36 Search Results

We present the result of a search for the charged-lepton-flavor violating decays τ⁻ → e^∓ℓ^±ℓ⁻, where ℓ is a muon or an electron, using a data sample with an integrated luminosity of 428 fb⁻¹ recorded by the Belle II experiment at the SuperKEKB e⁺e⁻ collider. The selection of e⁺e⁻ → τ⁺τ⁻ events containing a signal candidate is based on an inclusive-tagging reconstruction and on a boosted decision tree to suppress background. Upper limits on the branching fractions between 1.3 and 2.5 × 10⁻⁸ are set at the 90% confidence level. These results are the most stringent bounds to date formore » four of the modes.« less

We present a study of the rare charm meson decays 𝐷⁰ →𝐾⁺⁢𝐾⁻⁢𝑒⁺⁢𝑒⁻, 𝜋⁺⁢𝜋⁻⁢𝑒⁺⁢𝑒⁻, and 𝐾⁻⁢𝜋⁺⁢𝑒⁺⁢𝑒⁻ using a 942  fb⁻¹ dataset collected by the Belle detector at the KEKB asymmetric-energy 𝑒⁺⁢𝑒⁻ collider. We identify 𝐷⁰ candidates via the charge of the pion from 𝐷*⁺ →𝐷⁰⁢𝜋⁺ decays and normalize the branching fractions to 𝐷⁰ → 𝐾⁻⁢𝜋⁺⁢𝜋⁻⁢𝜋⁺ decays. The branching fraction for decay 𝐷⁰ → 𝐾⁻⁢𝜋⁺⁢𝑒⁺⁢𝑒⁻ is measured to be (39.6 ± 4.5 (stat) ± 2.9 (syst)) × 10⁻⁷, with the dielectron mass in the 𝜌/𝜔 mass region 675 < 𝑚_𝑒⁢𝑒 < 875  MeV/⁢𝑐². We also search for 𝐷⁰ → ℎ⁻⁡ℎ^(′)+⁡𝑒⁺⁢𝑒⁻ (ℎ^(′) = 𝐾, 𝜋) decays withmore » the dielectron mass near the 𝜂 and 𝜙 resonances, and away from these resonances for the 𝐾⁺⁢𝐾⁻⁢𝑒⁺⁢𝑒⁻ and 𝜋⁺⁢𝜋⁻⁢𝑒⁺⁢𝑒⁻ modes. For these modes, we find no significant signals and set 90% confidence level upper limits on their branching fractions at the 𝒪⁡(10⁻⁷) level.« less

We present a search for the rare flavor-changing neutral-current decay 𝐵⁰→𝐾^*0⁢𝜏⁺⁢𝜏⁻ with data collected by the Belle II experiment at the SuperKEKB electron-positron collider. The analysis uses a 365 fb⁻¹ data sample recorded at the center-of-mass energy of the ϒ⁡(4⁢𝑆) resonance. One of the 𝐵 mesons produced in the ϒ⁡(4⁢𝑆)→𝐵⁰$$⁢\overline{𝐵}$$⁰ process is fully reconstructed in a hadronic decay mode, while its companion 𝐵 meson is required to decay into a 𝐾^*0 and two 𝜏 leptons of opposite charge. The 𝜏 leptons are reconstructed in final states with a single electron, muon, charged pion or charged 𝜌 meson, and additional neutrinos.more » We set an upper limit on the branching fraction of ℬ⁡(𝐵⁰→𝐾^*0⁢𝜏⁺⁢𝜏⁻)<1.8×10⁻³ at the 90% confidence level, which is the most stringent constraint reported to date.« less

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 𝜒₁ and 𝜒₂, with 𝑚⁡(𝜒₂) >𝑚⁡(𝜒₁), 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,more » we observe no evidence for a signal. We set upper limits on the product of the production cross section 𝜎⁡(𝑒⁺⁢𝑒⁻ → ℎ′⁡𝜒₁⁢𝜒₂), and the product of branching fractions ℬ⁡(𝜒₂ → 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.« less

