Belle Detector Research Articles

Search for Rare b→dℓ+ℓ− Transitions at Belle

We present the results of a search for the b→dℓ+ℓ− flavor-changing neutral-current rare decays B+,0→(η,ω,π+,0,ρ+,0)e+e− and B+,0→(η,ω,π0,ρ+)μ+μ− using a 711 fb−1 data sample that contains 772×106 BB¯ events. The data were collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. We find no evidence for signal and set upper limits on branching fractions at the 90% confidence level in the range (3.8–47)×10−8 depending on the decay channel. The obtained limits are the world’s best results. This is the first search for the channels B+,0→(ω,ρ+,0)e+e− and B+,0→(ω,ρ+)μ+μ−. Published by the American Physical Society 2024

Search for charmed baryons in the Λc+η system and measurement of the branching fractions of Λc(2880)+ and Λc(2940)+ decaying to Λc+η and pD0 relative to Σc(2455)π

We search for excited charmed baryons in the Λc+η system using a data sample corresponding to an integrated luminosity of 980 fb−1. The data were collected by the Belle detector at the KEKB e+e− asymmetric-energy collider. No significant signals are found in the Λc+η mass spectrum, including the known Λc(2880)+ and Λc(2940)+. Clear Λc(2880)+ and Λc(2940)+ signals are observed in the pD0 mass spectrum. We set upper limits at 90% credibility level on ratios of branching fractions of Λc(2880)+ and Λc(2940)+ decaying to Λc+η relative to Σc(2455)π of <0.13 for the Λc(2880)+ and <1.11 for the Λc(2940)+. We measure ratios of branching fractions of Λc(2880)+ and Λc(2940)+ decaying to pD0 relative to Σc(2455)π of 0.75±0.03(stat)±0.07(syst) for the Λc(2880)+ and 3.59±0.21(stat)±0.56(syst) for the Λc(2940)+. Published by the American Physical Society 2024

Search for Baryon-Number-Violating Processes in B^{-} Decays to the Ξ[over ¯]_{c}^{0}Λ[over ¯]_{c}^{-} Final State.

We report the results of the first search for B^{-} decays to the Ξ[over ¯]_{c}^{0}Λ[over ¯]_{c}^{-} final state using 711 fb^{-1} of data collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider. The results are interpreted in terms of both direct baryon-number-violating B^{-} decay and Ξ_{c}^{0}-Ξ[over ¯]_{c}^{0} oscillations which follow the standard model decay B^{-}→Ξ_{c}^{0}Λ[over ¯]_{c}^{-}. We observe no evidence for baryon number violation and set the 95% confidence-level upper limits on the ratio of baryon-number-violating and standard model branching fractions B(B^{-}→Ξ[over ¯]_{c}^{0}Λ[over ¯]_{c}^{-})/B(B^{-}→Ξ_{c}^{0}Λ[over ¯]_{c}^{-}) to be <2.7% and on the effective angular frequency of mixing ω in Ξ_{c}^{0}-Ξ[over ¯]_{c}^{0} oscillations to be <0.76 ps^{-1} (equivalent to τ_{mix}>1.3 ps).

Search for two-body B meson decays to Λ0 and Ωc(*)0

We report the results of the first search for Standard Model and baryon-number-violating two-body decays of the neutral B mesons to Λ0 and Ωc(*)0 using 711 fb−1 of data collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. We observe no evidence of signal from any such decays and set 95% confidence-level upper limits on the products of B0 and B¯0 branching fractions for these two-body decays with B(Ωc0→π+Ω−) in the range between 9.5×10−8 and 31.2×10−8. Published by the American Physical Society 2024

Search for a heavy neutral lepton that mixes predominantly with the tau neutrino

We report a search for a heavy neutral lepton (HNL) that mixes predominantly with ντ. The search utilizes data collected with the Belle detector at the KEKB asymmetric energy e+e− collider. The data sample was collected at and just below the center-of-mass energies of the ϒ(4S) and ϒ(5S) resonances and has an integrated luminosity of 915 fb−1, corresponding to (836±12)×106 e+e−→τ+τ− events. We search for production of the HNL (denoted N) in the decay τ−→π−N followed by its decay via N→μ+μ−ντ. The search focuses on the parameter-space region in which the HNL is long-lived, so that the μ+μ− originate from a common vertex that is significantly displaced from the collision point of the KEKB beams. Consistent with the expected background yield, one event is observed in the data sample after application of all the event-selection criteria. We report limits on the mixing parameter of the HNL with the τ neutrino as a function of the HNL mass. Published by the American Physical Society 2024

