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  1. Free, publicly-accessible full text available August 1, 2025
  2. 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
  3. 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
  4. A<sc>bstract</sc>

    We report results from a study ofB±→ DK±decays followed byDdecaying to theCP-even final stateK+Kand CP-odd final state$$ {K}_S^0{\pi}^0 $$KS0π0, whereDis an admixture ofD0and$$ {\overline{D}}^0 $$D¯0states. These decays are sensitive to the Cabibbo-Kobayashi-Maskawa unitarity-triangle angleϕ3. The results are based on a combined analysis of the final data set of 772×106$$ B\overline{B} $$BB¯pairs collected by the Belle experiment and a data set of 198×106$$ B\overline{B} $$BB¯pairs collected by the Belle II experiment, both in electron-positron collisions at the Υ(4S) resonance. We measure the CP asymmetries to be$$ \mathcal{A} $$ACP+= (+12.5±5.8±1.4)% and$$ \mathcal{A} $$ACP−= (16.7±5.7±0.6)%, and the ratios of branching fractions to be$$ \mathcal{R} $$RCP+= 1.164±0.081±0.036 and$$ \mathcal{R} $$RCP−= 1.151±0.074±0.019. The first contribution to the uncertainties is statistical, and the second is systematic. The asymmetries$$ \mathcal{A} $$ACP+and$$ \mathcal{A} $$ACP−have similar magnitudes and opposite signs; their difference corresponds to 3.5 standard deviations. From these values we calculate 68.3% confidence intervals of (8.5°<ϕ3< 16.5°) or (84.5°<ϕ3< 95.5°) or (163.3°<ϕ3< 171.5°) and 0.321 <rB< 0.465.

     
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    Free, publicly-accessible full text available May 1, 2025
  5. Free, publicly-accessible full text available April 1, 2025
  6. A search for the nonresonant production of Higgs boson pairs in theHHbb¯τ+τchannel is performed using140fb1of proton-proton collisions at a center-of-mass energy of 13 TeV recorded by the ATLAS detector at the CERN Large Hadron Collider. The analysis strategy is optimized to probe anomalous values of the Higgs boson self-coupling modifierκλand of the quarticHHVV(V=W,Z) coupling modifierκ2V. No significant excess above the expected background from Standard Model processes is observed. An observed (expected) upper limitμHH<5.9(3.3)is set at 95% confidence-level on the Higgs boson pair production cross section normalized to its Standard Model prediction. The coupling modifiers are constrained to an observed (expected) 95% confidence interval of3.1<κλ<9.0(2.5<κλ<9.3) and0.5<κ2V<2.7(0.2<κ2V<2.4), assuming all other Higgs boson couplings are fixed to the Standard Model prediction. The results are also interpreted in the context of effective field theories via constraints on anomalous Higgs boson couplings and Higgs boson pair production cross sections assuming different kinematic benchmark scenarios.

    © 2024 CERN, for the ATLAS Collaboration2024CERN 
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    Free, publicly-accessible full text available August 1, 2025