Evidence for the decay ∑+ → pμ+ μ-

We report the first evidence for the decay ∑+ → pμ+ μ- from data taken by the HyperCP (E871) experiment at Fermilab. Based on three observed events, the branching ratio is B(∑+ → pμ+ μ-) = [8.6 -5.4 +6.6(stat) ± 5.5(syst)] × 10 -8. The narrow range of dimuon masses may indicate that the decay proceeds via a neutral intermediate state, ∑+ rarr; pp0, p0 → μ+ μ- with a P0 mass of 214.3 ± 0.5 MeV/c 2 and branching ratio B(∑+ rarr; pp 0, p0 → μ+ μ-) = [3.1-1.9 -2.4(stat) ± 1.5(syst)] × 10 -8. © 2005 The American Physical Society.