Effect of the tensor force on gamma-ray de-excitation angular distributions from dipole states populated in theC12(p,p′) reaction

Angular distributions have been measured for the de-excitation gamma rays from the isoscalar and isovector $1^{+}$ states populated in the ${}_{}{}^{12}{}_{}{}^{}\mathrm{C}$($p$,$p^{\prime }$) reaction at $E_p\left(\mathrm{lab}\right)=22-27\mathrm{}$ MeV. Fluctuations in the $A_0$ coefficients, which measure the total excitation cross sections, are observed to decrease with energy toward the high end of the region investigated. The $a_2$ coefficients are still fluctuating at 27 MeV, but exhibit reasonably well-defined average values of about -0.40 and 0.15 for the $T=0\mathrm{}$ and $T=1\mathrm{}$ levels, respectively. Microscopic distorted-wave calculations based on a realistic $G$ matrix interaction, while not sufficient to completely describe the present data, suggest that the isospin dependence of the $a_2$ coefficients is associated with the tensor force. The theoretical calculations have been extended to 50 MeV and this effect of the tensor force persists.