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Using a data sample of $427.9{\mathrm{fb}}^{-1}$collected by the Belle II detector at or near the $\mathrm{\Upsilon }\left(4S\right)$and $\mathrm{\Upsilon }\left(10753\right)$resonances, the cross sections for $e^{+}{e}^{-}\to h^{+}{h}^{-}J/\psi (h=\pi /K/p)$at center-of-mass energies ranging from 3.8 GeV or the production threshold to $5.5/6.0/7.0\mathrm{GeV}$have been measured via initial-state radiation. The cross sections for the processes $e^{+}{e}^{-}\to {\pi }^{+}{\pi }^{-}J/\psi$and $e^{+}{e}^{-}\to K^{+}{K}^{-}J/\psi$are consistent with previously published results. The cross sections for these channels obtained by combining with previous Belle results are also given. The process $e^{+}{e}^{-}\to p\overline{p}J/\psi$is investigated for the first time. The yields are small and no significant structure is observed in the cross section versus energy. Searches for vector charmoniumlike states in the $h^{+}{h}^{-}J/\psi$systems, and for associated intermediate states in the $h^{\pm }J/\psi$systems, are also presented. 

We report measurements of production cross sections for ${\rho }^{+}$, ${\rho }^0$, $\omega$, $K^{*+}$, $K^{*0}$, $\varphi$, $\eta$, $K_S^0$, $f_0\left(980\right)$, $D^{+}$, $D^0$, $D_s^{+}$, $D^{*+}$, $D^{*0}$, and $D_s^{*+}$in $e^{+}{e}^{-}$collisions at a center-of-mass energy near 10.58 GeV. The data were recorded by the Belle experiment, consisting of $571{\mathrm{fb}}^{-1}$at 10.58 GeV and $74{\mathrm{fb}}^{-1}$at 10.52 GeV. Production cross sections are extracted as a function of the fractional hadron momentum $x_p$. The measurements are compared to Monte Carlo generator predictions with various fragmentation settings, including those that have increased fragmentation into vector mesons over pseudoscalar mesons. The cross sections measured for light hadrons are consistent with no additional increase of vector over pseudoscalar mesons. The charmed-meson cross sections are compared to earlier measurements—when available—including older Belle results, which they supersede. They are in agreement before application of an improved initial-state radiation correction procedure that causes slight changes in their $x_p$shapes. Published by the American Physical Society2025 

A<sc>bstract</sc> We report a search for the lepton-flavor-violating decaysτ^±→ℓ^±α(ℓ=e,μ), whereαis an undetected spin-0 particle, such as an axion-like particle using 736 × 10⁶tau lepton pairs collected by the Belle detector at the KEKB asymmetric-energye⁺e⁻collider. We find no evidence of signal and obtain the most stringent upper limits on the branching fractions at 95% confidence level:$$\mathcal{B}$$(τ^±→ e^±α) < (0.4–6.4)×10⁻⁴and$$\mathcal{B}$$(τ^±→ μ^±α) < (0.2–3.5)×10⁻⁴at 95% confidence level for anαmass in the range 0.0≤ m_α≤1.6 GeV/c². 

