The parity- and isospin-forbidden ${\mathrm{\alpha }}_0$ decay from ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}_{}^{\mathrm{*}}{}_{}{}^{}$(${\mathit{J}}^{\mathrm{\pi }}$=$1^{+}$; T=1; ${\mathit{E}}_{\mathit{x}}$=13.482 MeV) to ${}_{}{}^{16}{}_{}{}^{}\mathrm{O}$(g.s.) has been investigated by measurement of the longitudinal analyzing power ${\mathit{A}}_{\mathit{z}}$ of the reaction ${}_{}{}^{19}{}_{}{}^{}\mathrm{F}$(p→,${\mathrm{\alpha }}_0$${)}^{16}$O around the $1^{+}$ resonance in ${}_{}{}^{20}{}_{}{}^{}\mathrm{Ne}_{}^{\mathrm{*}}{}_{}{}^{}$. With the data, the range for the expected interference effect has been reduced to ${\mathit{A}}_{\mathit{z}}$=(15.0±7.6)×${10}^{\mathrm{-}4}$ (68% c.l.), significantly smaller than a previous result. An analysis for the new data and those of related experiments has been performed to extract upper limits for the irregular ${\mathrm{\alpha }}_0$ width ${\mathrm{\Gamma }}_{\mathrm{\alpha }0}^{\mathrm{PNC}}$ and the weak pion nucleon coupling constant ${\mathit{f}}_{\mathrm{\pi }}$, being smaller than previous theoretical expectations.

• Received 6 April 1989

DOI:https://doi.org/10.1103/PhysRevC.41.R1336