1,362 research outputs found
B-flavor tagging at Belle II
We report on new flavor tagging algorithms developed to determine the quark-flavor content of bottom ([InlineMediaObject not available: see fulltext.]) mesons at Belle II. The algorithms provide essential inputs for measurements of quark-flavor mixing and charge-parity violation. We validate and evaluate the performance of the algorithms using hadronic [InlineMediaObject not available: see fulltext.] decays with flavor-specific final states reconstructed in a data set corresponding to an integrated luminosity of 62.8 fb- 1, collected at the [InlineEquation not available: see fulltext.] resonance with the Belle II detector at the SuperKEKB collider. We measure the total effective tagging efficiency to be εeff=(30.0±1.2(stat)±0.4(syst))%for a category-based algorithm and εeff=(28.8±1.2(stat)±0.4(syst))%for a deep-learning-based algorithm
Search for lepton-flavor-violating tau-lepton decays to ℓγ at Belle
Charged lepton flavor violation is forbidden in the Standard Model but possible in several new physics scenarios. In many of these models, the radiative decays τ± → ℓ±γ (ℓ = e, μ) are predicted to have a sizeable probability, making them particularly interesting channels to search at various experiments. An updated search via τ± → ℓ±γ using full data of the Belle experiment, corresponding to an integrated luminosity of 988 fb−1, is reported for charged lepton flavor violation. No significant excess over background predictions from the Standard Model is observed, and the upper limits on the branching fractions, B(τ± → μ±γ) ≤ 4.2 × 10−8 and B(τ± → e±γ) ≤ 5.6 × 10−8, are set at 90% confidence level. [Figure not available: see fulltext.]
Measurements of q2 moments of inclusive B →xcℓ+νℓ decays with hadronic tagging
We present the measurement of the first to fourth order moments of the four-momentum transfer squared, q2, of inclusive B→Xcℓ+νℓ decays using the full Belle dataset of 711 fb-1 of integrated luminosity at the (4S) resonance where ℓ=e, μ. The determination of these moments and their systematic uncertainties open new pathways to determine the absolute value of the Cabibbo-Kobayashi-Maskawa matrix element Vcb using a reduced set of matrix elements of the heavy quark expansion. In order to identify and reconstruct the Xc system, we reconstruct one of the two B-mesons using machine learning techniques in fully hadronic decay modes. The moments are measured with progressively increasing threshold selections on q2 starting with a lower value of 3.0 GeV2 in steps of 0.5 GeV2 up to a value of 10.0 GeV2. The measured moments are further unfolded, correcting for reconstruction and selection effects as well as QED final state radiation. We report the moments separately for electron and muon final states and observe no lepton flavor universality violating effects
Measurements of partial branching fractions of inclusive B →xuℓ+νℓ decays with hadronic tagging
We present measurements of partial branching fractions of inclusive semileptonic B→Xuℓ+νℓ decays using the full Belle dataset of 711 fb-1 of integrated luminosity at the ϒ(4S) resonance and for ℓ=e, μ. Inclusive semileptonic B→Xuℓ+νℓ decays are Cabibbo-Kobayashi-Maskawa (CKM) suppressed and measurements are complicated by the large background from CKM favored B→Xcℓ+νℓ transitions, which have a similar signature. Using machine learning techniques, we reduce this and other backgrounds effectively, while retaining access to a large fraction of the B→Xuℓ+νℓ phase space and high signal efficiency. We measure partial branching fractions in three phase-space regions covering about 31% to 86% of the accessible B→Xuℓ+νℓ phase space. The most inclusive measurement corresponds to the phase space with lepton energies of EℓB\u3e1 GeV, and we obtain ΔB(B→Xuℓ+νℓ)=(1.59±0.07±0.16)×10-3 from a two-dimensional fit of the hadronic mass spectrum and the four-momentum-transfer squared distribution, with the uncertainties denoting the statistical and systematic error. We find |Vub|=(4.10±0.09±0.22±0.15)×10-3 from an average of four calculations for the partial decay rate with the third uncertainty denoting the average theory error. This value is higher but compatible with the determination from exclusive semileptonic decays within 1.3 standard deviations. In addition, we report charmless inclusive partial branching fractions separately for B+ and B0 mesons as well as for electron and muon final states. No isospin breaking or lepton flavor universality violating effects are observed
Search for Z′ →μ+μ- in the Lμ-Lτ gauge-symmetric model at Belle
We search for a new gauge boson Z′ that couples only to heavy leptons and their corresponding neutrinos in the process e+e-→Z′(→μ+μ-)μ+μ-, using a 643 fb-1 data sample collected by the Belle experiment at or near the ϒ(1S,2S,3S,4S,5S) resonances at the KEKB collider. While previous searches for Z′ performed a data-based estimation of the initial state radiation effect, our search for the Z′ is the first to include effects due to initial state radiation in the signal simulated samples that were used in estimating the detection efficiency. No signal is observed in the Z′ mass range of 0.212-10 GeV/c2, and we set an upper limit on the coupling strength, g′, constraining the possible Z′ contribution to the anomalous magnetic dipole moment of the muon
