Coherent production of low-mass $p\pi^{+}\pi^{-}$ states in the interaction of 18.6 GeV/c protons with $^{4}He$ nuclei

A specially developed scintillator and spark chamber spectrometer has been used to measure recoiling /sup 4/He target nuclei, and a magnetic spark chamber spectrometer to measure forward-going p pi /sup +/ pi /sup -/ particles, selecting coherent events by four-momentum balance. Two enhancements in the p pi /sup +/ pi /sup -/ mass spectrum are found at 1.49 and 1.71 GeV/c/sup 2/. The branching ratio of the quasi- two-body state Delta /sup ++/ pi /sup -/ is 4.2+or-8% in the 1.71 GeV /c/sup 2/ mass region. The t slope, as a function of p pi /sup +/ pi /sup -/ mass, is consistent with what can be predicted using the impulse approximation. The decay angular analysis indicates the presence of several interfering states in the 1.5 GeV/ c/sup 2/ mass region and of one state in the 1.7 GeV/c/sup 2/ mass region. There is some preference shown for t-channel helicity conservation. (15 refs).