Observation of Eta-C and Other Structures in the Radiative Decay Psi to Gamma Eta Negative Pion Positive Pion.

The Mark III detector at SPEAR was used to observe the exclusive radiative decay (psi) (--->) (gamma)(eta)(pi)('+)(pi)(' -). The (eta)(,c) was seen as an intermediate state in this decay, (psi) (--->) (gamma)(eta)(,c), (eta)(,c) ( --->) (eta)(pi)('+)(pi)('-). The product branching ratio B((psi) (--->) (gamma)(eta)(,c)) B((eta)(,c) (--->) (eta)(pi)('+)(pi)(' -)) was measured to be 4.2 (+OR-) 0.5 (stat) (+OR-) 0.5 (sys) x 10('-4). Other resonant structure was observed in the (eta)(pi)('+)(pi)('-) spectrum in the range 1 < M(,(eta)(pi)+(pi)-) < 2 GeV/c('2), which appears to decay via X (--->) (delta)('(+OR-))(pi)('(-OR+)), (delta)('(+OR -)) (--->) (eta)(pi)('(+OR-)). A spin-parity analysis was performed to determine the quantum numbers of the resonances responsible for this structure. However, the result was seen to be model dependent, so that it was not possible to conclusively resolve the underlying structure in the spectrum. No evidence was seen for the decay (iota) ( --->) (eta)(pi)('+)(pi)('-). An upper limit was set for the ratio of branching ratios for (iota) (--->) (eta)(pi)(pi) relative to (iota) (--->) KK(pi), R = (B((psi)(--->) (gamma)(iota)) B((iota)(--->) (eta)(pi)(pi)))/(B((psi)(--->) (gamma)(iota)) B((iota)(--->) KK(pi))) < 0.2 (90% C.L.).