Electroexcitation of 4(-) states in 16O.

Abstract

We report electron scattering form factors for known ${4}^{\mathrm{\ensuremath{-}}}$ states in $^{16}\mathrm{O}$ at 17.79 and 18.98 MeV. We also report statistical upper bounds for the ${4}^{\mathrm{\ensuremath{-}}}$ state at 19.80 MeV. The ratios of these form factors are consistent with the isospin amplitudes derived from pion scattering data. The form factors of levels observed in (e,e') at 17.880\ifmmode\pm\else\textpm\fi{}0.015, 18.635\ifmmode\pm\else\textpm\fi{}0.020, and 20.510\ifmmode\pm\else\textpm\fi{}0.025 MeV are compatible with ${4}^{\mathrm{\ensuremath{-}}}$ T=1 assignments. Existing (\ensuremath{\pi},\ensuremath{\pi}') and (p,p') spectra do not contradict these identifications. The ${4}^{\mathrm{\ensuremath{-}}}$ form factors are well described by a harmonic-oscillator density with oscillator length b=1.58 fm. This value is substantially smaller than the value of 1.77 fm obtained from the ground state charge density. The form factor of the 18.98 MeV state can also be fitted by a Woods-Saxon transition density, with parameters fitted to the elastic M5 multipole of $^{17}\mathrm{O}$. The fit is improved by inclusion of meson-exchange currents. However, the isoscalar-isovector differences in the Woods-Saxon plus meson-exchange current form factors result in a poor fit to the 17.79 MeV state. The total isovector and isoscalar strengths observed for the established ${4}^{\mathrm{\ensuremath{-}}}$ states are 41% and 23%, respectively, of a pure single-particle prediction.