17 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 report a search for an axion-like particle a in B → K^(*)a decays using data collected with the Belle detector at the KEKB asymmetric-energy electron-positron collider. The search is based on a 711 fb⁻¹ data sample collected at the Υ(4S) resonance energy, corresponding to a sample of 772 × 10⁶ Υ(4S) events. In this study, we search for the decay of the axion-like particle into a pair of photons, a → γγ. We scan the two-photon invariant mass in the range 0.16 GeV–4.50 GeV for the K modes and 0.16 GeV–4.20 GeV for the K^* modes. No significant signalmore » is observed in any of the modes, and 90% confidence level upper limits are established on the coupling to the W boson, g_aW, as a function of a mass. The limits range from 3 × 10⁻⁶ GeV⁻¹ to 3 × 10⁻⁵ GeV⁻¹, improving the current constraints on g_aW by a factor of two over the most stringent previous experimental results.« 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 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

We present a measurement of the branching fraction of 𝐵⁺ →𝜏⁺⁢𝜈_𝜏 decays using 387±6×10⁶ ϒ⁡(4⁢𝑆) collected between 2019 and 2022 with the Belle II detector at the SuperKEKB 𝑒⁺⁢𝑒⁻ collider. We reconstruct the accompanying 𝐵⁻ meson using the hadronic tagging method, while 𝐵⁺ →𝜏⁺⁢𝜈_𝜏 candidates are identified in the recoil. We find evidence for 𝐵⁺ →𝜏⁺⁢𝜈_𝜏 decays at 3.0 standard deviations, including systematic uncertainties. The measured branching fraction is ℬ⁡(𝐵⁺ →𝜏⁺⁢𝜈_𝜏)=[1.24±0.41⁢(stat)±0.19⁢(syst)]×10⁻⁴.

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 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 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.

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