We report measurements of the ratios of branching fractions ℛ⁡(𝐷^(*)+) = ℬ⁡($$\bar{𝐵}$$⁰ → 𝐷^(*)+⁢𝜏^−⁢ $$\bar{𝜈}$$_𝜏)/ℬ⁡($$\bar{𝐵}$$⁰ → 𝐷^(*)+⁢ℓ⁻$$\bar{𝜈}$$_ℓ), where ℓ denotes either an electron or a muon. These ratios test the universality of the charged-current weak interaction. The results are based on a 365 fb⁻¹ data sample collected with the Belle II detector at the SuperKEKB 𝑒⁺⁢𝑒⁻ collider, which operates at a center-of-mass energy corresponding to the ϒ⁡(4⁢𝑆) resonance, just above the threshold for $$𝐵\bar{𝐵}$$ production. Signal candidates are reconstructed by selecting events in which the companion 𝐵 meson from the ϒ⁡(4⁢𝑆) → $$𝐵\bar{𝐵}$$ decay is identified in semileptonic modes. Themore » 𝜏 lepton is reconstructed via its leptonic decays. We obtain ℛ⁡(𝐷⁺) = 0.418$$^{+0.075}_{−0.073}$$⁢(stat)$$^{+0.049}_{−0.056}$$⁢(syst) and ℛ⁡(𝐷^*+) = 0.306$$^{+0.035}_{−0.033}$$⁢(stat)$$^{+0.016}_{−0.018⁢}$$(syst), which are consistent with world average values. Accounting for the correlation between them, these values differ from the Standard Model expectation by a collective significance of 1.7 standard deviations.« less

We present the results of a search for charged-lepton-flavor violating decays $${\tau }^{-}\to {{\ell}}^{-}{K}_{s}^{0}$$, where ℓ⁻ is either an electron or a muon. We combine e⁺e⁻ data samples recorded by the Belle II experiment at the SuperKEKB collider (428 fb⁻¹) with samples recorded by the Belle experiment at the KEKB collider (980 fb⁻¹) to obtain a sample of 1.3 billion e⁺e⁻ → τ⁺τ⁻ events. We observe 0 and 1 events and set 90% confidence level upper limits of 0.8 × 10⁻⁸ and 1.2 × 10⁻⁸ on the branching fractions of the decay modes $${\tau }^{-}\to {e}^{-}{K}_{S}^{0}$$ and $${\tau }^{-}\to {\mumore » }^{-}{K}_{S}^{0}$$, respectively. These are the most stringent upper limits to date.« less

We present the first search for the lepton-flavor-violating decay modes 𝐵⁰ →𝐾$$^0_𝑆$$⁢𝜏^±⁢ℓ^∓ (ℓ=𝜇,𝑒) using the 711 and 365 fb⁻¹ data samples recorded by the Belle and Belle II detectors, respectively. We use a hadronic 𝐵-tagging technique to fully reconstruct a 𝐵 meson and search for signal decays in the system recoiling against the tagged meson, considering 𝜏 decays to either light leptons, one charged hadron, or one charged hadron and a neutral pion. We find no evidence for 𝐵⁰ →𝐾$$^0_𝑆$$⁢𝜏^±⁢ℓ^∓ decays and set 90% confidence level upper limits on the branching fractions in the range of [0.8,3.6] ×10⁻⁵.

