65 Search Results

Using a sample of ( 2712.4 ± 14.3 ) × 10 6     ψ ( 3686 ) events collected with the BESIII detector, we perform a search for the isospin-violating decays χ c J → Λ Σ ¯   0 + c . c . ( J = 0 , 1 , 2 ) and η c → Λ Σ ¯   0 + c . c . No significant signal for χ c J or η c is observed in the Λ Σ ¯   0 invariant mass distribution. The upper limits on the branching fractions at the 90% confidencemore » level are set to be B ( χ c 0 → Λ Σ ¯   0 + c . c . ) < 1.5 × 10 − 6 , B ( χ c 1 → Λ Σ ¯   0 + c . c . ) < 1.6 × 10 − 6 , B ( χ c 2 → Λ Σ ¯   0 + c . c . ) < 1.7 × 10 − 6 , and B ( η c → Λ Σ ¯   0 + c . c . ) < 6.2 × 10 − 5 for the first time.« less

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

The first direct measurement of the relative phase between the strong and electromagnetic amplitudes for a J/ψ decaying into a vector-pseudoscalar final state is performed using 26 energy points of e⁺e⁻ annihilation data between 3.00 GeV and 3.12 GeV. The data sets were collected by the BESIII detector with a total integrated luminosity of 452 pb⁻¹. By investigating the interference pattern in the cross section lineshape of e⁺e⁻ → ϕη, the relative phase between the strong and electromagnetic amplitudes of J/ψ decay is determined to be within [133°, 228°] at 68% confidence level.

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

Using 983.0 fb⁻¹ and 427.9 fb⁻¹ data samples collected with the Belle and Belle II detectors at the KEKB and SuperKEKB asymmetric energy e⁺e⁻ colliders, respectively, we present studies of the Cabibbo-favored $${\Xi }_{c}^{+}$$ decays $${\Xi }_{c}^{+}\to {\Sigma }^{+}{K}_{S}^{0}$$ and $${\Xi }_{c}^{+}\to {\Xi }^{0}{\pi }^{+}$$, and the singly Cabibbo-suppressed decay $${\Xi }_{c}^{+}\to {\Xi }^{0}{K}^{+}$$. The ratios of branching fractions of $${\Xi }_{c}^{+}\to {\Sigma }^{+}{K}_{S}^{0}$$ and $${\Xi }_{c}^{+}\to {\Xi }^{0}{K}^{+}$$ relative to that of $${\Xi }_{c}^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+}$$ are measured for the first time, while the ratio $$\mathcal{B}({\Xi }_{c}^{+}\to {\Xi }^{0}{\pi }^{+})/\mathcal{B}({\Xi }_{c}^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+})$$ is alsomore » determined and improved by an order of magnitude in precision. The measured branching fraction ratios are $$\begin{array}{c}\frac{\mathcal{B}\left({\Xi }_{c}^{+}\to {\Sigma }^{+}{K}_{S}^{0}\right)}{\mathcal{B}\left({\Xi }_{c}^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+}\right)}=0.067\pm 0.007\pm 0.003,\\ \frac{\mathcal{B}\left({\Xi }_{c}^{+}\to {\Xi }^{0}{\pi }^{+}\right)}{\mathcal{B}\left({\Xi }_{c}^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+}\right)}=0.251\pm 0.005\pm 0.010,\\ \frac{\mathcal{B}\left({\Xi }_{c}^{+}\to {\Xi }^{0}{K}^{+}\right)}{\mathcal{B}\left({\Xi }_{c}^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+}\right)}=0.017\pm 0.003\pm 0.001.\end{array}$$ Additionally, the ratio $$\mathcal{B}({\Xi }_{c}^{+}\to {\Xi }^{0}{K}^{+})/\mathcal{B}({\Xi }_{c}^{+}\to {\Xi }^{0}{\pi }^{+})$$ is measured to be 0.068 ± 0.010 ± 0.004. Here, the first and second uncertainties are statistical and systematic, respectively. Multiplying the ratios by the branching fraction of the normalization mode, $$\mathcal{B}({\Xi }_{c}^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+})=(2.9\pm 1.3)%$$, we obtain the following absolute branching fractions $$\begin{array}{c}\mathcal{B}({\Xi }_{c}^{+}\to {\Sigma }^{+}{K}_{S}^{0})=(0.194\pm 0.021\pm 0.009\pm 0.087)\text{%},\\ \mathcal{B}({\Xi }_{c}^{+}\to {\Xi }^{0}{\pi }^{+})=(0.728\pm 0.014\pm 0.027\pm 0.326)\text{%},\\ \mathcal{B}({\Xi }_{c}^{+}\to {\Xi }^{0}{K}^{+})=(0.049\pm 0.007\pm 0.003\pm 0.022)\text{%},\end{array}$$ where the third uncertainties are from $$\mathcal{B}({\Xi }_{c}^{+}\to {\Xi }^{-}{\pi }^{+}{\pi }^{+})$$.« 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

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