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DOE OSTI2024

Measurement of C P asymmetries in B 0 → K S 0 K S 0 K S 0 decays at Belle II

We report a measurement of decay-time-dependent charge-parity ( C P ) asymmetries in B 0 → K S 0 K S 0 K S 0 decays. We use 387 × 10 6 B B ¯ pairs collected at the ϒ ( 4 S ) resonance with the Belle II detector at the SuperKEKB asymmetric-energy electron-positron collider. We reconstruct 220 signal events and extract the C P -violating parameters S and C from a fit to the distribution of the decay-time difference between the two B mesons. The resulting confidence region is consistent with previous measurements in B 0 → K S 0 K S 0 K S 0 and B 0 → ( c c ¯ ) K 0 decays and with predictions based on the standard model. Published by the American Physical Society 2024

Astronomy & Astrophysics↗

DOE OSTI2024

Test of lepton flavor universality with a measurement of R ( D * ) using hadronic B tagging at the Belle II experiment

The ratio of branching fractions R ( D * ) = B ( B ¯ → D * τ − ν ¯ τ ) / B ( B ¯ → D * ℓ − ν ¯ ℓ ) , where ℓ is an electron or muon, is measured using a Belle II data sample with an integrated luminosity of 189 fb − 1 at the SuperKEKB asymmetric-energy e + e − collider. Data is collected at the ϒ ( 4 S ) resonance, and one B meson in the ϒ ( 4 S ) → B B ¯ decay is fully reconstructed in hadronic decay modes. The accompanying signal B meson is reconstructed as B ¯ → D * τ − ν ¯ τ using leptonic τ decays. The normalization decay, B ¯ → D * ℓ − ν ¯ ℓ , produces the same observable final-state particles. The ratio of branching fractions is extracted in a simultaneous fit to two signal-discriminating variables in both channels and yields R ( D * ) = 0.262 − 0.039 + 0.041 ( stat ) − 0.032 + 0.035 ( syst ) . This result is consistent with the current world average and with Standard Model predictions. Published by the American Physical Society 2024

Adachi, I. (ORCID:0000000322870173)↗

DOE OSTI2025

Measurement of the 𝐶⁢𝑃 asymmetry in 𝐷 + → 𝜋 + ⁢𝜋 0 decays at Belle II

We measure the 𝐶⁢𝑃 asymmetry in 𝐷 + → 𝜋 + ⁢𝜋 0 decays reconstructed in 𝑒 + ⁢𝑒 − collisions at the Belle II experiment using a dataset corresponding to an integrated luminosity of 428 fb −1 . A control sample of 𝐷 + → 𝜋 + ⁢𝐾$^0_𝑆$ decays is used to correct for detection and production asymmetries. The result 𝐴 𝐶⁢𝑃 ⁡(𝐷 + → 𝜋 + ⁢𝜋 0 ) = (−1.8 ± 0.9 ± 0.1)%, where the first uncertainty is statistical and the second systematic, is the most precise determination to date. It agrees with the prediction of 𝐶⁢𝑃 symmetry from the standard model and with results of previous measurements.

CP violation↗

DOE OSTI2024

Measurement of branching fractions and direct C P asymmetries for B → K π and B → π π decays at Belle II

We report measurements of the branching fractions and direct C P asymmetries of the decays B 0 → K + π − , B + → K + π 0 , B + → K 0 π + , and B 0 → K 0 π 0 , and use these for testing the standard model through an isospin-based sum rule. In addition, we measure the branching fraction and direct C P asymmetry of the decay B + → π + π 0 and the branching fraction of the decay B 0 → π + π − . The data are collected with the elle II detector from e + e − collisions at the ϒ ( 4 S ) resonance produced by the SuperKEKB asymmetric-energy collider and contain 387 × 10 6 bottom-antibottom meson pairs. Signal yields are determined in two-dimensional fits to background-discriminating variables, and range from 500 to 3900 decays, depending on the channel. We obtain − 0.03 ± 0.13 ± 0.04 for the sum rule in agreement with the standard model expectation of zero and with a precision comparable to the best existing determinations. Published by the American Physical Society 2024

Astronomy & Astrophysics↗

DOE OSTI2025

Model-independent measurement of 𝐷 0 −$\overline{D}$ 0 mixing parameters in 𝐷 0 → 𝐾$^{0}_{𝑆}$⁢𝜋 + ⁢𝜋 − decays at Belle and Belle II

We perform a model-independent measurement of the 𝐷 0 −$\overline{D}$ 0 mixing parameters using samples of 𝑒 + ⁢𝑒 − -collision data collected by the Belle and Belle II experiments that have integrated luminosities of 951 fb −1 and 408 fb −1 , respectively. Approximately 2.05 × 10 6 neutral 𝐷 mesons are reconstructed in the 𝐷 0 → 𝐾$^{0}_{S}$⁢𝜋 + ⁢𝜋 − channel, with the neutral 𝐷 flavor tagged by the charge of the pion in the 𝐷* + → 𝐷 0⁢ 𝜋 + decay. Assuming charge-parity symmetry, the mixing parameters are measured to be 𝑥 = (4.0 ±1.7 ±0.4) × 10 −3 and 𝑦 = (2.9 ±1.4 ±0.3) × 10 −3 , where the first uncertainties are statistical and the second systematic. The results are consistent with previous determinations.

