_c^0 \to \Xi^0 \mu^+ \mu^-) / {\cal{B}}(\Xi_c^0\to \Xi^-\pi^+) < 4.3 \times
10^{-3}$ based on the phase-space assumption for signal decays. The 90\%
credibility upper limits on the absolute branching fractions of ${\cal{B}}
(\Xi_c^0 \to \Xi^0 e^+ e^-)$ and ${\cal{B}} (\Xi_c^0 \to \Xi^0 \mu^+ \mu^-)$
are found to be $9.9 \times 10^{-5}$ and $6.5 \times 10^{-5}$, respectively.
We search for the baryon and lepton number violating charm decays, $D
\rightarrow p\ell$, where $D$ is either a $D^0$ or a $\overline{D}^0$ and
$\ell$ is a muon or an electron, using a data sample of ...$921\,\mathrm{fb}^{-1}$
collected by the Belle detector at the KEKB asymmetric energy $e^{+}e^{-}$
collider. In the absence of significant signals, we set upper limits on the
branching fractions in the range $(5 - 8) \times 10^{-7}$ at a 90\% confidence
level, depending on the decay mode.
We report a search for the charged-lepton flavor violation in $\Upsilon(2S)
\to \ell^\mp\tau^\pm$ ($\ell=e,\mu$) decays using a $25~\fbi$ $\Upsilon(2S)$
sample collected by the Belle detector at the ...KEKB $e^{+}$$e^-$
asymmetric-energy collider. We find no evidence for a signal and set upper
limits on the branching fractions ($\mathcal{B}$) at 90\% confidence level. We
obtain the most stringent upper limits: $\mathcal{B}(\Ytomutau) < 0.23 \times
10^{-6}$ and $\mathcal{B}(\Ytoetau) < 1.12 \times 10^{-6}$.
We observe evidence for D(0)-D(0) mixing by measuring the difference in the apparent lifetime when a D(0) meson decays to the CP eigenstates K(+)K(-) and pi(+)pi(-) and when it decays to the final ...state K(-)pi(+). We find the relative difference of the lifetimes y(CP) to be 1.31+/-0.32(stat)+/-0.25(syst)%, 3.2 standard deviations from zero. We also search for a CP asymmetry between D(0) and D(0) decays; no evidence for CP violation is found. These results are based on 540 fb(-1) of data recorded by the Belle detector at the KEKB e(+)e(-) collider.
JHEP08(2023)178 We present a search for the lepton-flavor-violating decays $B{}^0_s
\rightarrow \ell^{\mp}\tau^{\pm}$, where $\ell = e, \mu$, using the full data
sample of $121~\mathrm{fb}^{-1}$ ...collected at the $\Upsilon(5S)$ resonance with
the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider. We use
$B{}^0_s \overline{B}{}^0_s$ events in which one $B{}^0_s$ meson is
reconstructed in a semileptonic decay mode and the other in the signal mode. We
find no evidence for $B{}^0_s \rightarrow \ell^{\mp}\tau^{\pm}$ decays and set
upper limits on their branching fractions at $90\%$ confidence level as
$\mathcal{B}(B{}^0_s \rightarrow e^{\mp}\tau^{\pm}) < 14 \times 10^{-4}$ and
$\mathcal{B}(B{}^0_s \rightarrow \mu^{\mp}\tau^{\pm}) < 7.3 \times 10^{-4}$.
Our result represents the first upper limit on the $B{}^0_s \rightarrow
e^{\mp}\tau^{\pm}$ decay rate.
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