The total decay widths of the charmed baryons are calculated by means of the ${}^3{P}_0$ model. Our calculations consider in the final states: the charmed baryon-(vector/pseudoscalar) meson pairs and the (octet/decuplet) baryon-(pseudoscalar/vector) charmed meson pairs, within a constituent quark model. Furthermore, we calculate the masses of the charmed baryon ground states and their excitations up to the $D$-wave in a constituent quark model both in the three-quark and in the quark-diquark schemes, utilizing a Hamiltonian model based on a harmonic oscillator potential plus a mass splitting term that encodes the spin, spin-orbit, isospin, and flavor interactions. The parameters of the Hamiltonian model are fitted to the experimental data of the charmed baryon masses and decay widths. As the experimental uncertainties of the data affect the fitted model parameters, we have thoroughly propagated these uncertainties into our predicted charmed baryon masses and decay widths via a Monte Carlo bootstrap approach, which is often absent in other theoretical studies on this subject. Our quantum number assignments and predictions of the masses and strong partial decay widths are in reasonable agreement with the available data. Thus, our results show the ability to guide future measurements in LHCb, Belle and Belle II experiments. Finally, the appendices provide some details of our calculations, in which we include the flavor coupling coefficients, which are useful for further theoretical investigations.

We search for lepton-flavor-violating ${\tau }^{-}\to e^{-}\alpha$ and ${\tau }^{-}\to {\mu }^{-}\alpha$ decays, where $\alpha$ is an invisible spin-0 boson. The search uses electron-positron collisions at 10.58 GeV center-of-mass energy with an integrated luminosity of $62.8{\mathrm{fb}}^{-1}$, produced by the SuperKEKB collider and collected with the Belle II detector. We search for an excess in the lepton-energy spectrum of the known ${\tau }^{-}\to e^{-}{\overline{\nu }}_e{\nu }_{\tau }$ and ${\tau }^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }{\nu }_{\tau }$ decays. We report 95% confidence-level upper limits on the branching-fraction ratio $\mathcal{B}({\tau }^{-}\to e^{-}\alpha )/\mathcal{B}({\tau }^{-}\to e^{-}{\overline{\nu }}_e{\nu }_{\tau })$ in the range $(1.1–9.7)\times {10}^{-3}$ and on $\mathcal{B}({\tau }^{-}\to {\mu }^{-}\alpha )/\mathcal{B}({\tau }^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }{\nu }_{\tau })$ in the range $(0.7–12.2)\times {10}^{-3}$ for $\alpha$ masses between 0 and $1.6\mathrm{GeV}/c^2$. These results provide the most stringent bounds on invisible boson production from $\tau$ decays.

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 $4.7{\mathrm{fb}}^{-1}$ of integrated luminosity, respectively. These data were collected with the Belle II detector during special operations of the SuperKEKB collider above the $\mathrm{\Upsilon }(4S)$ resonance. We report the first observation of $\omega {\chi }_{bJ}(1P)$ signals at $\sqrt{s}=10.745\mathrm{GeV}$. By combining Belle II data with Belle results at $\sqrt{s}=10.867\mathrm{GeV}$, 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 $\mathrm{\Upsilon }(10753)$ state. These data indicate that the internal structures of the $\mathrm{\Upsilon }(10753)$ and $\mathrm{\Upsilon }(10860)$ states may differ. Including data at $\sqrt{s}=10.653\mathrm{GeV}$, we also search for the bottomonium equivalent of the $X(3872)$ state decaying into $\omega \mathrm{\Upsilon }(1S)$. No significant signal is observed for masses between 10.45 and $10.65\mathrm{GeV}/c^2$.

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 $e^{+}{e}^{-}\to A^{\prime }{h}^{\prime }$ with $A^{\prime }\to {\mu }^{+}{\mu }^{-}$ and $h^{\prime }$ invisible in electron-positron collisions at a center-of-mass energy of 10.58 GeV in data collected by the Belle II experiment in 2019. With an integrated luminosity of $8.34{\mathrm{fb}}^{-1}$, 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 $1.7\times {10}^{-8}–200\times {10}^{-8}$ on the effective coupling ${ϵ}^2\times {\alpha }_D$ for the $A^{\prime }$ mass in the range of $4.0\mathrm{GeV}/c^2<M_{A^{\prime }}<9.7\mathrm{GeV}/c^2$ and for the $h^{\prime }$ mass $M_{h^{\prime }}<M_{A^{\prime }}$, where $ϵ$ 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.

The $L_{\mu }-L_{\tau }$ extension of the standard model predicts the existence of a lepton-flavor-universality-violating $Z^{\prime }$ boson that couples only to the heavier lepton families. We search for such a $Z^{\prime }$ through its invisible decay in the process $e^{+}{e}^{-}\to {\mu }^{+}{\mu }^{-}{Z}^{\prime }$. We use a sample of electron-positron collisions at a center-of-mass energy of 10.58 GeV collected by the Belle II experiment in 2019–2020, corresponding to an integrated luminosity of $79.7{\mathrm{fb}}^{-1}$. We find no excess over the expected standard-model background. We set 90%-confidence-level upper limits on the cross section for this process as well as on the coupling of the model, which ranges from $3\times {10}^{-3}$ at low $Z^{\prime }$ masses to 1 at $Z^{\prime }$ masses of $8\mathrm{GeV}/c^2$.

We report on a measurement of the ${\mathrm{\Omega }}_c^0$ lifetime using ${\mathrm{\Omega }}_c^0\to {\mathrm{\Omega }}^{-}{\pi }^{+}$ decays reconstructed in $e^{+}{e}^{-}\to c\overline{c}$ data collected by the Belle II experiment and corresponding to $207{\mathrm{fb}}^{-1}$ of integrated luminosity. The result, $\tau ({\mathrm{\Omega }}_c^0)=243\pm 48(\mathrm{stat})\pm 11(\mathrm{syst})\mathrm{fs}$, agrees with recent measurements indicating that the ${\mathrm{\Omega }}_c^0$ is not the shortest-lived weakly decaying charmed baryon.