Belle Detector Research Articles

Measurement of the mass and width of the Λc(2625)+ charmed baryon and the branching ratios of Λc(2625)+→Σc0π+ and Λc(2625)+→Σc++π−

Using the entire data sample of 980 fb−1 collected at or near the ϒ(4S) resonance with the Belle detector operating at the KEKB asymmetric-energy e+e− collider, we report the measurement of the mass, width, and the branching ratios of the Λc(2625)+ charmed baryon. The mass difference between Λc(2625)+ and Λc+ is measured to be M(Λc(2625)+)−M(Λc+)=341.518±0.006±0.049 MeV/c2. The upper limit on the width is measured to be Γ(Λc(2625)+)<0.52 MeV/c2 at 90% confidence level. Based on a full Dalitz plot fit, branching ratios with respect to the mode Λc(2625)+→Λc+π+π− are measured to be B(Λc(2625)+→Σc0π+)B(Λc(2625)+→Λc+π+π−)=(5.19±0.23±0.40)% and B(Λc(2625)+→Σc++π−)B(Λc(2625)+→Λc+π+π−)=(5.13±0.26±0.32)%, where the first and second uncertainties are statistical and systematic, respectively. These measurements can be used to further constrain the parameters of the underlying theoretical models.Received 13 December 2022Accepted 20 January 2023DOI:https://doi.org/10.1103/PhysRevD.107.032008Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsCharmed baryonsPropertiesParticle propertiesTechniquesParticle decaysParticles & Fields

Measurement of branching fractions of Λc+→pKS0KS0 and Λc+→pKS0η at Belle

We present a study of a singly Cabibbo-suppressed decay Λc+→pKS0KS0 and a Cabibbo-favored decay Λc+→pKS0η based on 980 fb−1 of data collected by the Belle detector, operating at the KEKB energy-asymmetric e+e− collider. We measure their branching fractions relative to Λc+→pKS0: B(Λc+→pKS0KS0)/B(Λc+→pKS0)=(1.48±0.08±0.04)×10−2 and B(Λc+→pKS0η)/B(Λc+→pKS0)=(2.73±0.06±0.13)×10−1. Combining with the world average B(Λc+→pKS0), we have the absolute branching fractions, B(Λc+→pKS0KS0)=(2.35±0.12±0.07±0.12)×10−4 and B(Λc+→pKS0η)=(4.35±0.10±0.20±0.22)×10−3. The first and second uncertainties are statistical and systematic, respectively, while the third ones arise from the uncertainty on B(Λc+→pKS0). The mode Λc+→pKS0KS0 is observed for the first time and has a statistical significance of >10σ. The branching fraction of Λc+→pKS0η has been measured with a threefold improvement in precision over previous results and is found to be consistent with the world average.Received 6 October 2022Accepted 14 November 2022DOI:https://doi.org/10.1103/PhysRevD.107.032004Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBranching fractionPhysical SystemsCharmed baryonsTechniquesLepton collidersParticles & Fields

Measurement of the branching fraction of Ξc0→Λc+π− at Belle

Based on a data sample of 983 fb−1 collected with the Belle detector at the KEKB asymmetric-energy e+e− collider, we present the study of the heavy-flavor-conserving decay Ξc0→Λc+π− with Λc+ reconstructed via its pK−π+ decay mode. The branching fraction ratio B(Ξc0→Λc+π−)/B(Ξc0→Ξ−π+) is measured to be 0.38±0.04±0.04. Combining with the world average value of B(Ξc0→Ξ−π+), the branching fraction B(Ξc0→Λc+π−) is deduced to be (0.54±0.05±0.05±0.12)%. Here, the uncertainties above are statistical, systematic, and from B(Ξc0→Ξ−π+), respectively.Received 17 June 2022Accepted 28 July 2022DOI:https://doi.org/10.1103/PhysRevD.107.032005Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsCharmed baryonsGeneral Physics

Measurements of branching fractions of Λc+→Σ+η and Λc+→Σ+η′ and asymmetry parameters of Λc+→Σ+π0 , Λc+→Σ+η , and Λc+→Σ+η′

