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 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.
Using the entire data sample of \(980\,\textrm{fb}^{-1}\) collected with the Belle detector operating at the KEKB asymmetric-energy collider, we report the measurement of the mass, width, and the ...relative branching ratios of the \(\Lambda_c(2625)^+\) charmed baryon. The mass difference between \(\Lambda_c(2625)^+\) and \(\Lambda_c^+\) is measured to be \(M(\Lambda_c(2625)^{+}) - M(\Lambda_c^{+}) = 341.518 \pm 0.006 \pm 0.049\ \mathrm{MeV}/\mathit{c}^2\). The upper limit on the width is measured to be \(\Gamma(\Lambda_c(2625)^+) < 0.52\,\mathrm{MeV}/\textit{c}^2\) at 90\% credibility level. Based on a full Dalitz plot fit, branching ratios with respect to the mode \(\Lambda_c(2625)^+ \to \Lambda_c^+ \pi^+ \pi^-\) are measured to be \(\frac{\mathcal{B}(\Lambda_c(2625)^+ \to \Sigma_c^{0} \pi^{+})} {\mathcal{B}(\Lambda_c(2625)^+ \to \Lambda_c^+ \pi^{+} \pi^{-})} = (5.19 \pm 0.23 \pm 0.40) \%\) and \(\frac{\mathcal{B}(\Lambda_c(2625)^+ \to \Sigma_c^{++} \pi^{-})} {\mathcal{B}(\Lambda_c(2625)^+ \to \Lambda_c^+ \pi^{+} \pi^{-})} = (5.13 \pm 0.26 \pm 0.32) \%\). These measurements can be used to further constrain the parameters of the underlying theoretical models.
We present the analysis of two-particle angular correlations using coordinate systems defined with the conventional beam axis and the event thrust axis, and propose the latter to be a useful ...representation for the correlation structure interpretation in the \(e^+ e^-\) collision system. The \(e^+ e^-\) collisions to hadronic final states at center-of-mass energies of \(\sqrt{s} = 10.52\) GeV and \(10.58\) GeV are recorded by the Belle detector at KEKB. In this paper, results on the first dataset are supplementary to the previous Belle publication arXiv:2201.01694 while the latter one is the first two-particle correlation measurement at a collision energy on the \(\Upsilon(4S)\) resonance and sensitive to its decay products. Measurements are reported as a function of the charged-particle multiplicity. Finally, a qualitative understanding of the correlation structure is discussed using a combination of Monte Carlo simulations and experimental data.
Using the data sample of 980 fb\(^{-1}\) collected with the Belle detector operating at the KEKB asymmetric-energy \(e^+e^-\) collider, we present the results of an investigation of the ...\(\Lambda\pi^+\) and \(\Lambda\pi^-\) invariant mass distributions looking for substructure in the decay \(\Lambda_c^+\rightarrow\Lambda\pi^+\pi^+\pi^-\). We find a significant signal in each mass dis\ tribution. When interpreted as resonances, we find for the \(\Lambda\pi^+\) (\(\Lambda\pi^-\)) combination a mass of \(1434.3 \pm 0.6 (\mathrm{stat}) \pm 0.9(\mathrm{syst})\) MeV/\(c^2\) \(1438.5 \pm 0.9 (\mathrm{stat}) \pm 2.5(\mathrm{syst})\) MeV/\(c^2\), an intrinsic width of \(11.5 \pm 2.8 (\mathrm{stat}) \pm 5.3(\mathrm{syst})\) MeV/\(c^2\) \(33.0 \pm 7.5 (\mathrm{stat}) \pm 23.6(\mathrm{syst})\) MeV/\(c^2\) with a significance of 7.5\(\sigma\) (6.2\(\sigma\)). As these two signals are very close to the \(\bar{K}N\) threshold, we also investigate the possibility of a \(\bar{K}N\) cusp, and find that \ we cannot discriminate between these two interpretations due to the limited size of the data sample.
The processes \( e^+e^-\rightarrow \Sigma^0\overline{\Sigma}{}^0 \) and \( e^+e^-\rightarrow\Sigma^+\overline{\Sigma}{}^-\) are studied using initial-state-radiation events in a sample of 980 ...\(\,\mbox{fb}^{-1}\) collected with the Belle detector at the KEKB asymmetric-energy \( e^+e^- \) collider. The cross sections from the mass threshold to \( 3{\mathrm{\,Ge\kern -0.1em V\!/}c^2} \) and the effective form factors of \( \Sigma^0 \) and \( \Sigma^+ \) are measured. In the charmonium region, we observe the decays \(J/\psi\rightarrow\Sigma^0\overline{\Sigma}{}^0\) and \(J/\psi\rightarrow\Sigma^+\overline{\Sigma}{}^-\) and determine the respective branching fractions.
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/\(c^2\).
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 measure the ratio of branching fractions for the \(\Upsilon (4S)\) decays to \(B^+B^-\) and \(B^0\bar{B}{}^0\) using \(B^+ \rightarrow J/\psi(\ell\ell) K^+\) and \(B^0 \rightarrow J/\psi(\ell\ell) ...K^0\) samples, where \(J/\psi(\ell\ell)\) stands for \(J/\psi \to \ell^+\ell^-\) (\(\ell = e\) or \(\mu\)), with \(711\) fb\(^{-1}\) of data collected at the \(\Upsilon(4S)\) resonance with the Belle detector. We find the decay rate ratio of \(\Upsilon(4S) \rightarrow B^+B^-\) over \(\Upsilon(4S) \rightarrow B^0\bar{B}{}^0\) to be \(1.065\pm0.012\pm 0.019 \pm 0.047\), which is the most precise measurement to date. The first and second uncertainties are statistical and systematic, respectively, and the third uncertainty is systematic due to the assumption of isospin symmetry in \(B \to J/\psi(\ell\ell) K\).
We present a search for the decay \(X(3872) \to \pi^+\pi^-\pi^0\) in the \((772\pm11)\times10^6\) \(\Upsilon(4S)\to B \bar B\) data sample collected at the Belle detector, where the \(X(3872)\) is ...produced in \(B^{\pm}\to K^{\pm}X(3872)\) and \(B^{0}\to K_{S}^0 X(3872)\) decays. We do not observe a signal, and set 90\% credible upper limits for two different models of the decay processes: if the decay products are distributed uniformly in phase space, \(\mathcal{B}(X(3872) \to \pi^+\pi^-\pi^0) < 1.3\%\); if \(M(\pi^+\pi^-)\) is concentrated near the mass of the \(D^0 \bar D^0\) pair in the process \(X(3872)\to D^0\bar{D}^{*0}+c.c.\to D^0 \bar D^{0}\pi^0\to\pi^+ \pi^- \pi^0\), \(\mathcal{B}(X(3872) \to \pi^+\pi^-\pi^0) < 1.2\times10^{-3}\).
