Branching ratio and form of interaction for the β-decay of the Λ-hyperon

Abstract

102 events of the decay Λ→p+e−+ν have been found among 218 000 photos taken with the Ecole Polytechnique (1×1/2×1/2) m3, 17 kG heavy liquid bubble chamber in a 1.45 GeV/c K− beam (<2% π−) at the CERN PS. The liquid was genetron C2F5Cl, density 1.2 g/cm3, radiation length 25 cm, giving ∼87% electron detection efficiency and stopping ∼85% of protons from Λ→p+e−+ν. The branching ratio (Λ→p+e−+ν)/(all Λ) is found to be (0.78±0.12)·10−3 in agreement with previous results (1). The proton transverse momentum (and proton kinetic energy in the Λ rest system) spectra, and the e-ν correlation are incompatible (<1% probability) with those expected for either pure vector or pure scalar Λ→p+e−+ν interaction. Pure tensor or pure axial vector are less probable thanV±A, but are not excluded by the data. Assuming aV andA theory with two form factors independent of the momentum transfer, the best fit is for |C V /C A |=0.8 −0.8 +0.9 and with 95% confidence 0<|C V /C A |<3. Since our Λ’s are unpolarized, there are too few events to determine the sign ofC V /C A .