Search for π0 → vμvμ decays in the LSND detector

We observe a net beam excess of 8.7 ′ 6.3(stat) ′ 2.4(syst) events, above 160 MeV, resulting from the charged-current reaction of v μ and/or v μ on C and H in the LSND detector. No beam-related muon background is expected in this energy regime. Within an analysis framework of π 0 → v μ v μ , we set a direct upper limit for this branching ratio of Γ(π 0 → v μ v μ )/Γ(π 0 → all) < 1.6 × 10 - 6 at 90% confidence level.