Test of lepton flavor universality and search for lepton flavor violation in B → Kℓℓ decays
We present measurements of the branching fractions for the decays B → Kμ+μ− and B → Ke+e−, and their ratio (RK), using a data sample of 711 fb−1 that contains 772 × 106BB¯ events. The data were collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. The ratio RK is measured in five bins of dilepton invariant-mass-squared (q2): q2 ∈ (0.1, 4.0), (4.00, 8.12), (1.0, 6.0), (10.2, 12.8) and (\u3e 14.18) GeV2/c4, along with the whole q2 region. The RK value for q2 ∈ (1.0, 6.0) GeV2/c4 is 1.03−0.24+0.28 ± 0.01. The first and second uncertainties listed are statistical and systematic, respectively. All results for RK are consistent with Standard Model predictions. We also measure CP-averaged isospin asymmetries in the same q2 bins. The results are consistent with a null asymmetry, with the largest difference of 2.6 standard deviations occurring for the q2 ∈ (1.0, 6.0) GeV2/c4 bin in the mode with muon final states. The measured differential branching fractions, dℬ /dq2, are consistent with theoretical predictions for charged B decays, while the corresponding values are below the expectations for neutral B decays. We have also searched for lepton-flavor-violating B → Kμ±e∓ decays and set 90% confidence-level upper limits on the branching fraction in the range of 10−8 for B+ → K+μ±e∓, and B0 → K0μ±e∓ modes. [Figure not available: see fulltext.
Search for B0 meson decays into Λ and missing energy with a hadronic tagging method at Belle
We present a search for the decays of B0 mesons into a final state containing a Λ baryon and missing energy. These results are obtained from a 711 fb-1 data sample that contains 772×106 BB¯ pairs and was collected near the (4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e- collider. We use events in which one B meson is fully reconstructed in a hadronic decay mode and require the remainder of the event to consist of only a single Λ. No evidence for these decays is found, and we set 90% confidence level upper limits on the branching fractions in the range 2.1-3.8×10-5. This measurement provides the world\u27s most restrictive limits, with implications for baryogenesis and dark matter production
Measurements of the branching fractions of Λc+ →pη and Λc+ →pπ0 decays at Belle
We report measurements of the branching fractions of singly Cabibbo-suppressed decays Λc+→pη and Λc+→pπ0 using the full Belle data sample corresponding to an integrated luminosity of 980.6 fb-1. The data were collected by the Belle detector at the KEKB e+e- asymmetric energy collider. A clear Λc+ signal is seen in the invariant mass distribution of pη. The fitted number of signal events of the Λc+→pη process is 7734±263; from this, we measure the ratio of branching fractions B(Λc+→pη)/B(Λc+→pK-π+)=[2.258±0.077(stat)±0.122(syst)]×10-2, from which we infer the branching fraction B(Λc+→pη)=[1.42±0.05(stat)±0.11(syst)]×10-3. In addition, no significant signal for Λc+→pπ0 is found, so an upper limit on the branching fraction of B(Λc+→pπ0)\u3c8.0×10-5 at a 90% credibility level is set, more than 3 times better than the best current upper limit
Search for Axionlike Particles Produced in e+e- Collisions at Belle II
© 2020 authors. Published by the American Physical Society. Published by the American Physical Society under the terms of the https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article\u27s title, journal citation, and DOI. Funded by SCOAP3. We present a search for the direct production of a light pseudoscalar a decaying into two photons with the Belle II detector at the SuperKEKB collider. We search for the process e+e-→γa, a→γγ in the mass range 0.
First test of lepton flavor universality in the charmed baryon decays ωc0 → ω-ℓ+νℓ using data of the Belle experiment
We present the first observation of the ωc0→ω-μ+νμ decay and present measurements of the branching fraction ratios of the ωc0→ω-ℓ+νℓ decays compared to the reference mode ωc0→ω-π+, (ℓ=e or μ). This analysis is based on 89.5 fb-1, 711 fb-1, and 121.1 fb-1 data samples collected with the Belle detector at the KEKB asymmetric-energy e+e- collider at the center-of-mass energies of 10.52 GeV, 10.58 GeV, and 10.86 GeV, respectively. The ωc0 signal yields are extracted by fitting Mωℓ and Mωπ spectra. The branching fraction ratios B(ωc0→ω-e+νe)/B(ωc0→ω-π+) and B(ωc0→ω-μ+νμ)/B(ωc0→ω-π+) are measured to be 1.98±0.13(stat)±0.08(syst) and 1.94±0.18(stat)±0.10(syst), respectively. The ratio of B(ωc0→ω-e+νe)/B(ωc0→ω-μ+νμ) is measured to be 1.02±0.10(stat)±0.02(syst), which is consistent with the expectation of lepton flavor universality
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