Observation of the spin isovector monopole resonance using the Pb ( p → , n → ) Bi reaction at 795 MeV

We examine a broad resonance in the $0\ifmmode^\circ\else\textdegree\fi{}$ ${}^{208}\mathrm{Pb}{(p,n)}^{208}\mathrm{Bi}$ and ${}^{90}\mathrm{Zr}{(p,n)}^{90}\mathrm{Nb}$ cross section spectra obtained at 795 MeV incident energy that appears at 32 MeV excitation energy in ${}^{208}\mathrm{Bi}$ and ${}^{90}\mathrm{Nb}.$ Earlier data taken at 200 MeV incident energy using the same reactions show no evidence for this resonance. We present theoretical results indicating that the spin isovector monopole predicted at this excitation energy is barely excited at 200 MeV while it is strongly excited at the higher energy. To further investigate this structure we have measured the first complete set of polarization-transfer observables for ${}^{\mathrm{nat}}\mathrm{Pb}$ at $0\ifmmode^\circ\else\textdegree\fi{}$ using the $(\stackrel{\ensuremath{\rightarrow}}{p},\stackrel{\ensuremath{\rightarrow}}{n})$ reaction at 795 MeV. The data show this resonance is predominantly a spin flip $\ensuremath{\Delta}S=1$ excitation giving strong supporting evidence for assigning this resonance as the spin isovector monopole.