Using data samples of 102 million ϒ⁡(1⁢𝑆) events and 158 million ϒ⁡(2⁢𝑆) events collected by the Belle detector at the KEKB asymmetric-energy 𝑒+⁢𝑒− collider, we search for [𝑢⁢𝑑⁢𝑠⁢𝑐$$\overline{𝑐}$$ ] pentaquark states decaying to 𝐽/𝜓⁢Λ. Using the first observations of ϒ⁡(1⁢𝑆,2⁢𝑆) inclusive decays to 𝐽/𝜓⁢Λ, we find evidence of the 𝑃_{𝑐$$\overline{𝑐}$$𝑠}⁢(4459)⁰ state with a local significance of 3.3 standard deviations, including statistical and systematic uncertainties. We measure the mass and width of the 𝑃_{𝑐$$\overline{𝑐}$$𝑠}⁢(4459)⁰ to be (4471.7±4.8±0.6) MeV/𝑐² and (22 ±13 ±3) MeV, respectively. The branching fractions for 𝑃_{𝑐$$\overline{𝑐}$$𝑠}⁢(4459)⁰ production are measured to be ℬ⁡[ϒ⁡(1⁢𝑆)→𝑃_{𝑐$$\overline{𝑐}$$𝑠}⁢(4459)0/ $$\overline{𝑃}$$_{𝑐$$\overline{𝑐}$$𝑠}⁢(4459)⁰+anything]=(3.5±2.0±0.2)×10⁻⁶ and ℬ⁡[ϒ⁡(2⁢𝑆)→𝑃_{𝑐$$\overline{𝑐}$$𝑠}⁢(4459)⁰/$$\overline{𝑃}$$_{𝑐$$\overline{𝑐}$$𝑠}⁢(4459)⁰+anything]=(2.9±1.7±0.4)×10⁻⁶. The inclusive branchingmore » fractions of ϒ⁡(1⁢𝑆,2⁢𝑆) →𝐽/𝜓⁢Λ/$$\overline{Λ}$$ are measured to be ℬ⁡[ϒ⁡(1⁢𝑆) →𝐽/𝜓⁢Λ/$$\overline{Λ}$$ +anything] =(36.9 ±5.3 ±2.4) ×10⁻⁶ and ℬ⁡[ϒ⁡(2⁢𝑆) →𝐽/𝜓⁢Λ/$$\overline{Λ}$$ +anything] =(22.3 ±5.7 ±3.1) ×10⁻⁶. We measure the visible cross section 𝜎⁡(𝑒+⁢𝑒−→𝐽/𝜓⁢Λ/ $$\overline{Λ}$$ +anything)=(90±14±6) fb for the continuum production at √𝑠 =10.52 GeV. In all cases, the first uncertainties are statistical and the second are systematic.« less

We perform a model-independent measurement of the 𝐷⁰−$$\overline{D}$$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 × 10⁶ neutral 𝐷 mesons are reconstructed in the 𝐷⁰ → 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 themore » second systematic. The results are consistent with previous determinations.« less

We present a measurement of the branching fraction and fraction of longitudinal polarization of $B^0\to {\rho }^{+}{\rho }^{-}$ decays, which have two ${\pi }^0$ ’s in the final state. We also measure time-dependent $CP$ violation parameters for decays into longitudinally polarized ${\rho }^{+}{\rho }^{-}$ pairs. This analysis is based on a data sample containing $(387\pm 6)more »$ × 10 6 $\mathrm{\Upsilon }\left(4S\right)$ mesons collected with the Belle II detector at the SuperKEKB asymmetric-energy $e^{+}{e}^{-}$ collider in 2019–2022. We obtain $\mathcal{B}(B^0\to {\rho }^{+}{\rho }^{-})=\left(2.89_{-0.22}^{+0.23}{}_{-0.27}^{+0.29}\right)\times {10}^{-5}$ , $f_L=0.921_{-0.025}^{+0.024}{}_{-0.015}^{+0.017}$ , $S=-0.26\pm 0.19\pm 0.08$ , and $C=-0.02\pm 0.12_{-0.05}^{+0.06}$ , where the first uncertainties are statistical and the second are systematic. We use these results to perform an isospin analysis to constrain the Cabibbo-Kobayashi-Maskawa angle ${\varphi }_2$ and obtain two solutions; the result consistent with other Standard Model constraints is ${\varphi }_2=\left({92.6}_{-4.7}^{+4.5}\right)°$ . Published by the American Physical Society 2025« less