Charmed mesons↗

DOE OSTI2025

Search for a Dark Higgs Boson Produced in Association with Inelastic Dark Matter at the Belle II Experiment

Inelastic dark matter models that have two dark matter particles and a massive dark photon can reproduce the observed relic dark matter density without violating cosmological limits. The mass splitting between the two dark matter particles 𝜒 1 and 𝜒 2 , with 𝑚⁡(𝜒 2 ) >𝑚⁡(𝜒 1 ), is induced by a dark Higgs field and a corresponding dark Higgs boson ℎ′. We present a search for dark matter in events with two vertices, at least one of which must be displaced from the interaction region, and missing energy. Using a 365 fb −1 data sample collected at Belle II, which operates at the SuperKEKB 𝑒 + ⁢𝑒 − collider, we observe no evidence for a signal. We set upper limits on the product of the production cross section 𝜎⁡(𝑒 + ⁢𝑒 − → ℎ′⁡𝜒 1 ⁢𝜒 2 ), and the product of branching fractions ℬ⁡(𝜒 2 → 𝜒 1⁢ 𝑒 + ⁢𝑒 − ) ×ℬ⁡(ℎ′ → 𝑥 + ⁢𝑥 − ), where 𝑥 + ⁢𝑥 − indicates 𝜇 + ⁢𝜇 − , 𝜋 + ⁢𝜋 − , or 𝐾 + ⁢𝐾 − , as functions of ℎ′ mass and lifetime at the level of 10 −1 fb. We set model-dependent upper limits on the dark Higgs mixing angle at the level of 10 −5 and on the dark photon kinetic mixing parameter at the level of 10 −3 . This is the first search for dark Higgs bosons in association with inelastic dark matter.

dark matter↗

DOE OSTI2025

Model-agnostic likelihood for the reinterpretation of the 𝐵 + → 𝐾 + ⁢$𝑣\bar{𝑣}$ measurement at Belle II

We recently measured the branching fraction of the 𝐵 + → 𝐾 + ⁢$𝑣\bar{𝑣}$ decay using 362 fb −1 of on-resonance 𝑒 + ⁢𝑒 − collision data under the assumption of Standard Model kinematics, providing the first evidence for this decay. To facilitate future reinterpretations and maximize the scientific impact of this measurement, we publicly release the full analysis likelihood along with all necessary material required for reinterpretation under arbitrary theoretical models sensitive to this measurement. In this work, we demonstrate how the measurement can be reinterpreted within the framework of the weak effective theory. Using a kinematic reweighting technique in combination with the published likelihood, we derive marginal posterior distributions for the Wilson coefficients, construct credible intervals, and assess the goodness of fit to the Belle II data. For the weak effective theory Wilson coefficients, the posterior mode of the magnitudes |𝐶 VL +𝐶 VR |, |𝐶 SL +𝐶 SR |, and |𝐶 TL | corresponds to the point (11.3, 0.0, 8.2). The respective 95% credible intervals are [1.9, 16.2], [0.0, 15.4], and [0.0, 11.2].

bottom quark↗

DOE OSTI2024

Measurement of the branching fractions of $$ \overline{B} $$ → D()K−$$ {K}_{(S)}^{\left(\ast \right)0} $$ and $$ \overline{B} $$ → D()$$ {D}_s^{-} $$ decays at Belle II

Abstract We present measurements of the branching fractions of eight$$ {\overline{B}}^0 $$ B ¯ 0 →D (*)+ K − $$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 ,B − →D (*)0 K − $$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 decay channels. The results are based on data from SuperKEKB electron-positron collisions at the Υ(4S) resonance collected with the Belle II detector, corresponding to an integrated luminosity of 362 fb −1 . The event yields are extracted from fits to the distributions of the difference between expected and observedBmeson energy, and are efficiency-corrected as a function ofm(K − $$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 ) andm(D (*) $$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 ) in order to avoid dependence on the decay model. These results include the first observation of$$ {\overline{B}}^0 $$ B ¯ 0 →D + K − $$ {K}_S^0 $$ K S 0 ,B − →D* 0 K − $$ {K}_S^0 $$ K S 0 , and$$ {\overline{B}}^0 $$ B ¯ 0 →D* + K − $$ {K}_S^0 $$ K S 0 decays and a significant improvement in the precision of the other channels compared to previous measurements. The helicity-angle distributions and the invariant mass distributions of theK − $$ {K}_{(S)}^{\left(\ast \right)0} $$ K S ∗ 0 systems are compatible with quasi-two-body decays via a resonant transition with spin-parityJ P = 1 − for theK − $$ {K}_S^0 $$ K S 0 systems andJ P = 1 + for theK − K* 0 systems. We also present measurements of the branching fractions of four$$ {\overline{B}}^0 $$ B ¯ 0 →D (*)+ $$ {D}_s^{-} $$ D s − ,B − →D (*)0 $$ {D}_s^{-} $$ D s − decay channels with a precision compatible to the current world averages.

Physics↗