412 Search Results

Accurate measurement of the cosmogenic muon-induced neutron yield is crucial for constraining a significant background in a wide range of low-energy physics searches. Although previous underground experiments have measured this yield across various cosmogenic muon energies, SNO+ is uniquely positioned due to its exposure to one of the highest average cosmogenic muon energies at 364 GeV. Using ultrapure water, we have determined a neutron yield of 𝑌_𝑛 = (3.3⁢8$$^{+0.23}_{−0.30}$$) × 10⁻⁴ cm² g⁻¹ 𝜇⁻¹ at SNO+. Comparison with simulations demonstrates clear agreement with the FLUKA neutron production model, highlighting discrepancies with the widely used GEANT4 model. Furthermore, this measurement reveals a lowermore » cosmogenic neutron yield than that observed by the SNO experiment, which used heavy water under identical muon flux conditions. This result provides new evidence that nuclear structure and target material composition significantly influence neutron production by cosmogenic muons, offering fresh insight with important implications for the design and background modeling of future underground experiments.« less

The SNO+ Collaboration reports the first evidence of ⁸B solar neutrinos interacting on ¹³C nuclei. The charged current interaction proceeds through ¹³C +𝜈_𝑒→¹³N+𝑒⁻ which is followed, with a 10 minute half life, by ¹³N →¹³C +𝑒⁺+𝜈_𝑒. The detection strategy is based on the delayed coincidence between the electron and the positron. Evidence for the charged current signal is presented with a significance of 4.2⁢𝜎. Using the natural abundance of ¹³C present in the scintillator, 5.7 metric tons of ¹³C over 231 days of data were used in this analysis. The 5.6$$^{+3.0}_{−2.3}$$ observed events in the data set are consistent withmore » the expectation of 4.7$$^{+0.6}_{−1.3}$$ events. This result is the second real-time measurement of CC interactions of ⁸B neutrinos with nuclei and constitutes the lowest energy observation of neutrino interactions on ¹³C generally. This enables the first direct measurement of the CC 𝜈_𝑒 reaction to the ground state of ¹³N , yielding an average cross section of (16.1$$^{+8.5}_{−6.7}$$⁢(stat.)$$^{+1.6}_{−2.7}$$⁢(syst.))×10⁻⁴³ cm² over the relevant ⁸B solar neutrino energies.« 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

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

We recently measured the branching fraction of the 𝐵⁺→ 𝐾⁺⁢$$𝑣\bar{𝑣}$$ decay using 362 fb⁻¹ of on-resonance 𝑒⁺⁢𝑒⁻ collision data under the assumption of Standard Model kinematics, providing the first evidence for this decay. To facilitate future reinterpretations and maximize the scientific impact of this measurement, we publicly release the full analysis likelihood along with all necessary material required for reinterpretation under arbitrary theoretical models sensitive to this measurement. In this work, we demonstrate how the measurement can be reinterpreted within the framework of the weak effective theory. Using a kinematic reweighting technique in combination with the published likelihood, we derivemore » marginal posterior distributions for the Wilson coefficients, construct credible intervals, and assess the goodness of fit to the Belle II data. For the weak effective theory Wilson coefficients, the posterior mode of the magnitudes |𝐶_VL+𝐶_VR|, |𝐶_SL +𝐶_SR|, and |𝐶_TL| corresponds to the point (11.3, 0.0, 8.2). The respective 95% credible intervals are [1.9, 16.2], [0.0, 15.4], and [0.0, 11.2].« 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 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

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