Phys. Rev. Lett. 131, 051804 (2023) We present the first measurement of the ratio of branching fractions of
inclusive semileptonic $B$-meson decays, $R(X_{e/\mu}) = \mathcal{B}(B\to X \,
e \, \nu) / ...\mathcal{B}(B\to X \, \mu \, \nu)$, a precision test of
electron-muon universality, using data corresponding to $189\,\mathrm{fb}^{-1}$
from electron-positron collisions collected with the Belle II detector. In
events where the partner $B$ meson is fully reconstructed, we use fits to the
lepton momentum spectra above $1.3\,\mathrm{GeV}/c$ to obtain $R(X_{e/\mu}) =
1.007 \pm 0.009~(\mathrm{stat}) \pm 0.019~(\mathrm{syst})$, which is the most
precise lepton-universality test of its kind and agrees with the standard-model
expectation.
We present a measurement of the differential shapes of exclusive $B\to D^*
\ell \bar{\nu}_\ell$ ($B = B^-, \bar{B}^0 $ and $\ell = e, \mu$) decays with
hadronic tag-side reconstruction for the full ...Belle data set of
$711\,\mathrm{fb}^{-1}$ integrated luminosity. We extract the
Caprini-Lellouch-Neubert (CLN) and Boyd-Grinstein-Lebed (BGL) form factor
parameters and use an external input for the absolute branching fractions to
determine the Cabibbo-Kobayashi-Maskawa matrix element and find
$|V_{cb}|_\mathrm{CLN} = (40.1\pm0.9)\times 10^{-3}$ and $|V_{cb}|_\mathrm{BGL}
= (40.6\pm 0.9)\times 10^{-3}$ with the zero-recoil lattice QCD point
$\mathcal{F}(1) = 0.906 \pm 0.013$. We also perform a study of the impact of
preliminary beyond zero-recoil lattice QCD calculations on the $|V_{cb}|$
determinations. Additionally, we present the lepton flavor universality ratio
$R_{e\mu} = \mathcal{B}(B \to D^* e \bar{\nu}_e) / \mathcal{B}(B \to D^* \mu
\bar{\nu}_\mu) = 0.990 \pm 0.021 \pm 0.023$, the electron and muon
forward-backward asymmetry and their difference $\Delta A_{FB}=0.022\pm0.026\pm
0.007$, and the electron and muon $D^*$ longitudinal polarization fraction and
their difference $\Delta F_L^{D^*} = 0.034 \pm 0.024 \pm 0.007$. The
uncertainties quoted correspond to the statistical and systematic
uncertainties, respectively.
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.
We present a search for the lepton-flavour-violating decays $B^+ \to K^+
\tau^\pm \ell^\mp$, with $\ell = (e, \mu)$, using the full data sample of $772
\times 10^6$ $B\overline{B}$ pairs recorded by ...the Belle detector at the KEKB
asymmetric-energy $e^+ e^-$ collider. We use events in which one $B$ meson is
fully reconstructed in a hadronic decay mode. We find no evidence for $B^\pm
\to K^\pm \tau \ell$ decays and set upper limits on their branching fractions
at the 90% confidence level in the $(1$-$3) \times 10^{-5}$ range. The obtained
limits are the world's best results.
We present the novel implementation of a non-differentiable metric approximation and a corresponding loss-scheduling aimed at the search for new particles of unknown mass in high energy physics ...experiments. We call the loss-scheduling, based on the minimisation of a figure-of-merit related function typical of particle physics, a Punzi-loss function, and the neural network that utilises this loss function a Punzi-net. We show that the Punzi-net outperforms standard multivariate analysis techniques and generalises well to mass hypotheses for which it was not trained. This is achieved by training a single classifier that provides a coherent and optimal classification of all signal hypotheses over the whole search space. Our result constitutes a complementary approach to fully differentiable analyses in particle physics. We implemented this work using PyTorch and provide users full access to a public repository containing all the codes and a training example.
Phys. Rev. D 107, L031103 (2023) We report on a measurement of the $\Omega_c^0$ lifetime using $\Omega_c^0 \to
\Omega^-\pi^+$ decays reconstructed in $e^+e^-\to c\bar{c}$ data collected by
the Belle ...II experiment and corresponding to $207~{\rm fb^{-1}}$ of integrated
luminosity. The result, $\rm\tau(\Omega_c^0)=243\pm48( stat)\pm11(syst)~fs$,
agrees with recent measurements indicating that the $\Omega_c^0$ is not the
shortest-lived weakly decaying charmed baryon.
