Symmetry breaking and the decays ψ′ → J/ψπ(η) and J/ψ → η(η′) γ

The theoretical mechanisms for the decays ψ′ → ψπ⁰ and ψ′ → ψη, which violate SU₂ and SU₃, respectively discussed. It is argued that symmetry breaking in the decay amplitudes may be as important as π⁰ −η −η′ mixing. The π⁰ −η mechanism ψ′ → ψν → ψπ⁰ leads to Γ(ψ′ → ψπ⁰) = (3.3±1.0) × 10⁻³ Γ(ψ′ → ψν), but this number may be enhanced by a factor as large as 12 by π⁰−η′ mixing and isospin violation in the decay amplitude. The related decays ψ → ηγ and ψ → η′γ are also discussed.     

A highly significant deviation from the Cabibbo theory in hyperon leptonic decays

A very recently reported measurement of the transition rate of Σ⁻ → Γeν shows a 4 to 5 standard deviation discrepancy from the prediction of the Cabibbo theory. It could be attributed to symmetry breaking effects.     

Properties of the proposed τ charged lepton

The anomalous eμ and 2-prong μx events produced in e⁺e⁻ annihilation are used to determine the properties of the proposed τ charged lepton. We find the τ mass is 1.90 ± 0.10 GeV/c²; the mass of the associated neutrino, ν_τ, is less than 0.6 GeV/c² with 95% confidence; V - A coupling is favored over V + A coupling for the τ − ν_τ current; and the leptonic branching ratios are 0.186 ± 0.010 ± 0.028 from the eμ events and 0.175 ± 0.027 ± 0.030 from the μx events where the first error is statistical and the second is systematic.     

A measurement of the η radiative width Γη→γγ

The radiative width of the η meson has been measured at PETRA in photon-photon collisions. The resulting value is Γ_η→γγ = 0.53±0.04±0.04 keV.     

The radiative decay of the Λ(1405)

A recent result for the branching ratio for the decay of the kaonic hydrogen atom to Λγ is used to derive a value for the transition moment for the decay Λ(1405) → Λγ. Two solutions are found, −0.29 < κ < −0.07 and 0.60 < κ < 0.81 proton magnetons, giving decay widths of Γ(Λ(1405) → Λγ) = 6 ± 6 and 74 ± 22 keV, respectively.     

Electron-muon permutation symmetry and multiplicatively conserved lepton number in gauge theories

The similarity between the weak interactions of electron and muon is extended to the principle that all e and μ interactions in gauge models are invariant under e ↔ μ exchange. This necessitates the existence of two Higgs doublets φ_e and φ_μ, and an extended e ↔ μ permutation invariance. After symmetry-breaking, a multiplicatively conserved “permutation parity” π = ± 1 for all particles naturally emerges, with π_μ = −π_e = 1. The model forbids μ → eγ but predicts e⁻e⁻ → μ⁻μ⁻ mediated by π = −1 Higgs bosons at 10⁻¹¹ times the rate of typical weak cross sections.     

B→ηlν decays and the flavor-singlet form factors

We study semileptonic decays B→η^(′)lν, taking into account the flavor-singlet contribution (F^singlet₊) to the B→η^(′) form factors, which arises from the two-gluon emission in a decaying B meson. It has been recently pointed out that, in addition to large weak annihilation effects, the unknown value of F^singlet₊ prevents accurate theoretical estimates in the analysis of B→η′K decays in QCD factorization. We present a certain method to determine F^singlet₊ with a reasonable accuracy, using B→η^(′)lν and B→πlν decays. We also investigate the possible effect of F^singlet₊ on the estimated branching ratios (BRs) for B→η^(′)lν and find that the BR for B→η′lν is particularly sensitive to the effect of F^singlet₊.     

The contribution of Bc mesons to the search for B → τ ντ decays at LEP

We study the contribution of B_c mesons to the search for B → τν_τ decays. We find that at LEP the contributions from B_u and B_c mesons can be comparable. This observation can have a relevant impact on the extraction of constraints on new physics (such as charged-Higgs contributions) from current LEP limits on B → τν final states. Inclusion of the B_c contribution can reduce the current L3 limit on tan β/M_H from 0.38 GeV⁻¹ (90% CL) down to 0.27 GeV⁻¹ (90% CL).     

Tests of second-class currents

We argue that the presence of second-class neutral currents can be tested from the observations of and electron polarisation in η→π⁰e⁺e^-. Consequences of this model in η→π^±eν_e,τ^±→ηπ^±ν_τ and τ^±→ωπ^±ν_τ decays which would establish second-class charged currents are also discussed.     

Upper limit on the decay Σ(1385)→Σγ and cross section for γΣ→Λπ

Coherent Λπ⁻ production on Pb of 600 GeV Σ⁻ hyperons has been studied with the SELEX facility at Fermilab. Using the Primakoff formalism, we set a 90% CL upper limit on the radiative decay width Γ[Σ(1385)⁻→Σ⁻γ]<9.5 keV, and estimate the cross section for γΣ⁻→Λπ⁻ at  GeV to be 56±16 μb.