Search for the semileptonic decays Ξc0→Ξ0ℓ+ℓ− at Belle

Using the full data sample of 980 fb−1 collected with the Belle detector at the KEKB asymmetric energy electron-positron collider, we report the results of the first search for the rare semileptonic decays Ξc0→Ξ0ℓ+ℓ− (ℓ=e or μ). No significant signals are observed in the Ξ0ℓ+ℓ− invariant-mass distributions. Taking the decay Ξc0→Ξ−π+ as the normalization mode, we report 90% credibility upper limits on the branching fraction ratios B(Ξc0→Ξ0e+e−)/B(Ξc0→Ξ−π+)&lt;6.7×10−3 and B(Ξc0→Ξ0μ+μ−)/B(Ξc0→Ξ−π+)&lt;4.3×10−3 based on the phase-space assumption for signal decays. The 90% credibility upper limits on the absolute branching fractions of B(Ξc0→Ξ0e+e−) and B(Ξc0→Ξ0μ+μ−) are found to be 9.9×10−5 and 6.5×10−5, respectively. Published by the American Physical Society 2024

Search for charged-lepton flavor violation in Υ(2S) → ℓ∓τ± (ℓ = e, μ) decays at Belle

We report a search for the charged-lepton flavor violation in Υ(2S) → ℓ∓τ± (ℓ = e, μ) decays using a 25 fb−1 Υ(2S) sample collected by the Belle detector at the KEKB e+e− asymmetric-energy collider. We find no evidence for a signal and set upper limits on the branching fractions (B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document}) at 90% confidence level. We obtain the most stringent upper limits: B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document}(Υ(2S) → μ∓τ±) < 0.23 × 10−6 and B\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\mathcal{B} $$\\end{document}(Υ(2S) → e∓τ±) < 1.12 × 10−6.

Search for baryon and lepton number violating decays D→pℓ

We search for the baryon and lepton number violating charm decays, D→pℓ, where D is either a D0 or a D¯0 and ℓ is a muon or an electron, using a data sample of 921 fb−1 collected by the Belle detector at the KEKB asymmetric energy e+e− collider. In the absence of significant signals, we set upper limits on the branching fractions in the range (5–8)×10−7 at a 90% confidence level, depending on the decay mode. Published by the American Physical Society 2024

Search for {textrm{B}}_{textrm{s}}^0 → ℓ∓τ± with the semi-leptonic tagging method at Belle

We present a search for the lepton-flavor-violating decays {B}_s^0 → ℓ∓τ±, where ℓ = e, μ, using the full data sample of 121 fb−1 collected at the Υ(5S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. We use {B}_s^0{overline{B}}_s^0 events in which one {B}_s^0 meson is reconstructed in a semileptonic decay mode and the other in the signal mode. We find no evidence for {B}_s^0 → ℓ∓τ± decays and set upper limits on their branching fractions at 90% confidence level as mathcal{B} ( {B}_s^0 → e∓τ±) < 14 × 10−4 and mathcal{B} ( {B}_s^0 → μ∓τ±) < 7.3 × 10−4. Our result represents the first upper limit on the {B}_s^0 → e∓τ± decay rate.

Measurement of the e+e−→ {B}_s^0{overline{B}}_s^0X cross section in the energy range from 10.63 to 11.02 GeV using inclusive {D}_s^{+} and D0 production

We report the first measurement of the inclusive e+e−→ boverline{b} → {D}_s^{pm } X and e+e−→ boverline{b} → D0/ {overline{D}}^0 X cross sections in the energy range from 10.63 to 11.02 GeV. Based on these results, we determine σ(e+e−→ {B}_s^0{overline{B}}_s^0 X) and σ(e+e−→ Boverline{B} X) in the same energy range. We measure the fraction of {B}_s^0 events at Υ(10860) to be fs = ( {22.0}_{-2.1}^{+2.0} )%. We determine also the ratio of the {B}_s^0 inclusive branching fractions mathcal{B} ( {B}_s^0 → D0/ {overline{D}}^0 X)/ mathcal{B} ( {B}_s^0 → {D}_s^{pm } X) = 0.416 ± 0.018 ± 0.092. The results are obtained using the data collected with the Belle detector at the KEKB asymmetric-energy e+e− collider.

Search for the double-charmonium state with ηcJ/ψ at Belle

We measure the cross section of e+e− → ηcJ/ψ at the Υ(nS)(n = 1–5) on-resonance and 10.52 GeV off-resonance energy points using the full data sample collected by the Belle detector with an integrated luminosity of 955 fb−1. We also search for double charmonium production in e+e− → ηcJ/ψ via initial state radiation near the ηcJ/ψ threshold. No evident signal of the double charmonium state is found, but evidence for the e+e−→ ηcJ/ψ process is found with a statistical significance greater than 3.3σ near the ηcJ/ψ threshold. The average cross section near the threshold is measured and upper limits of cross sections are set for other regions.