We report a study of Λc+→Σ+π0, Λc+→Σ+η, and Λc+→Σ+η′ using the data sample corresponding to an integrated luminosity of 980 fb−1 collected with the Belle detector at the KEKB asymmetric-energy e+e− collider. The branching fractions relative to Λc+→Σ+π0 are measured as BΛc+→Σ+η/BΛc+→Σ+π0=0.25±0.03±0.01 and BΛc+→Σ+η′/BΛc+→Σ+π0=0.33±0.06±0.02. Using BΛc+→Σ+π0=(1.25±0.10)%, we obtain BΛc+→Σ+η=(3.14±0.35±0.17±0.25)×10−3 and BΛc+→Σ+η′=(4.16±0.75±0.25±0.33)×10−3. Here the uncertainties are statistical, systematic, and from BΛc+→Σ+π0, respectively. The ratio of the branching fraction of Λc+→Σ+η′ with respect to that of Λc+→Σ+η is measured to be BΛc+→Σ+η′/BΛc+→Σ+η=1.34±0.28±0.08. We update the asymmetry parameter of Λc+→Σ+π0, αΣ+π0=−0.48±0.02±0.02, with a considerably improved precision. The asymmetry parameters of Λc+→Σ+η and Λc+→Σ+η′ are measured to be αΣ+η=−0.99±0.03±0.05 and αΣ+η′=−0.46±0.06±0.03 for the first time.1 MoreReceived 23 August 2022Accepted 8 December 2022DOI:https://doi.org/10.1103/PhysRevD.107.032003Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBranching fractionPhysical SystemsCharmed baryonsTechniquesLepton collidersParticles & Fields

First search for the weak radiative decays Λc+→Σ+γ and Ξc0→Ξ0γ

We present the first search for the weak radiative decays Λc+→Σ+γ and Ξc0→Ξ0γ using a data sample of 980 fb−1 collected by the Belle detector operating at the KEKB asymmetric-energy e+e− collider. There are no evident Λc+→Σ+γ or Ξc0→Ξ0γ signals. Taking the decays Λc+→pK−π+ and Ξc0→Ξ−π+ as normalization channels, the upper limits at 90% credibility level on the ratios of branching fractions B(Λc+→Σ+γ)/B(Λc+→pK−π+)<4.0×10−3 and B(Ξc0→Ξ0γ)/B(Ξc0→Ξ−π+)<1.2×10−2 are determined. We obtain the upper limits at 90% credibility level on the absolute branching fractions B(Λc+→Σ+γ)<2.6×10−4 and B(Ξc0→Ξ0γ)<1.8×10−4.Received 27 June 2022Accepted 2 August 2022DOI:https://doi.org/10.1103/PhysRevD.107.032001Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBranching fractionElectroweak radiative correctionsPhysical SystemsCharmed baryonsParticles & Fields

Measurement of two-photon decay width of χc2(1P) in γγ → χc2(1P) → J/ψγ at Belle

We report the measurement of the two-photon decay width of χc2(1P) in two-photon processes at the Belle experiment. We analyze the process γγ → χc2(1P) → J/ψγ, J/ψ → ℓ+ℓ− (ℓ = e or μ) using a data sample of 971 fb−1 collected with the Belle detector at the KEKB e+e− collider. In this analysis, the product of the two-photon decay width of χc2(1P) and the branching fraction is determined to be {Gamma}_{gamma gamma}left({chi}_{c2}(1P)right)mathcal{B}left({chi}_{c2}(1P)to J/psi gamma right)mathcal{B}left(J/psi to {ell}^{+}{ell}^{-}right)=14.8pm 0.3left(textrm{stat}.right)pm 0.7left(textrm{syst}.right) eV, which corresponds to Γγγ(χc2(1P)) = 653 ± 13(stat.) ± 31(syst.) ± 17(B.R.) eV, where the third uncertainty is from mathcal{B} (χc2(1P) → J/ψγ) and mathcal{B} (J/ψ → ℓ+ℓ−).

