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  1. A<sc>bstract</sc>

    We report a search for the charged-lepton flavor violation in Υ(2S) →τ±(=e, μ) decays using a 25 fb1Υ(2S) sample collected by the Belle detector at the KEKBe+easymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions ($$ \mathcal{B} $$B) at 90% confidence level. We obtain the most stringent upper limits:$$ \mathcal{B} $$B(Υ(2S)→ μτ±)<0.23×106and$$ \mathcal{B} $$B(Υ(2S)→ eτ±)<1.12×106.

     
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    Free, publicly-accessible full text available February 1, 2025
  2. We measure the branching fraction of the decayBD0ρ(770)using data collected with the Belle II detector. The data contain 387 millionBB¯pairs produced ine+ecollisions at theϒ(4S)resonance. We reconstruct8360±180decays from an analysis of the distributions of theBenergy and theρ(770)helicity angle. We determine the branching fraction to be(0.939±0.021(stat)±0.050(syst))%, in agreement with previous results. Our measurement improves the relative precision of the world average by more than a factor of two.

    Published by the American Physical Society2024 
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    Free, publicly-accessible full text available June 1, 2025
  3. We report on a search for a resonanceXdecaying to a pair of muons ine+eμ+μXevents in the0.2129.000GeV/c2mass range, using178fb1of data collected by the Belle II experiment at the SuperKEKB collider at a center of mass energy of 10.58 GeV. The analysis probes two different models ofXbeyond the standard model: aZvector boson in theLμLτmodel and a muonphilic scalar. We observe no evidence for a signal and set exclusion limits at the 90% confidence level on the products of cross section and branching fraction for these processes, ranging from 0.046 fb to 0.97 fb for theLμLτmodel and from 0.055 fb to 1.3 fb for the muonphilic scalar model. For masses below6GeV/c2, the corresponding constraints on the couplings of these processes to the standard model range from 0.0008 to 0.039 for theLμLτmodel and from 0.0018 to 0.040 for the muonphilic scalar model. These are the first constraints on the muonphilic scalar from a dedicated search.

    Published by the American Physical Society2024 
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    Free, publicly-accessible full text available June 1, 2025
  4. We report a measurement of decay-time-dependent charge-parity (CP) asymmetries inB0KS0KS0KS0decays. We use387×106BB¯pairs collected at theϒ(4S)resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract theCP-violating parametersSandCfrom a fit to the distribution of the decay-time difference between the twoBmesons. The resulting confidence region is consistent with previous measurements inB0KS0KS0KS0andB0(cc¯)K0decays and with predictions based on the standard model.

    Published by the American Physical Society2024 
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    Free, publicly-accessible full text available June 1, 2025
  5. We search for the rare decayB+K+νν¯in a362fb1sample of electron-positron collisions at theϒ(4S)resonance collected with the Belle II detector at the SuperKEKB collider. We use the inclusive properties of the accompanyingBmeson inϒ(4S)BB¯events to suppress background from other decays of the signalBcandidate and light-quark pair production. We validate the measurement with an auxiliary analysis based on a conventional hadronic reconstruction of the accompanyingBmeson. 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 theB+K+νν¯branching fraction of[2.7±0.5(stat)±0.5(syst)]×105and[1.10.8+0.9(stat)0.5+0.8(syst)]×105, respectively. Combining the results, we determine the branching fraction of the decayB+K+νν¯to be[2.3±0.5(stat)0.4+0.5(syst)]×105, 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