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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 

The ratio of branching fractions $R\left(D^{*}\right)=\mathcal{B}(\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau })/\mathcal{B}(\overline{B}\to D^{*}{\ell }^{-}{\overline{\nu }}_{\ell })$, where $\ell$is an electron or muon, is measured using a Belle II data sample with an integrated luminosity of $189{\mathrm{fb}}^{-1}$at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$collider. Data is collected at the $\mathrm{\Upsilon }\left(4\mathrm{S}\right)$resonance, and one $B$meson in the $\mathrm{\Upsilon }\left(4\mathrm{S}\right)\to B\overline{B}$decay is fully reconstructed in hadronic decay modes. The accompanying signal $B$meson is reconstructed as $\overline{B}\to D^{*}{\tau }^{-}{\overline{\nu }}_{\tau }$using leptonic $\tau$decays. The normalization decay, $\overline{B}\to D^{*}{\ell }^{-}{\overline{\nu }}_{\ell }$, produces the same observable final-state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields $R\left(D^{*}\right)={0.262}_{-0.039}^{+0.041}\left(\mathrm{stat}{)}_{-0.032}^{+0.035}\right(\mathrm{syst})$. This result is consistent with the current world average and with Standard Model predictions. Published by the American Physical Society2024 

We present the results of a search for the $b\to d{\ell }^{+}{\ell }^{-}$flavor-changing neutral-current rare decays $B^{+,0}\to (\eta ,\omega ,{\pi }^{+,0},{\rho }^{+,0})e^{+}{e}^{-}$and $B^{+,0}\to (\eta ,\omega ,{\pi }^0,{\rho }^{+}){\mu }^{+}{\mu }^{-}$using a $711{\mathrm{fb}}^{-1}$data sample that contains $772\times {10}^{6}B\overline{B}$events. The data were collected at the $\mathrm{\Upsilon }\left(4S\right)$resonance with the Belle detector at the KEKB asymmetric-energy $e^{+}{e}^{-}$collider. We find no evidence for signal and set upper limits on branching fractions at the 90% confidence level in the range $\left(3.8–47\right)\times {10}^{-8}$depending on the decay channel. The obtained limits are the world’s best results. This is the first search for the channels $B^{+,0}\to (\omega ,{\rho }^{+,0})e^{+}{e}^{-}$and $B^{+,0}\to (\omega ,{\rho }^{+}){\mu }^{+}{\mu }^{-}$. Published by the American Physical Society2024 

A<sc>bstract</sc> We report measurements of thee⁺e⁻→$$ B\overline{B} $$ $B\overline{B}$,$$ B{\overline{B}}^{\ast } $$ $B{\overline{B}}^{\ast }$, and$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{\ast }{\overline{B}}^{\ast }$cross sections at four energies, 10653, 10701, 10746 and 10805 MeV, using data collected by the Belle II experiment. We reconstruct oneBmeson in a large number of hadronic final states and use its momentum to identify the production process. In the first 2 – 5 MeV above$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{\ast }{\overline{B}}^{\ast }$threshold, thee⁺e⁻→$$ {B}^{\ast }{\overline{B}}^{\ast } $$ $B^{\ast }{\overline{B}}^{\ast }$cross section increases rapidly. This may indicate the presence of a pole close to the threshold. 

A<sc>bstract</sc> We present the result of a search for the charged-lepton-flavor violating decayτ⁻→μ⁻μ⁺μ⁻using a 424 fb⁻¹sample of data recorded by the Belle II experiment at the SuperKEKBe⁺e⁻collider. The selection ofe⁺e⁻→τ⁺τ⁻events is based on an inclusive reconstruction of the non-signal tau decay, and on a boosted decision tree to suppress background. We observe one signal candidate, which is compatible with the expectation from background processes. We set a 90% confidence level upper limit of 1.9×10⁻⁸on the branching fraction of theτ⁻→ μ⁻μ⁺μ⁻decay, which is the most stringent bound to date. 

A<sc>bstract</sc> We present a search for the lepton-flavor-violating decays$$ {B}_s^0 $$ $B_s^0$→ℓ^∓τ^±, whereℓ=e, μ, using the full data sample of 121 fb⁻¹collected at the Υ(5S) resonance with the Belle detector at the KEKB asymmetric-energye⁺e⁻collider. We use$$ {B}_s^0{\overline{B}}_s^0 $$ $B_s^0{\overline{B}}_s^0$events in which one$$ {B}_s^0 $$ $B_s^0$meson is reconstructed in a semileptonic decay mode and the other in the signal mode. We find no evidence for$$ {B}_s^0 $$ $B_s^0$→ ℓ^∓τ^±decays and set upper limits on their branching fractions at 90% confidence level as$$ \mathcal{B} $$ $B$($$ {B}_s^0 $$ $B_s^0$→ e^∓τ^±)<14×10⁻⁴and$$ \mathcal{B} $$ $B$($$ {B}_s^0 $$ $B_s^0$→ μ^∓τ^±)<7.3×10⁻⁴. Our result represents the first upper limit on the$$ {B}_s^0 $$ $B_s^0$→ e^∓τ^±decay rate. 

A<sc>bstract</sc> We present a measurement of the ratio$$ {R}_{\mu }=\mathcal{B}\left({\tau}^{-}\to {\mu}^{-}{\overline{\nu}}_{\mu }{\nu}_{\tau}\right)/\mathcal{B}\left({\tau}^{-}\to {e}^{-}{\overline{\nu}}_e{\nu}_{\tau}\right) $$ $R_{\mu }=B\left({\tau }^{-}\to {\mu }^{-}{\overline{\nu }}_{\mu }{\nu }_{\tau }\right)/B\left({\tau }^{-}\to e^{-}{\overline{\nu }}_e{\nu }_{\tau }\right)$of branching fractions$$ \mathcal{B} $$ $B$of theτlepton decaying to muons or electrons using data collected with the Belle II detector at the SuperKEKBe⁺e⁻collider. The sample has an integrated luminosity of 362 ± 2 fb⁻¹at a centre-of-mass energy of 10.58 GeV. Using an optimised event selection, a binned maximum likelihood fit is performed using the momentum spectra of the electron and muon candidates. The result,R_μ= 0.9675 ± 0.0007 ± 0.0036, where the first uncertainty is statistical and the second is systematic, is the most precise to date. It provides a stringent test of the light-lepton universality, translating to a ratio of the couplings of the muon and electron to theWboson inτdecays of 0.9974 ± 0.0019, in agreement with the standard model expectation of unity.