We report decay-rate $CP$asymmetries of the singly-Cabibbo-suppressed decays ${\mathrm{\Xi }}_c^{+}\to {\mathrm{\Sigma }}^{+}{h}^{+}{h}^{-}$and ${\mathrm{\Lambda }}_c^{+}\to p{h}^{+}{h}^{-}$, with $h=K$, $\pi$, measured using $428{\mathrm{fb}}^{-1}$of $e^{+}{e}^{-}$collisions collected by the Belle II experiment at the SuperKEKB collider. The results, $A_{CP}({\mathrm{\Xi }}_c^{+}\to {\mathrm{\Sigma }}^{+}{K}^{+}{K}^{-})=(3.7\pm 6.6\pm 0.6)\%$, $A_{CP}({\mathrm{\Xi }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^{+}{\pi }^{-})=(9.5\pm 6.8\pm 0.5)\%$, $A_{CP}({\mathrm{\Lambda }}_c^{+}\to p{K}^{+}{K}^{-})=(3.9\pm 1.7\pm 0.7)\%$, $A_{CP}({\mathrm{\Lambda }}_c^{+}\to p{\pi }^{+}{\pi }^{-})=(0.3\pm 1.0\pm 0.2)\%$, where the first uncertainties are statistical and the second systematic, agree with $CP$symmetry. From these results we derive the sums $A_{CP}({\mathrm{\Xi }}_c^{+}\to {\mathrm{\Sigma }}^{+}{\pi }^{+}{\pi }^{-})+A_{CP}({\mathrm{\Lambda }}_c^{+}\to p{K}^{+}{K}^{-})=(13.4\pm 7.0\pm 0.9)\%$, $A_{CP}({\mathrm{\Xi }}_c^{+}\to {\mathrm{\Sigma }}^{+}{K}^{+}{K}^{-})+A_{CP}({\mathrm{\Lambda }}_c^{+}\to p{\pi }^{+}{\pi }^{-})=(4.0\pm 6.6\pm 0.7)\%$, which are consistent with the $U$-spin symmetry prediction of zero. These are the first measurements of $CP$asymmetries for individual hadronic three-body charmed-baryon decays. 

We perform an angular analysis of the $B\to K^{*}{e}^{+}{e}^{-}$decay for the dielectron mass squared, $q^2$, range of $0.0008–1.1200{\mathrm{GeV}}^2/c^4$using the full Belle dataset in the $K^{*0}\to K^{+}{\pi }^{-}$and $K^{*+}\to K_S^0{\pi }^{+}$channels, incorporating new methods of electron identification to improve the statistical power of the dataset. This analysis is sensitive to contributions from right-handed currents from physics beyond the Standard Model by constraining the Wilson coefficients ${\mathcal{C}}_7^{(\prime )}$. We perform a fit to the $B\to K^{*}{e}^{+}{e}^{-}$differential decay rate and measure the imaginary component of the transversality amplitude to be $A_T^{\mathrm{Im}}=-1.27\pm 0.52\pm 0.12$, and the $K^{*}$transverse asymmetry to be $A_T^{\left(2\right)}=0.52\pm 0.53\pm 0.11$, with $F_L$and $A_T^{\mathrm{Re}}$fixed to the Standard Model values. The resulting constraints on the value of ${\mathcal{C}}_7^{\prime }$are consistent with the Standard Model within a $2\sigma$confidence interval. Published by the American Physical Society2024 

We present a determination of the Cabibbo-Kobayashi-Maskawa matrix element $\left|V_{cb}\right|$from the decay $B\to D\ell {\nu }_{\ell }$using a $365{\mathrm{fb}}^{-1}$ $e^{+}{e}^{-}\to \mathrm{\Upsilon }\left(4S\right)\to B\overline{B}$data sample recorded by the Belle II experiment at the SuperKEKB collider. The semileptonic decay of one $B$meson is reconstructed in the modes $B^0\to D^{-}(\to K^{+}{\pi }^{-}{\pi }^{-}){\ell }^{+}{\nu }_{\ell }$and $B^{+}\to {\overline{D}}^0(\to K^{+}{\pi }^{-}){\ell }^{+}{\nu }_{\ell }$, where $\ell$denotes either an electron or a muon. Charge conjugation is implied. The second $B$meson in the $\mathrm{\Upsilon }\left(4S\right)$event is not reconstructed explicitly. Using an inclusive reconstruction of the unobserved neutrino momentum, we determine the recoil variable $w=v_B·v_D$, where $v_B$and $v_D$are the four-velocities of the $B$and $D$mesons. We measure the total decay branching fractions to be $\mathcal{B}(B^0\to D^{-}{\ell }^{+}{\nu }_{\ell })=(2.06\pm 0.05(\mathrm{stat})\pm 0.10(\mathrm{sys}\left)\right)\%$and $\mathcal{B}(B^{+}\to {\overline{D}}^0{\ell }^{+}{\nu }_{\ell })=(2.31\pm 0.04(\mathrm{stat})\pm 0.09(\mathrm{sys}\left)\right)\%$. We probe lepton flavor universality by measuring $\mathcal{B}(B\to De{\nu }_e)/\mathcal{B}(B\to D\mu {\nu }_{\mu })=1.020\pm 0.020\left(\mathrm{stat}\right)\pm 0.022\left(\mathrm{sys}\right)$. Fitting the partial decay branching fraction as a function of $w$and using the average of lattice QCD calculations of the $B\to D$form factor, we obtain $\left|V_{cb}\right|=(39.2\pm 0.4(\mathrm{stat})\pm 0.6(\mathrm{sys})\pm 0.5(\mathrm{th}\left)\right)\times {10}^{-3}$. 

