71 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

The Belle II experiment’s ability to identify particles critically affects the sensitivity of its measurements. We describe Belle II’s algorithms for identifying charged particles and evaluate their performance in separating pions, kaons, and protons using $$426\,\text {fb}^{-1}$$ of data collected at the energy-asymmetric $$e^{+}e^{-}$$ collider SuperKEKB in 2019–2022 at center-of-mass energies at and near the mass of the $$\Upsilon$$ (4S).

We report the first observation of the two-body baryonic decays 𝐵⁺ → Σ_𝑐⁢(2455)⁺⁺$$\bar{Ξ}^-_c$$ and 𝐵⁰ → Σ_𝑐⁢(2455)^0⁢$$\bar{Ξ}^0_𝑐$$ with significances of 7.3⁢𝜎 and 6.2⁢𝜎, respectively, including statistical and systematic uncertainties. The branching fractions are measured to be ℬ⁡(𝐵⁺ → Σ_𝑐⁢(2455)⁺⁺$$\bar{Ξ}^-_c$$) = (5.74 ± 1.11 ± 0.4⁢2$$^{+2.47}_{−1.53}$$) ×10⁻⁴ and ℬ⁡(𝐵⁰ → Σ_𝑐⁢(2455)^0⁢$$\bar{Ξ}^0_𝑐$$) = (4.83 ± 1.12 ± 0.3⁢7$$^{+0.72}_{−0.60}$$) ×10⁻⁴. The first and second uncertainties are statistical and systematic, respectively, while the third ones arise from the absolute branching fractions of $$\bar{Ξ}^-_c$$ or $$\bar{Ξ}^0_𝑐$$ decays. The data samples used for this analysis have integrated luminosities of 711 fb⁻¹ and 365 fb⁻¹, and weremore » collected at the ϒ⁡(4⁢𝑆) resonance by the Belle and Belle II detectors operating at the KEKB and SuperKEKB asymmetric-energy 𝑒⁺⁢𝑒⁻ colliders, respectively.« 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 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 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

A study is presented of $B^{+}\to K_{\mathrm{S}}^0{K}^{-}{\pi }^{+}{K}^{+}$ and $B^{+}\to K_{\mathrm{S}}^0{K}^{+}{\pi }^{-}{K}^{+}$ decays based on the analysis of proton-proton collision data collected with the LHCb detector at center-of-mass energies of 7, 8 and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . The ${more »}_{}^{}$ K S 0 K π invariant-mass distributions of both $B^{+}$ decay modes show, in the $m\left(K_{\mathrm{S}}^{0}K\pi \right)<1.85\mathrm{GeV}$ mass region, large activity which is resolved using an amplitude analysis. A simple model, where $J^{PC}$ amplitudes are described by multiple Breit-Wigner functions with appropriate angular distributions, provides a good description of the experimental data. In this approach a complex mixture of $J^{PC}=0^{-+}$ , $1^{++}$ and $1^{+-}$ amplitudes is observed that is dominated by $\eta \left(1405\right)$ , $\eta \left(1470\right)$ , $\eta \left(1760\right)$ , $f_1\left(1285\right)$ , $f_1\left(1420\right)$ and $h_1\left(1405\right)$ resonances. The $K_{\mathrm{S}}^{0}K\pi$ Dalitz plots are dominated by asymmetric crossing $K^{*}\overline{K}$ bands which are different for the two $B^{+}$ decay modes. This is due to a different interference pattern between the $1^{++}$ and $1^{+-}$ amplitudes in the two channels. Branching fractions are measured for each resonant contribution. © 2025 CERN, for the LHCb Collaboration 2025 CERN« less

The branching fraction of the decay $B^{+}\to \psi \left(2S\right)\varphi \left(1020\right)K^{+}$ , relative to the topologically similar decay $B^{+}\to J/\psi \varphi \left(1020\right)K^{+}$ , is measured using proton-proton collision data collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of $9{\mathrm{fb}}^{-1}$ . The ratio is found to be $0.061\pm 0.004more »$ ± 0.009 , where the first uncertainty is statistical and the second systematic. Using the world-average branching fraction for $B^{+}\to J/\psi \varphi \left(1020\right)K^{+}$ , the branching fraction for the decay $B^{+}\to \psi \left(2S\right)\varphi \left(1020\right)K^{+}$ is found to be $(3.0\pm 0.2\pm 0.5\pm 0.2)\times {10}^{-6}$ , where the first uncertainty is statistical, the second systematic, and the third is due to the branching fraction of the normalization channel. © 2025 CERN, for the LHCb Collaboration 2025 CERN« less

The first measurement of the $CP$ asymmetry of the decay rate ( $A_{CP}$ ) and the $CP$ average ( $\mathrm{\Sigma }{A}_{\mathrm{FB}}$ ) and $CP$ asymmetry ( $\mathrm{\Delta }{A}_{\mathrm{FB}}$ ) of the forward-backward asymmetry in the muon system of ${\mathrm{\Lambda }}_c^{+}\to p{\mu }^{+}{\mu }^{-}$ decays is reported. The measurement is performed using a data sample of proton-proton collisions, recordedmore » by the LHCb experiment from 2016 to 2018 at a center-of-mass energy of 13 TeV, which corresponds to an integrated luminosity of $5.4{\mathrm{fb}}^{-1}$ . The asymmetries are measured in two regions of dimuon mass near the  $\varphi$ -meson mass peak. The dimuon-mass integrated results are $A_{CP}=(-1.1\pm 4.0\pm 0.5)\%$ , $\mathrm{\Sigma }{A}_{\mathrm{FB}}=(3.9\pm 4.0\pm 0.6)\%$ , $\mathrm{\Delta }{A}_{\mathrm{FB}}=(3.1\pm 4.0\pm 0.4)\%$ , where the first uncertainty is statistical and the second systematic. The results are consistent with the conservation of $CP$ symmetry and the Standard Model expectations. © 2025 CERN, for the LHCb Collaboration 2025 CERN« less