Physical Review Letters 130, 091902 (2023) We study the processes $e^+e^-\to\omega\chi_{bJ}(1P)$ ($J$ = 0, 1, or 2)
using samples at center-of-mass energies $\sqrt{s}$ = 10.701, 10.745, and
10.805 ...GeV, corresponding to 1.6, 9.8, and 4.7 fb$^{-1}$ of integrated
luminosity, respectively. These data were collected with the Belle II detector
during special operations of the SuperKEKB collider above the $\Upsilon(4S)$
resonance. We report the first observation of $\omega\chi_{bJ}(1P)$ signals at
$\sqrt{s}$ = 10.745 GeV. By combining Belle II data with Belle results at
$\sqrt{s}$ = 10.867 GeV, we find energy dependencies of the Born cross sections
for $e^+e^-\to \omega\chi_{b1,b2}(1P)$ to be consistent with the shape of the
$\Upsilon(10753)$ state. These data indicate that the internal structures of
the $\Upsilon(10753)$ and $\Upsilon(10860)$ states may differ. Including data
at $\sqrt{s}$ = 10.653 GeV, we also search for the bottomonium equivalent of
the $X(3872)$ state decaying into $\omega\Upsilon(1S)$. No significant signal
is observed for masses between 10.45 and 10.65 GeV/$c^2$.
Phys. Rev. Lett. 130, 071802 (2023) An absolute measurement of the $\Lambda^{+}_c$ lifetime is reported using
$\Lambda_c^+\rightarrow pK^-\pi^+$ decays in events reconstructed from data
collected by ...the Belle II experiment at the SuperKEKB asymmetric-energy
electron-positron collider. The total integrated luminosity of the data sample,
which was collected at center-of-mass energies at or near the $\Upsilon(4S)$
resonance, is $207.2~\mbox{fb}^{-1}$. The result, $\tau(\Lambda^{+}_c) = 203.20
\pm 0.89 \,\mathrm{(stat)} \pm 0.77 \,\mathrm{(syst)}$ fs, is the most precise
measurement to date and is consistent with previous determinations.
Phys. Rev. D 109, 032002 (Published 5 February 2024) A dark leptophilic scalar $(\phi_L)$ is a hypothetical particle that couples
only to leptons rather than quarks. We report on a search for ...$\phi_L$ in the
$e^+e^- \to \tau^+ \tau^- \phi_L, ~\phi_L \to \ell^+ \ell^- ~(\ell = e, \mu)$
process using 626~\invfb of data collected by the Belle experiment near the
\Y4S resonance. We validate the backgrounds with multiple control regions in
data, using a novel multiclass multivariate event classifier. In absence of a
signal, we quote upper limits at the 90\% confidence level on the coupling
between $\phi_L$ and leptons. Our bounds, obtained in a blinded approach, are
19\% more constraining than the previous limits, averaged over the mass range
$0.04 \leq m_{\phi_L} \leq 6.5~\gev$. We exclude the parameter space below
4~\gev favored by measurement of the anomalous magnetic moment of the muon.
Phys. Rev. Lett. 130, 071804 (2023) The dark photon $A^\prime$ and the dark Higgs boson $h^\prime$ are
hypothetical particles predicted in many dark sector models. We search for the
simultaneous ...production of $A^\prime$ and $h^\prime$ in the dark Higgsstrahlung
process $e^{+}e^{-}\rightarrow A^\prime \, h^\prime$ with $A^\prime \rightarrow
\mu^+\mu^-$ and $ h^\prime$ invisible in electron-positron collisions at a
center-of-mass energy of 10.58 GeV collected by the Belle II experiment in
2019. With an integrated luminosity of 8.34 fb$^{-1}$, we observe no evidence
for signal. We obtain exclusion limits at 90% Bayesian credibility in the range
of 1.7--5.0 fb on the cross section and in the range of $1.7
\times10^{-8}$--$200 \times10^{-8}$ on the effective coupling $\varepsilon^2
\times \alpha_D$ for the $A^\prime$ mass in the range of 4.0 GeV/$c^2$ $<
M_{A^\prime}< 9.7$ GeV/$c^2$ and for the $h^\prime$ mass $M_{h^\prime} <
M_{A^\prime}$, where $\varepsilon$ is the mixing strength between the standard
model and the dark photon and $\alpha_D$ is the coupling of the dark photon to
the dark Higgs boson. Our limits are the first in this mass range.
