60 Search Results

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 the charged-lepton-flavor violating decays B⁰ → K^*0τ^±ℓ^∓, where ℓ^∓ is either an electron or a muon. The results are based on 365 fb⁻¹ and 711 fb⁻¹ datasets collected with the Belle II and Belle detectors, respectively. We use an exclusive hadronic B-tagging technique, and search for a signal decay in the system recoiling against a fully reconstructed B meson. We find no evidence for B⁰ → K*⁰τ^±ℓ^∓ decays and set upper limits on the branching fractions in the range of (2.9–6.4)×10⁻⁵ at 90% confidence level.

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 measure the 𝐶⁢𝑃 asymmetry in 𝐷⁺ → 𝜋⁺⁢𝜋⁰ 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⁺⁢𝜋⁰) = (−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.

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 measure the time-integrated 𝐶⁢𝑃 asymmetry in 𝐷⁰ → 𝐾$$^{0}_{S}$$𝐾$$^{0}_{S}$$ decays reconstructed in 𝑒⁺⁢𝑒⁻→ $$c\bar{c}$$ events collected by the Belle and Belle II experiments. The corresponding data samples have integrated luminosities of 980 and 428 fb⁻¹, respectively. To infer the flavor of the 𝐷⁰ meson, we exploit the correlation between the flavor of the reconstructed decay and the electric charges of particles reconstructed in the rest of the 𝑒⁺⁢𝑒⁻→ $$c\bar{c}$$ event. This results in a sample which is independent from any other previously used at Belle or Belle II. The result, 𝐴_𝐶⁢𝑃⁡(𝐷⁰ → 𝐾$$^{0}_{S}$$𝐾$$^{0}_{S}$$)=(1.3±2.0±0.2)%, where the first uncertainty is statisticalmore » and the second systematic, is consistent with previous determinations and with 𝐶⁢𝑃 symmetry.« less

We report a precise measurement of the ratio of branching fractions ℬ⁡(Λ$$^+_𝑐$$ → 𝑝⁢𝐾$$^0_𝑆$$⁢𝜋⁰)/ℬ⁡(Λ$$^+_𝑐$$ → 𝑝⁢𝐾⁻⁢𝜋⁺) using 980 fb⁻¹ of 𝑒⁺⁢𝑒⁻ data from the Belle experiment. We obtain a value of ℬ⁡(Λ$$^+_𝑐$$ → 𝑝⁢𝐾$$^0_𝑆$$⁢𝜋⁰)/ℬ⁡(Λ$$^+_𝑐$$ → 𝑝⁢𝐾⁻⁢𝜋⁺) =0.339 ±0.002 ±0.009, where the first and second uncertainties are statistical and systematic, respectively. This Belle result is consistent with the previous measurement from the CLEO experiment but has a fivefold improvement in precision. By combining our result with the world average ℬ⁡(Λ$$^+_𝑐$$ → 𝑝⁢𝐾⁻⁢𝜋⁺), we obtain the absolute branching fraction ℬ⁡(Λ$$^+_𝑐$$ → 𝑝⁢𝐾$$^0_𝑆$$⁢𝜋⁰) =(2.12 ± 0.01 ±0.05 ±0.10)%, where the uncertainties are statistical,more » systematic, and the uncertainty in the absolute branching fraction scale ℬ⁡(Λ$$^+_𝑐$$ → 𝑝⁢𝐾⁻⁢𝜋⁺), respectively. This measurement can shed light on hadronic decay mechanisms in charmed baryon decays.« less

We measure the time-integrated 𝐶⁢𝑃 asymmetry, 𝐴_𝐶⁢𝑃, in 𝐷⁰ → 𝜋⁰⁢𝜋⁰ decays reconstructed in 𝑒⁺⁢𝑒⁻ → $$𝑐⁢\bar{𝑐}$$ events collected by Belle II during 2019–2022. The data corresponds to an integrated luminosity of 428 fb⁻¹. The 𝐷⁰ decays are required to originate from the flavor-conserving 𝐷^*+ → 𝐷⁰⁢𝜋⁺ decay to determine the charm flavor at production time. Control samples of 𝐷⁰ → 𝐾⁻⁢𝜋⁺ decays, with or without an associated pion from a 𝐷^*+ decay, are used to correct for detection asymmetries. The result, 𝐴_𝐶⁢𝑃⁡(𝐷⁰ → 𝜋⁰⁢𝜋⁰) = (0.30 ± 0.72 ± 0.20)%, where the first uncertainty is statistical and the secondmore » systematic, is consistent with 𝐶⁢𝑃 symmetry.« 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