Study of e+e−→ηϕ via initial state radiation at Belle

Using 980 fb−1 of data collected on and around the ϒ(nS) (n=1, 2, 3, 4, 5) resonances with the Belle detector at the KEKB collider, we measure the cross section of e+e−→ηϕ from threshold to 3.95 GeV via initial state radiation. There are clear ϕ(1680) and J/ψ signals but no significant ϕ(2170) signal in the ηϕ final state. The branching fraction B[J/ψ→ηϕ] is measured to be (7.2±0.8±0.5)×10−4. The resonant parameters of ϕ(1680) are determined to be mϕ(1680)=(1696±8±10) MeV/c2 (statistical and systematic errors, respectively), Γϕ(1680)=(175±13±16) MeV and, depending on the possible presence of predominantly constructive or destructive interference between ϕ(1680) and continuum production, Γϕ(1680)e+e−·B[ϕ(1680)→ηϕ] and B[ϕ(1680)→ηϕ] are determined to be (75±10±11) eV and (25±12±2)% or (207±16±20) eV and (23±10±2)%, respectively. The upper limit for Γϕ(2170)e+e−·B[ϕ(2170)→ηϕ] is determined to be either 0.17 or 18.6 eV at the 90% confidence level.Received 1 September 2022Accepted 10 October 2022DOI:https://doi.org/10.1103/PhysRevD.107.012006Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHadronic decaysQuark modelParticles & Fields

Evidence of a New Excited Charmed Baryon Decaying to Σc(2455)0,++π±

We present the study of B[over ¯]^{0}→Σ_{c}(2455)^{0,++}π^{±}p[over ¯] decays based on 772×10^{6} BB[over ¯] events collected with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider. The Σ_{c}(2455)^{0,++} candidates are reconstructed via their decay to Λ_{c}^{+}π^{∓} and Λ_{c}^{+} decays to pK^{-}π^{+}, pK_{S}^{0}, and Λπ^{+} final states. The corresponding branching fractions are measured to be B(B[over ¯]^{0}→Σ_{c}(2455)^{0}π^{+}p[over ¯])=(1.09±0.06±0.07)×10^{-4} and B(B[over ¯]^{0}→Σ_{c}(2455)^{++}π^{-}p[over ¯])=(1.84±0.11±0.12)×10^{-4}, which are consistent with the world average values with improved precision. A new structure is found in the M_{Σ_{c}(2455)^{0,++}π^{±}} spectrum with a significance of 4.2σ including systematic uncertainty. The structure is possibly an excited Λ_{c}^{+} and is tentatively named Λ_{c}(2910)^{+}. Its mass and width are measured to be (2913.8±5.6±3.8) MeV/c^{2} and (51.8±20.0±18.8) MeV, respectively. The products of branching fractions for the Λ_{c}(2910)^{+} are measured to be B(B[over ¯]^{0}→Λ_{c}(2910)^{+}p[over ¯])×B(Λ_{c}(2910)^{+}→Σ_{c}(2455)^{0}π^{+})=(9.5±3.6±1.6)×10^{-6} and B(B[over ¯]^{0}→Λ_{c}(2910)^{+}p[over ¯])×B(Λ_{c}(2910)^{+}→Σ_{c}(2455)^{++}π^{-})=(1.24±0.35±0.10)×10^{-5}. Here, the first and second uncertainties are statistical and systematic, respectively.

Evidence for the singly Cabibbo-suppressed decay {Omega}_c^0to {Xi}^{-}{pi}^{+} and search for {Omega}_c^0to {Xi}^{-}{K}^{+} and Ω−K+ decays at Belle

