Coherent production of low-masspπ+π−states in the interaction of 18.6GeV/cprotons with4He nuclei

Abstract A specially developed scintillator and spark chamber spectrometer has been used to measure recoiling 4 He target nuclei, and a magnetic spark chamber spectrometer to measure forward-going p π + π − particles, selecting coherent events by four-momentum balance. Two enhancements in the p π + π − mass spectrum are found at 1.49 and 1.71 GeV/ c 2 . The branching ratio of the quasi-two-body stateΔ ++ π − is42 ± 8%in the 1.71 GeV/ c 2 mass region. The t slope, as a function of p π + π − 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 2 mass region and of one state of spin3/2, possibly5/2, in the 1.7 GeV/c 2 mass region. There is some preference shown for t -channelhelicity conservation. The data are very similar to results obtained with photon targets, which provides independent support for the hypothesis that vacuum exchange is dominant in the production of low-mass p π + π − states in pp interactions around 20 GeV/c .