Three-nucleon interaction in heavy-ion collisions at intermediate energies

The effect of a three-nucleon (3N) interaction is studied for the production of high energy protons in heavyion collisions in the incident energy range of 44 to 400 A MeV. The 3N interaction is incorporated into the antisymmetrized molecular dynamics transport model of Ono [A. Ono, Phys. Rev. C 59, 853 (1999)] as a 3N collision term, following a diagram of three consecutive binary collisions. For the theoretical Ar-40 + V-51 reaction studies, no contribution from the 3N collisions is observed for high energy proton production at the incident energy of 44 A MeV. However when the incident energy increases, the contribution increases gradually. At 200 A MeV and above, the contribution is observed as distinctly harder energy slopes in the proton energy spectra. The model is applied to the available Bevalac data for Ar-40 + Ca-40 at 42,92, and 137 A MeV. The experimental proton energy spectra are reasonably well reproduced at angles theta >= 70 degrees for all three incident energies, showing negligible 3N contributions at 42 A MeV and significant contributions at 137 A MeV at the large laboratory angles. Good agreement at these large angles, where the 3N collision is a major mechanism to produce such protons, strongly indicates for the first time the importance of the 3N interaction in intermediate heavy ion reactions in a full transport calculation. The possible relation between the 3N collision term and the short range and the tensor interactions is suggested.