Using a data sample of 980 fb−1 collected with the Belle detector at the KEKB asymmetric-energy e+e− collider, we study for the first time the singly Cabibbo-suppressed decays {Omega}_c^0to {Xi}^{-}{pi}^{+} and Ω−K+ and the doubly Cabibbo-suppressed decay {Omega}_c^0to {Xi}^{-}{K}^{+} . Evidence for an {Omega}_c^0 signal in the {Omega}_c^0 → Ξ−π+ mode is reported with a significance of 4.5σ including systematic uncertainties. The ratio of branching fractions to the normalization mode {Omega}_c^0 → Ω−π+ is measured to beBΩc0→Ξ−π+/BΩc0→Ω−π+=0.253±0.052stat.±0.030syst..\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B}\\left({\\Omega}_c^0\ o {\\Xi}^{-}{\\pi}^{+}\\right)/\\mathcal{B}\\left({\\Omega}_c^0\ o {\\Omega}^{-}{\\pi}^{+}\\right)=0.253\\pm 0.052\\left(\ extrm{stat}.\\right)\\pm 0.030\\left(\ extrm{syst}.\\right). $$\\end{document}No significant signals of {Omega}_c^0to {Xi}^{-}{K}^{+} and Ω−K+ modes are found. The upper limits at 90% confidence level on ratios of branching fractions are determined to beBΩc0→Ξ−K+/BΩc0→Ω−π+<0.070\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B}\\left({\\Omega}_c^0\ o {\\Xi}^{-}{K}^{+}\\right)/\\mathcal{B}\\left({\\Omega}_c^0\ o {\\Omega}^{-}{\\pi}^{+}\\right)<0.070 $$\\end{document}andBΩc0→Ω−K+/BΩc0→Ω−π+<0.29.\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B}\\left({\\Omega}_c^0\ o {\\Omega}^{-}{K}^{+}\\right)/\\mathcal{B}\\left({\\Omega}_c^0\ o {\\Omega}^{-}{\\pi}^{+}\\right)<0.29. $$\\end{document}

Lepton Flavor Universality at Belle and Belle II

We report the test of lepton flavor universality at Belle and Belle II experiments considering b → c and b → s transitions. We use 711 fb−1 and 189 fb−1 of data collected at 𝒴(4S ) resonance from electron-positron collisions for Belle and Belle II, respectively. For the b → c transition, we present the measurement of lepton-flavor-universality in B0 → D*−ℓ+νℓ decay, exclusive measurement of ℛ(D) &amp; ℛ(D*) in Belle data, and inclusive ℛ(Xe/µ) in Belle II. For the b → s transitions, we provide measurements of RK and RK* using Belle data, measurement of the branching fraction for the decay B → K*ℓℓ, and test of lepton-flavor-universality in B → J/ψK using Belle II data. Additionally, we report the lepton-flavor-universality in $ \Omega_{c}^{0} $ → Ω−ℓ+νℓ decay using 922 fb−1 of data collected by the Belle detector.

Observation of $\Omega(2012) \to \Xi(1530)\bar{K}$ and measurement of the effective couplings of $\Omega(2012)$ to $\Xi(1530)\bar{K}$ and $\Xi\bar{K}$

Abstract Using $\Upsilon(1S)$, $\Upsilon(2S)$, and $\Upsilon(3S)$ data collected by the Belle detector, we discover a new resonant three-body decay $\Omega(2012)^-\to \Xi(1530)^0K^-\to\Xi^-\pi^+K^-$ with a significance of 5.2$\sigma$. The mass of the $\Omega(2012)^-$ is $(2012.5\pm0.7\pm0.5)$ MeV and its effective couplings to $\Xi(1530)\bar{K}$ and $\Xi\bar{K}$ are $(41.1\pm35.8\pm6.0)\times10^{-2}$ and $(1.7\pm0.3\pm0.3)\times10^{-2}$, where the first uncertainties are statistical and the second are systematic. The ratio of the branching fraction for the resonant three-body decay to that for the two-body decay to $\Xi\bar{K}$ is $0.97\pm0.24\pm0.07$ assuming isospin symmetry, consistent with the molecular model of the $\Omega(2012)$, which predicts comparable rates for $\Omega(2012)$ decay to $\Xi(1530)\bar{K}$ and $\Xi\bar{K}$. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.

BELLE AEROGEL CHERENKOV COUNTER – A DETECTOR FOR PARTICLE IDENTIFICATION

Particle identification, in particular the identification of charged pions and kaons, plays an important role for the studies of CP violation in B Factory experiments. For the BELLE detector at the KEK B-Factory in Japan, a threshold aerogel Cherenkov counter (ACC) has been used to separate pions and kaons in the momentum range between 1.0 GeV/c and 3.5 GeV/c. This paper gives a brief description of the ACC design, construction, monitoring system and its performance of the beam test and Monte-Carlo simulation.

