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We present the design of a pair spectrometer for use at FACET-II, where there is a need for spectroscopy of photons having energies up to 10 GeV. Incoming gammas are converted to high-energy positron-electron pairs, which are then subsequently analyzed in a dipole magnet. These charged particles are then recorded in arrays of acrylic Cherenkov counters, which are significantly less sensitive to background x-rays than scintillator counters in this case. To reconstruct energies of single high-energy photons, the spectrometer has a sensitivity to single positron-electron pairs. Even in this single-photon limit, there is always some low-energy continuum present, so spectral deconvolution is not trivial, for which we demonstrate a maximum likelihood reconstruction. Finally, end-to-end simulations of experimental scenarios, together with anticipated backgrounds, are presented. 

We present a measurement of the branching fraction and fraction of longitudinal polarization of $B^0\to {\rho }^{+}{\rho }^{-}$decays, which have two ${\pi }^0$’s in the final state. We also measure time-dependent $CP$violation parameters for decays into longitudinally polarized ${\rho }^{+}{\rho }^{-}$pairs. This analysis is based on a data sample containing $(387\pm 6)\times {10}^6$ $\mathrm{\Upsilon }\left(4S\right)$mesons collected with the Belle II detector at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$collider in 2019–2022. We obtain $\mathcal{B}(B^0\to {\rho }^{+}{\rho }^{-})=\left(2.89_{-0.22}^{+0.23}{}_{-0.27}^{+0.29}\right)\times {10}^{-5}$, $f_L=0.921_{-0.025}^{+0.024}{}_{-0.015}^{+0.017}$, $S=-0.26\pm 0.19\pm 0.08$, and $C=-0.02\pm 0.12_{-0.05}^{+0.06}$, where the first uncertainties are statistical and the second are systematic. We use these results to perform an isospin analysis to constrain the Cabibbo-Kobayashi-Maskawa angle ${\varphi }_2$and obtain two solutions; the result consistent with other Standard Model constraints is ${\varphi }_2=\left({92.6}_{-4.7}^{+4.5}\right)°$. Published by the American Physical Society2025 

A<sc>bstract</sc> We perform the first search forCPviolation in$$ {D}_{(s)}^{+}\to {K}_S^0{K}^{-}{\pi}^{+}{\pi}^{+} $$ $D_{\left(s\right)}^{+}\to K_S^0{K}^{-}{\pi }^{+}{\pi }^{+}$decays. We use a combined data set from the Belle and Belle II experiments, which studye⁺e⁻collisions at center-of-mass energies at or near the Υ(4S) resonance. We use 980 fb⁻¹of data from Belle and 428 fb⁻¹of data from Belle II. We measure sixCP-violating asymmetries that are based on triple products and quadruple products of the momenta of final-state particles, and also the particles’ helicity angles. We obtain a precision at the level of 0.5% for$$ {D}^{+}\to {K}_S^0{K}^{-}{\pi}^{+}{\pi}^{+} $$ $D^{+}\to K_S^0{K}^{-}{\pi }^{+}{\pi }^{+}$decays, and better than 0.3% for$$ {D}_s^{+}\to {K}_S^0{K}^{-}{\pi}^{+}{\pi}^{+} $$ $D_s^{+}\to K_S^0{K}^{-}{\pi }^{+}{\pi }^{+}$decays. No evidence ofCPviolation is found. Our results for the triple-product asymmetries are the most precise to date for singly-Cabibbo-suppressedD⁺decays. Our results for the other asymmetries are the first such measurements performed for charm decays. 

A<sc>bstract</sc> We report the first measurement of the inclusivee⁺e⁻→$$ b\overline{b} $$ $b\overline{b}$→$$ {D}_s^{\pm } $$ $D_s^{\pm }$Xande⁺e⁻→$$ b\overline{b} $$ $b\overline{b}$→ D⁰/$$ {\overline{D}}^0 $$ ${\overline{D}}^0$Xcross sections in the energy range from 10.63 to 11.02 GeV. Based on these results, we determineσ(e⁺e⁻→$$ {B}_s^0{\overline{B}}_s^0 $$ $B_s^0{\overline{B}}_s^0$X) andσ(e⁺e⁻→$$ B\overline{B} $$ $B\overline{B}$X) in the same energy range. We measure the fraction of$$ {B}_s^0 $$ $B_s^0$events at Υ(10860) to bef_s= ($$ {22.0}_{-2.1}^{+2.0} $$ ${22.0}_{-2.1}^{+2.0}$)%. We determine also the ratio of the$$ {B}_s^0 $$ $B_s^0$inclusive branching fractions$$ \mathcal{B} $$ $B$($$ {B}_s^0 $$ $B_s^0$→ D⁰/$$ {\overline{D}}^0 $$ ${\overline{D}}^0$X)/$$ \mathcal{B} $$ $B$($$ {B}_s^0 $$ $B_s^0$→$$ {D}_s^{\pm } $$ $D_s^{\pm }$X) = 0.416 ± 0.018 ± 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energye⁺e⁻collider. 

We measure the tau-to-light-lepton ratio of inclusive $B$-meson branching fractions $R\left(X_{\tau /\ell }\right)\equiv \mathcal{B}(B\to X\tau \nu )/\mathcal{B}(B\to X\ell \nu )$, where $\ell$indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed $B$meson and a charged lepton candidate from $189{\mathrm{fb}}^{-1}$of electron-positron collision data collected with the Belle II detector. We find $R\left(X_{\tau /\ell }\right)=0.228\pm 0.016\left(\mathrm{stat}\right)\pm 0.036\left(\mathrm{syst}\right)$, in agreement with standard-model expectations. This is the first direct measurement of $R\left(X_{\tau /\ell }\right)$. Published by the American Physical Society2024 

We search for the rare decay $B^{+}\to K^{+}\nu \overline{\nu }$in a $362{\mathrm{fb}}^{-1}$sample of electron-positron collisions at the $\mathrm{\Upsilon }\left(4S\right)$resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying $B$meson in $\mathrm{\Upsilon }\left(4S\right)\to B\overline{B}$events to suppress background from other decays of the signal $B$candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying $B$meson. For background suppression, we exploit distinct signal features using machine learning methods tuned with simulated data. The signal-reconstruction efficiency and background suppression are validated through various control channels. The branching fraction is extracted in a maximum likelihood fit. Our inclusive and hadronic analyses yield consistent results for the $B^{+}\to K^{+}\nu \overline{\nu }$branching fraction of $[2.7\pm 0.5(\mathrm{stat})\pm 0.5(\mathrm{syst}\left)\right]\times {10}^{-5}$and $\left[{1.1}_{-0.8}^{+0.9}\right(\mathrm{stat}{)}_{-0.5}^{+0.8}\left(\mathrm{syst}\right)]\times {10}^{-5}$, respectively. Combining the results, we determine the branching fraction of the decay $B^{+}\to K^{+}\nu \overline{\nu }$to be $[2.3\pm 0.5(\mathrm{stat}{)}_{-0.4}^{+0.5}\left(\mathrm{syst}\right)]\times {10}^{-5}$, providing the first evidence for this decay at 3.5 standard deviations. The combined result is 2.7 standard deviations above the standard model expectation. Published by the American Physical Society2024