Photoproduction of neutral pions from complex nuclei: an indirect measurement of the π° lifetime

[摘要] The cross section for the photoproduction of neutral pions from complex nuclei at small angles has been measured in an attempt to determine the π° lifetime from the reaction γ + γ → π° where one of the photons is provided by the Coulomb field of the nucleus. Data were taken at energies near 950 Mev using targets of lead, copper, aluminum, carbon and oxygen. Both of the decay gamma rays from the pions were detected in total absorption counters so that an angular distribution of the pions could be calculated from the detector geometry and the partition of energy between the two counters. The cross section appears to be dominated by production from the individual nucleons adding coherently, leaving the nucleus in its ground state. The effects from the Coulomb production at the smallest angles allow us to exclude lifetimes shorter than 2 x 10^(-17) sec and longer than 1.5 x 10^(-16) sec. Because there is the possibility of systematic effects we quote the best value for the lifetime with large errors as 7.6^(+6.0)_(-4.5) x 10^(-17) sec. The large coherent nuclear production indicates that the non-spin flip part of the nucleon cross section is peaked at small angles. If this is attributed to a pole in the scattering amplitude due to the exchange of a neutral vector meson, then the meson must be an isoscalar. We cannot distinguish between contributions from the mass 550 Mev and the mass 780 Mev particles.

[发布机构] University:California Institute of Technology;Department:Physics, Mathematics and Astronomy

[关键词] Physics