Search for the decay Bs0→η′KS0

We report the results of the first search for the decay Bs0→η′KS0 using 121.4 fb−1 of data collected at the ϒ(5S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. We observe no signal and set a 90% confidence-level upper limit of 8.16×10−6 on the Bs0→η′KS0 branching fraction.Received 5 January 2022Accepted 13 September 2022DOI:https://doi.org/10.1103/PhysRevD.106.L051103Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBranching fractionHadronic decaysParticle decaysParticle phenomenaRare decaysParticles & Fields

Measurement of the branching fractions of the B+→ηℓ+νℓ and B+→η′ℓ+νℓ decays with signal-side only reconstruction in the full q2 range

The branching fractions of the decays B+→ηℓ+νℓ and B+→η′ℓ+νℓ are measured, where ℓ is either an electron or a muon, using a data sample of 711 fb−1 containing 772×106BB¯ pairs collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. To reduce the dependence of the result on the form factor model, the measurement is performed over the entire q2 range. The resulting branching fractions are B(B+→ηℓ+νℓ)=(2.83±0.55(stat.)±0.34(syst.))×10−5 and B(B+→η′ℓ+νℓ)=(2.79±1.29(stat.)±0.30(syst.))×10−5.Received 28 April 2021Accepted 4 August 2022DOI:https://doi.org/10.1103/PhysRevD.106.032013Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP3.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasBranching fractionLeptonic, semileptonic & radiative decaysPhysical SystemsBottom quarkTechniquesLepton collidersParticles & Fields

Study of γγ→γψ(2S) at Belle

Using $980~\rm fb^{-1}$ of data on and around the $\Upsilon(nS)(n=1,2,3,4,5)$ resonances collected with the Belle detector at the KEKB asymmetric-energy $e^+e^-$ collider, the two-photon process $\gamma\gamma\to \gamma\psi(2S)$ is studied from the threshold to $4.2~{\rm GeV}$ for the first time. Two structures are seen in the invariant mass distribution of $\gamma\psi(2S)$: one at $M_{R_1} = 3922.4\pm 6.5 \pm 2.0~{\rm MeV}/c^2$ with a width of $\Gamma_{R_1} = 22\pm 17\pm 4~{\rm MeV}$, and another at $M_{R_2} = 4014.3\pm 4.0 \pm 1.5~{\rm MeV}/c^2$ with a width of $\Gamma_{R_2} = 4\pm 11 \pm 6~{\rm MeV}$; the signals are parametrized with the incoherent sum of two Breit-Wigner functions. The first structure is consistent with the $X(3915)$ or the $\chi_{c2}(3930)$, and the local statistical significance is determined to be $3.1\sigma$ with the systematic uncertainties included. The second matches none of the known charmonium or charmoniumlike states, and its global significance is determined to be $2.8\sigma$ including the look-elsewhere effect. The production rates are $\Gamma_{\gamma\gamma}{\cal B}(R_1\to\gamma\psi(2S)) = 9.8\pm 3.6\pm 1.2~{\rm eV}$ assuming $(J^{PC}, |\lambda|) =(0^{++}, 0)$ or $2.0\pm 0.7\pm 0.2~{\rm eV}$ with $(2^{++}, 2)$ for the first structure and $\Gamma_{\gamma\gamma}{\cal B}(R_2\to\gamma\psi(2S)) = 6.2\pm 2.2\pm 0.8~{\rm eV}$ with $(0^{++}, 0)$ or $1.2\pm 0.4\pm 0.2~{\rm eV}$ with $(2^{++}, 2)$ for the second one. Here, the first errors are statistical and the second systematic, and $\lambda$ is the helicity.

Search for charged lepton flavor violating decays of ϒ (1S)

We present a search for the charged lepton-flavor-violating decays ϒ(1S) →ℓ±ℓ′∓ and radiative charged lepton-flavour-violating decays ϒ(1S) → γℓ±ℓ′∓ [ℓ,ℓ′ = e, μ, τ] using the 158 million ϒ(2S) sample collected by the Belle detector at the KEKB collider. This search uses ϒ(1S) mesons produced in ϒ(2S) → π+π− ϒ(1S) transitions. We do not find any significant signal, so we provide upper limits on the branching fractions at the 90% confidence level.

