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

Search for the decay Bs0 →η′η

We report the results of the first search for the decay Bs0→η′η using 121.4 fb-1 of data collected at the I (5S) resonance with the Belle detector at the KEKB asymmetric-energy e+e-collider. We observe no significant signal and set a 90% confidence-level upper limit of 6.5×10-5 on the branching fraction of this decay

Measurement of B (Bs →dsX) with Bs semileptonic tagging

We report the first direct measurement of the inclusive branching fraction B(Bs→DsX) via Bs tagging in e+e-→ (5S) events. Tagging is accomplished through a partial reconstruction of semileptonic decays Bs→DsXℓν, where X denotes unreconstructed additional hadrons or photons and ℓ is an electron or muon. With 121.4 fb-1 of data collected at the (5S) resonance by the Belle detector at the KEKB asymmetric-energy e+e- collider, we obtain B(Bs→DsX)=(60.2±5.8±2.3)%, where the first uncertainty is statistical and the second is systematic

Measurement of branching fractions and search for CP violation in D \u3csup\u3e0\u3c/sup\u3e → π \u3csup\u3e+\u3c/sup\u3e π \u3csup\u3e −\u3c/sup\u3e η, D \u3csup\u3e0\u3c/sup\u3e → K \u3csup\u3e+\u3c/sup\u3e K \u3csup\u3e −\u3c/sup\u3e η, and D \u3csup\u3e0\u3c/sup\u3e → ϕη at Belle

We measure the branching fractions and CP asymmetries for the singly Cabibbo-suppressed decays D0 → π+π−η, D0 → K+K−η, and D0 → ϕη, using 980 fb−1 of data from the Belle experiment at the KEKB e+e− collider. We obtainB(D0→π+π−η)=[1.22±0.02(stat)±0.02(syst)±0.03(Bref)]×10−3,B(D0→K+K−η)=[1.80−0.06+0.07(stat)±0.04(syst)±0.05(Bref)]×10−4,B(D0→ϕη)=[1.84±0.09(stat)±0.06(syst)±0.05(Bref)]×10−4, where the third uncertainty (Bref) is from the uncertainty in the branching fraction of the reference mode D0 → K−π+η. The color-suppressed decay D0 → ϕη is observed for the first time, with very high significance. The results for the CP asymmetries areACP(D0π+π−η)=[0.9±1.2(stat)±0.5(syst)]%,ACP(D0→K+K−η)=[−1.4±3.3(stat)±1.1(syst)]%,ACP(D0→ϕη)=[−1.9±4.4(stat)±0.6(syst)]%. The results for D0 → π+π−η are a significant improvement over previous results. The branching fraction and ACP results for D0 → K+K−η, and the ACP result for D0 → ϕη, are the first such measurements. No evidence for CP violation is found in any of these decays. [Figure not available: see fulltext.

Search for the dark photon in B \u3csup\u3e0\u3c/sup\u3e → A′A′, A′ → e \u3csup\u3e+\u3c/sup\u3e e \u3csup\u3e −\u3c/sup\u3e, μ \u3csup\u3e+\u3c/sup\u3e μ \u3csup\u3e −\u3c/sup\u3e, and π \u3csup\u3e+\u3c/sup\u3e π \u3csup\u3e −\u3c/sup\u3e decays at Belle

We present a search for the dark photon A′ in the B0 → A′A′ decays, where A′ subsequently decays to e+e−, μ+μ−, and π+π−. The search is performed by analyzing 772 × 106BB¯ events collected by the Belle detector at the KEKB e+e− energy-asymmetric collider at the ϒ(4S) resonance. No signal is found in the dark photon mass range 0.01 GeV/c2 ≤ mA′ ≤ 2.62 GeV/c2, and we set upper limits of the branching fraction of B0 → A′A′ at the 90% confidence level. The products of branching fractions, ℬ(B0→A′A′)×ℬ(A′→e+e−)2 and ℬ(B0→A′A′)×ℬ(A′→μ+μ−)2, have limits of the order of 10−8 depending on the A′ mass. Furthermore, considering A′ decay rate to each pair of charged particles, the upper limits of ℬ (B→ A′ A′) are of the order of 10−8–10−5. From the upper limits of ℬ (B→ A′ A′) , we obtain the Higgs portal coupling for each assumed dark photon and dark Higgs mass. The Higgs portal couplings are of the order of 10−2–10−1 at mh′≃mB0 ± 40 MeV/c2 and 10−1–1 at mh′≃mB0 ± 3 GeV/c2. [Figure not available: see fulltext.