We measure the complete set of angular coefficients $J_i$for exclusive $\overline{B}\to D^{*}\ell {\overline{\nu }}_{\ell }$decays ( $\ell =e$, $\mu$). Our analysis uses the full $711{\mathrm{fb}}^{-1}$Belle dataset with hadronic tag-side reconstruction. The results allow us to extract the form factors describing the $\overline{B}\to D^{*}$transition and the Cabibbo-Kobayashi-Maskawa matrix element $\left|V_{\mathrm{cb}}\right|$. Using recent lattice QCD calculations for the hadronic form factors, we find $\left|V_{\mathrm{cb}}\right|=(40.7\pm 0.7)\times {10}^{-3}$using the Boyd-Grinstein-Lebed parametrization, compatible with determinations from inclusive semileptonic decays. We search for lepton flavor universality violation as a function of the hadronic recoil parameter $w$and investigate the differences of the electron and muon angular distributions. We find no deviation from standard model expectations. Published by the American Physical Society2024 

We report the results of the first search for $B^{-}$decays to the ${\overline{\mathrm{\Xi }}}_c^0{\overline{\mathrm{\Lambda }}}_c^{-}$final state using $711{\mathrm{fb}}^{-1}$of data collected at the $\mathrm{\Upsilon }\left(4S\right)$resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$collider. The results are interpreted in terms of both direct baryon-number-violating $B^{-}$decay and ${\mathrm{\Xi }}_c^0-{\overline{\mathrm{\Xi }}}_c^0$oscillations which follow the standard model decay $B^{-}\to {\mathrm{\Xi }}_c^0{\overline{\mathrm{\Lambda }}}_c^{-}$. We observe no evidence for baryon number violation and set the 95% confidence-level upper limits on the ratio of baryon-number-violating and standard model branching fractions $\mathcal{B}(B^{-}\to {\overline{\mathrm{\Xi }}}_c^0{\overline{\mathrm{\Lambda }}}_c^{-})/\mathcal{B}(B^{-}\to {\mathrm{\Xi }}_c^0{\overline{\mathrm{\Lambda }}}_c^{-})$to be $<2.7\%$and on the effective angular frequency of mixing $\omega$in ${\mathrm{\Xi }}_c^0-{\overline{\mathrm{\Xi }}}_c^0$oscillations to be $<0.76{\mathrm{ps}}^{-1}$(equivalent to ${\tau }_{\mathrm{mix}}>1.3\mathrm{ps}$). Published by the American Physical Society2024 

We report a search for a heavy neutral lepton (HNL) that mixes predominantly with ${\nu }_{\tau }$. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy $e^{+}{e}^{-}$collider. The data sample was collected at and just below the center-of-mass energies of the $\mathrm{\Upsilon }\left(4S\right)$and $\mathrm{\Upsilon }\left(5S\right)$resonances and has an integrated luminosity of $915{\mathrm{fb}}^{-1}$, corresponding to $(836\pm 12)\times {10}^6$ $e^{+}{e}^{-}\to {\tau }^{+}{\tau }^{-}$events. We search for production of the HNL (denoted $N$) in the decay ${\tau }^{-}\to {\pi }^{-}N$followed by its decay via $N\to {\mu }^{+}{\mu }^{-}{\nu }_{\tau }$. The search focuses on the parameter-space region in which the HNL is long-lived, so that the ${\mu }^{+}{\mu }^{-}$originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the $\tau$neutrino as a function of the HNL mass. Published by the American Physical Society2024