Abstract

The following decays of η 0 mesons produced in 350 000 photographs of a heavy liquid bubble chamber have been observed: η 0 → π + π − π 0, η 0 → 3 π 0 → 6 γ, η 0 → γγ. All the γ-rays coming from these decays have been observed and measured; this provides a direct separation of the two neutral modes and independent measurements of the two following branching ratios: R 1 = Γ ( η 0 → 3 π 0)/ Γ( η 0 → π + π − π 0 and R 2 = Γ( gh 0 → γγ)/ Γ( η 0 → π + π − π 0). This analysis has been done with 526 η 0 → π + π − π 0( π 0 → γγ), 199 η 0 → 3 π 0 → 6 γ and 401 η 0 → γγ events. The result R 1 = 1.50 −0.29 +0.15 is in agreement with the theoretical predictions assuming that η 0 → 3 π decays are pure ΔI = 1 transitions. The result R 2 = 1.72 ± 0.25, combined with with R 1 and compared with the present world average of the ratio Γ( η 0 → neutrals)/gG(η 0 → π +π −π 0) is compatible with the absence of the mode η 0 → π 0 γγ or with only a small decay rate for this last mode. Then η 0 → 3 π 0 Dalitz plot has been studied, the three π 0 being reconstructed from the measurements of the 6γ-rays, using kinematical fits. No significant variation of the η 0 → 3 π 0 amplitude has been found. The coefficient of the term quadratic in pion energies in this amplitude is found to be α = −0.32 ± 0.37.

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