First Measurement of the Michel Parameter ξ^{'} in the τ^{-}→μ^{-}ν[over ¯]_{μ}ν_{τ} Decay at Belle.

We report the first measurement of the Michel parameter ξ^{'} in the τ^{-}→μ^{-}ν[over ¯]_{μ}ν_{τ} decay with a new method proposed just recently. The measurement is based on the reconstruction of the τ^{-}→μ^{-}ν[over ¯]_{μ}ν_{τ} events with subsequent muon decay in flight in the Belle central drift chamber. The analyzed data sample of 988 fb^{-1} collected by the Belle detector corresponds to approximately 912×10^{6} τ^{+}τ^{-} pairs. We measure ξ^{'}=0.22±0.94(stat)±0.42(syst), which is in agreement with the standard model prediction of ξ^{'}=1. Statistical uncertainty dominates in this study, being a limiting factor, while systematic uncertainty is well under control. Our analysis proved the practicability of this promising method and its prospects for further precise measurement in future experiments.

Search for lepton-flavor-violating τ decays into a lepton and a vector meson using the full Belle data sample

Charged-lepton-flavor-violation is predicted in several new physics scenarios. We update the analysis of τ lepton decays into a light charged lepton (ℓ = e± or μ±) and a vector meson (V0 = ρ0, ϕ, ω, K*0, or overline{K} *0) using 980 fb−1 of data collected with the Belle detector at the KEKB collider. No significant excess of such signal events is observed, and thus 90% credibility level upper limits are set on the τ → ℓV0 branching fractions in the range of (1.7–4.3) × 10−8. These limits are improved by 30% on average from the previous results.

First Measurement of the B^{+}→π^{+}π^{0}π^{0} Branching Fraction and CP Asymmetry.

We study B^{+}→π^{+}π^{0}π^{0} using 711 fb^{-1} of data collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e^{+}e^{-} collider. We measure an inclusive branching fraction of (19.0±1.5±1.4)×10^{-6} and an inclusive CP asymmetry of (9.2±6.8±0.7)%, where the first uncertainties are statistical and the second are systematic, and a B^{+}→ρ(770)^{+}π^{0} branching fraction of (11.2±1.1±0.9_{-1.6}^{+0.8})×10^{-6}, where the third uncertainty is due to possible interference with B^{+}→ρ(1450)^{+}π^{0}. We present the first observation of a structure around 1 GeV/c^{2} in the π^{0}π^{0} mass spectrum, with a significance of 6.4σ, and measure a branching fraction to be (6.9±0.9±0.6)×10^{-6}. We also report a measurement of local CP asymmetry in this structure.

First Observation of Λπ^{+} and Λπ^{-} Signals near the K[over ¯]N(I=1) Mass Threshold in Λ_{c}^{+}→Λπ^{+}π^{+}π^{-} Decay.

Using the data sample of 980 fb^{-1} collected with the Belle detector operating at the KEKB asymmetric-energy e^{+}e^{-} collider, we present the results of an investigation of the Λπ^{+} and Λπ^{-} invariant mass distributions looking for substructure in the decay Λ_{c}^{+}→Λπ^{+}π^{+}π^{-}. We find a significant signal in each mass distribution. When interpreted as resonances, we find for the Λπ^{+} (Λπ^{-}) combination a mass of 1434.3±0.6(stat)±0.9(syst) MeV/c^{2} [1438.5±0.9(stat)±2.5(syst) MeV/c^{2}], an intrinsic width of 11.5±2.8(stat)±5.3(syst) MeV/c^{2} [33.0±7.5(stat)±23.6(syst) MeV/c^{2}] with a significance of 7.5σ (6.2σ). As these two signals are very close to the K[over ¯]N threshold, we also investigate the possibility of a K[over ¯]N cusp, and find that we cannot discriminate between these two interpretations due to the limited size of the data sample.

