Pion Induced Pion Production on Oxygen at 280 Mev.

A first coincident measurement of the pion induced pion production reaction cross-section on a complex nucleus (A > 2) has been successfully performed. In particular, the reaction ^{16}O( pi^+,pi^+pi^-) was measured at 280 MeV incident pion energy. The only previous published measures of this reaction on nuclei consisted of a dated measurement done on emulsion nuclei (BBD*69) and did not provide very stringent limits to the nuclear cross section. Single arm experiments have previously been done elsewhere on the proton (BJK*80) and the deuteron (PGM*84). The reaction was measured at TRIUMF using the QQD magnetic spectrometer in coincidence with the CARUZ (RGR88), a total absorption scintillator range telescope. The measured four-fold differential cross sections were extrapolated to the unmeasured portions of the phase-space to extract the total reaction cross-section at 280 MeV, which was found to be sigma_{tot} = 2.250 +/-.350mb . The (pi,2pi ) cross-section is thus observed to provide approximately 40% of the inclusive double charge exchange cross section (Woo84) at this energy. The model of (OV86) is found to explain many of the features of the data, including sigma _{tot}. The present data do not preclude effects due to pion condensate precursor phenomena as proposed by (CE83) but they do not support the existence of a strong effect. The data are also compared to kinematical Monte Carlo simulations of some possible reaction mechanisms and it is found that the presence of an intermediate Delta can aid the explanation of the low energy features of the pi^+ energy spectrum.