The Belle II SVD detector

The Silicon Vertex Detector (SVD) is one of the main detectors in the Belle II experiment at KEK, Japan. In combination with a pixel detector, the SVD determines precise decay vertex and low-momentum track reconstruction. The SVD ladders are being developed at several institutes. For the development of the tracking algorithm as well as the performance estimation of the ladders, beam tests for the ladders were performed. We report an overview of the SVD development, its performance measured in the beam test, and the prospect of its assembly and commissioning until installation

Performance studies of the Belle II Silicon Vertex Detector with data taken at the DESY test beam in April 2016

Belle II is a multipurpose detector currently under construction which will be operated at the next generation B-factory SuberKEKB in Japan. Its main devices for the vertex reconstruction are the Silicon Vertex Detector (SVD) and the Pixel Detector (PXD). In April 2016 a sector of the Belle II SVD and PXD have been tested in a beam of high energetic electrons at the test beam facility at DESY Hamburg (Germany). We report here the results for the hit efficiency estimation and the measurement of the resolution for the Belle II silicon vertex etector. We find that the hit efficiencies are on average above 99.5% and that the measured resolution is within the expectations

Performance studies of the Belle II Silicon Vertex Detector with data taken at the DESY test beam in April 2016

Belle II is a multipurpose detector currently under construction which will be operated at the next generation B-factory SuberKEKB in Japan. Its main devices for the vertex reconstruction are the Silicon Vertex Detector (SVD) and the Pixel Detector (PXD). In April 2016 a sector of the Belle II SVD and PXD have been tested in a beam of high energetic electrons at the test beam facility at DESY Hamburg (Germany). We report here the results for the hit efficiency estimation and the measurement of the resolution for the Belle II silicon vertex etector. We find that the hit efficiencies are on average above 99.5% and that the measured resolution is within the expectations

Measurement of the Branching Fraction of the Decay B+→π+π−ℓ+νℓ\boldsymbol{B^{+}\to\pi^{+}\pi^{-}\ell^{+}\nu_\ell} in Fully Reconstructed Events at Belle

We present an analysis of the exclusive $B^{+}\to\pi^{+}\pi^{-}\ell^{+}\nu_{\ell}$ decay, where $\ell$ represents an electron or a muon, with the assumption of charge-conjugation symmetry and lepton universality. The analysis uses the full $\Upsilon(4S)$ data sample collected by the Belle detector, corresponding to 711 fb$^{-1}$ of integrated luminosity. We select the events by fully reconstructing one $B$ meson in hadronic decay modes, subsequently determining the properties of the other $B$ meson. We extract the signal yields using a binned maximum-likelihood fit to the missing-mass squared distribution in bins of the invariant mass of the two pions or the momentum transfer squared. We measure a total branching fraction of ${{\cal B}(B^{+}\to \pi^{+}\pi^{-}\ell^{+}\nu_{\ell})= [22.7 ^{+1.9}_{-1.6} (\mathrm{stat}) \pm 3.5(\mathrm{syst}) ]\times 10^{-5}}$, where the uncertainties are statistical and systematic, respectively. This result is the first reported measurement of this decay.Comment: 23 pages, 19 figure

Search for lepton-number- And baryon-number-violating tau decays at Belle

© 2020 authors. Published by the American Physical Society. We search for lepton-number- and baryon-number-violating decays τ-→p¯e+e-, pe-e-, p¯e+μ-, p¯e-μ+, p¯μ+μ-, and pμ-μ- using 921 fb-1 of data, equivalent to (841±12)×106 τ+τ- events, recorded with the Belle detector at the KEKB asymmetric-energy e+e- collider. In the absence of a signal, 90% confidence-level upper limits are set on the branching fractions of these decays in the range (1.8-4.0)×10-8. We set the world\u27s first limits on the first four channels and improve the existing limits by an order of magnitude for the last two channels