Search for the decay Bs0→π0π0 at Belle

We report the results of the first search for the decay Bs0→π0π0 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 7.7×10−6 on the Bs0→π0π0 decay branching fraction.Received 23 January 2023Accepted 2 March 2023DOI:https://doi.org/10.1103/PhysRevD.107.L051101Published 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 AreasParticle decaysParticle interactionsPhysical SystemsBottom quarkMesonsStrange quarkPropertiesFlavor symmetriesTechniquesArtificial neural networksBayesian methodsCalorimetersGrid computingLepton collidersMonte Carlo methodsParticle data analysisParticles & Fields

Two-particle angular correlations in e+e− collisions to hadronic final states in two reference coordinates at Belle

We present the analysis of two-particle angular correlations using coordinate systems defined with the conventional beam axis and the event thrust axis. We propose the latter as a good representation for the correlation structure interpretation in the e+e− collision system. The e+e− collisions to hadronic final states at center-of-mass energies of sqrt{s} = 10.52 GeV and 10.58 GeV are recorded by the Belle detector at KEKB. In this paper, results on the first dataset are supplementary to the previous Belle publication [1]. At the same time, the latter is the first two-particle correlation measurement at collision energy on the Υ(4S) resonance and is sensitive to its decay products. Measurements are reported as a function of the charged-particle multiplicity. Finally, a qualitative understanding of the correlation structure is discussed using a combination of Monte Carlo simulations and experimental data.

Search for X(3872)→π+π−π0 at Belle

We present a search for the decay X(3872)→π+π−π0 in the (772±11)×106 ϒ(4S)→BB¯ data sample collected at the Belle detector, where the X(3872) is produced in B±→K±X(3872) and B0→KS0X(3872) decays. We do not observe a signal, and set 90% credible upper limits for two different models of the decay processes: if the decay products are distributed uniformly in phase space, B(X(3872)→π+π−π0)<1.3%, and if M(π+π−) is concentrated near the mass of the D0D¯0 pair in the process X(3872)→D0D¯*0+c.c.→D0D¯0π0→π+π−π0, B(X(3872)→π+π−π0)<1.2×10−3.Received 17 June 2022Accepted 26 September 2022DOI:https://doi.org/10.1103/PhysRevD.107.052004Published 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 decaysQuantum chromodynamicsQuark modelStrong interactionParticles & Fields

Angular analysis of the low K+K− invariant mass enhancement in B+→K+K−π+ decays

We study the decay B+→K+K−π+ and investigate the angular distribution of K+K− pairs with invariant mass below 1.1 GeV/c2. This region exhibits both a strong enhancement in signal and very large direct CP violation. We construct a coherent sum model for the angular distribution of the S- and P-wave, and report the ratio of their amplitudes, the relative phase and the forward-backward asymmetry. We also report absolute differential branching fractions and direct CP asymmetry for the decay in bins of MK+K− and the differential branching fractions in bins of MK+π−. The results are based on a data sample that contains 772×106 BB¯ pairs collected at the ϒ(4S) resonance with the Belle detector at the KEKB asymmetric-energy e+e− collider. The result favors the presence of S- and D-waves in low MK+K− region to the detriment of a P-wave.1 MoreReceived 24 June 2022Accepted 3 February 2023DOI:https://doi.org/10.1103/PhysRevD.107.032013Published 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 fractionHadronic decaysParticle decaysRare decaysPhysical SystemsMesonsPropertiesCP violationParticles & Fields

Search for [formula omitted] violation and measurement of branching fractions and decay asymmetry parameters for [formula omitted] and [formula omitted] ([formula omitted

We report a study of Λc+→Λh+ and Λc+→Σ0h+ (h=K,π) decays based on a data sample of 980fb-1 collected with the Belle detector at the KEKB energy-asymmetric e+e- collider. The first results of direct CP asymmetry in two-body singly Cabibbo-suppressed (SCS) decays of charmed baryons are measured, ACPdir(Λc+→ΛK+)=+0.021±0.026±0.001 and ACPdir(Λc+→Σ0K+)=+0.025±0.054±0.004. We also make the most precise measurement of the decay asymmetry parameters (α) for the four modes of interest and search for CP violation via the α-induced CP asymmetry (ACPα). We measure ACPα(Λc+→ΛK+)=-0.023±0.086±0.071 and ACPα(Λc+→Σ0K+)=+0.08±0.35±0.14, which are the first ACPα results for SCS decays of charmed baryons. We search for Λ-hyperon CP violation in Λc+→(Λ,Σ0)π+ and find ACPα(Λ→pπ-)=+0.013±0.007±0.011. This is the first time that hyperon CP violation has been measured via Cabibbo-favored charm decays. No evidence of baryon CP violation is found. We also obtain the most precise branching fractions for two SCS Λc+ decays, B(Λc+→ΛK+)=(6.57±0.17±0.11±0.35)×10-4 and B(Λc+→Σ0K+)=(3.58±0.19±0.06±0.19)×10-4. The first uncertainties are statistical and the second systematic, while the third uncertainties come from the uncertainties on the world average branching fractions of Λc+→(Λ,Σ0)π+.