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  1. 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. 
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  2. A<sc>bstract</sc> We report the first measurement of the inclusivee+e→$$ b\overline{b} $$ b b ¯ →$$ {D}_s^{\pm } $$ D s ± Xande+e→$$ b\overline{b} $$ b b ¯ → D0/$$ {\overline{D}}^0 $$ 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 B ¯ s 0 X) andσ(e+e→$$ B\overline{B} $$ B B ¯ X) in the same energy range. We measure the fraction of$$ {B}_s^0 $$ B s 0 events at Υ(10860) to befs= ($$ {22.0}_{-2.1}^{+2.0} $$ 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/$$ {\overline{D}}^0 $$ D ¯ 0 X)/$$ \mathcal{B} $$ B ($$ {B}_s^0 $$ B s 0 →$$ {D}_s^{\pm } $$ D s ± X) = 0.416 ± 0.018 ± 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energye+ecollider. 
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  3. We measure the tau-to-light-lepton ratio of inclusive B -meson branching fractions R ( X τ / ) B ( B X τ ν ) / B ( B X ν ) , where 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 fb 1 of electron-positron collision data collected with the Belle II detector. We find R ( X τ / ) = 0.228 ± 0.016 ( stat ) ± 0.036 ( syst ) , in agreement with standard-model expectations. This is the first direct measurement of R ( X τ / ) . Published by the American Physical Society2024 
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  4. We search for the rare decay B + K + ν ν ¯ in a 362 fb 1 sample of electron-positron collisions at the ϒ ( 4 S ) resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanying B meson in ϒ ( 4 S ) B 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 + K + ν ν ¯ branching fraction of [ 2.7 ± 0.5 ( stat ) ± 0.5 ( syst ) ] × 10 5 and [ 1.1 0.8 + 0.9 ( stat ) 0.5 + 0.8 ( syst ) ] × 10 5 , respectively. Combining the results, we determine the branching fraction of the decay B + K + ν ν ¯ to be [ 2.3 ± 0.5 ( stat ) 0.4 + 0.5 ( syst ) ] × 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 
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    Free, publicly-accessible full text available June 1, 2025