Measurement of Two-Particle Correlations of Hadrons in e^{+}e^{-} Collisions at Belle.

The measurement of two-particle angular correlation functions in high-multiplicity e^{+}e^{-} collisions at sqrt[s]=10.52 GeV is reported. In this study, the 89.5 fb^{-1} of hadronic e^{+}e^{-} annihilation data collected by the Belle detector at KEKB are used. Two-particle angular correlation functions are measured in the full relative azimuthal angle (Δϕ) and three units of pseudorapidity (Δη), defined by either the electron beam axis or the event-shape thrust axis, and are studied as a function of charged-particle multiplicity. The measurement in the thrust axis analysis, with mostly outgoing quark pairs determining the reference axis, is sensitive to the region of additional soft gluon emissions. No significant anisotropic collective behavior is observed with either coordinate analyses. Near-side jet correlations appear to be absent in the thrust axis analysis. The measurements are compared to predictions from various event generators and are expected to provide new constraints to the phenomenological models in the low-energy regime.

An improved search for the electric dipole moment of the τ lepton

We report a measurement of the electric dipole moment of the τ lepton (dτ) using an 833 fb−1 data sample collected near the Υ(4S) resonance, with the Belle detector at the KEKB asymmetric-energy e+e− collider. Using an optimal observable method, we obtain the real and imaginary parts of dτ as Re(dτ) = (−0.62 ± 0.63) × 10−17ecm and Im(dτ) = (−0.40 ± 0.32) × 10−17ecm, respectively. These results are consistent with null electric dipole moment at the present level of experimental sensitivity and improve the sensitivity by about a factor of three.

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 $B^0$ mesons into a final state containing a $\Lambda$ baryon and missing energy. These results are obtained from a $711\text{fb}^{-1}$ data sample that contains $772 \times 10^6$ $B{\kern 0.18em}\overline{\kern -0.18em B}$ pairs and was collected near the $\Upsilon(4S)$ 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 $\Lambda$. No evidence for these decays is found and we set $90\%$ confidence level upper limits on the branching fractions in the range $2.1$-$3.8\times 10^{-5}$. This measurement provides the world's most restrictive limits, with implications for baryogenesis and dark matter production.

First measurement of the {Lambda}_c^{+} → pη′ decay

We present the first measurement of the branching fraction of the singly Cabibbo-suppressed (SCS) decay {Lambda}_c^{+} → pη′ with η′ → ηπ+π−, using a data sample corresponding to an integrated luminosity of 981 fb−1, collected by the Belle detector at the KEKB e+e− asymmetric-energy collider. A significant {Lambda}_c^{+} → pη′ signal is observed for the first time with a signal significance of 5.4σ. The relative branching fraction with respect to the normalization mode {Lambda}_c^{+} → pK−π+ is measured to beBΛc+→pη′BΛc+→pK−π+=7.54±1.32±0.73×10−3,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\frac{\\mathcal{B}\\left({\\Lambda}_c^{+}\ o p\\eta^{\\prime}\\right)}{\\mathcal{B}\\left({\\Lambda}_c^{+}\ o {pK}^{-}{\\pi}^{+}\\right)}=\\left(7.54\\pm 1.32\\pm 0.73\\right)\ imes {10}^{-3}, $$\\end{document}where the uncertainties are statistical and systematic, respectively. Using the world-average value of mathcal{B}left({Lambda}_c^{+}to {pK}^{-}{pi}^{+}right) = (6.28 ± 0.32) × 10−2, we obtainBΛc+→pη′=4.73±0.82±0.46±0.24×10−4,\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B}\\left({\\Lambda}_c^{+}\ o p\\eta^{\\prime}\\right)=\\left(4.73\\pm 0.82\\pm 0.46\\pm 0.24\\right)\ imes {10}^{-4}, $$\\end{document}where the uncertainties are statistical, systematic, and from mathcal{B}left({Lambda}_c^{+}to {pK}^{-}{pi}^{+}right) , respectively.