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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.more » « less
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A<sc>bstract</sc> We report the first measurement of the inclusivee+e−→$$ b\overline{b} $$ →$$ {D}_s^{\pm } $$ Xande+e−→$$ b\overline{b} $$ → D0/$$ {\overline{D}}^0 $$ Xcross sections in the energy range from 10.63 to 11.02 GeV. Based on these results, we determineσ(e+e−→$$ {B}_s^0{\overline{B}}_s^0 $$ X) andσ(e+e−→$$ B\overline{B} $$ X) in the same energy range. We measure the fraction of$$ {B}_s^0 $$ events at Υ(10860) to befs= ($$ {22.0}_{-2.1}^{+2.0} $$ )%. We determine also the ratio of the$$ {B}_s^0 $$ inclusive branching fractions$$ \mathcal{B} $$ ($$ {B}_s^0 $$ → D0/$$ {\overline{D}}^0 $$ X)/$$ \mathcal{B} $$ ($$ {B}_s^0 $$ →$$ {D}_s^{\pm } $$ 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.more » « less
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We measure the tau-to-light-lepton ratio of inclusive -meson branching fractions , where indicates an electron or muon, and thereby test the universality of charged-current weak interactions. We select events that have one fully reconstructed meson and a charged lepton candidate from of electron-positron collision data collected with the Belle II detector. We find , in agreement with standard-model expectations. This is the first direct measurement of . Published by the American Physical Society2024more » « less
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We search for the rare decay in a sample of electron-positron collisions at the resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying meson in events to suppress background from other decays of the signal candidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanying 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 branching fraction of and , respectively. Combining the results, we determine the branching fraction of the decay to be , 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 Society2024more » « lessFree, publicly-accessible full text available June 1, 2025