Ab initio study of the photoabsorption of 4He

There are some discrepancies in the low-energy data on the photoabsorption cross section of He-4. We calculate the cross section with realistic nuclear forces and explicitly correlated Gaussian functions. Final-state interactions and two-and three-body decay channels are taken into account. The cross section is evaluated using two methods: With the complex scaling method the total absorption cross section is obtained up to the rest energy of a pion, and with the microscopic R-matrix method cross sections for both He-4(gamma, p)H-3 and He-4(gamma, n)He-3 are calculated below 40 MeV. Both methods give virtually the same result. The cross section rises sharply from the H-3 + p threshold, reaching a giant resonance peak at 26-27 MeV. Our calculation reproduces almost all the data above 30 MeV. We stress the importance of H-3 + p and He-3 + n cluster configurations on the cross section as well as the effect of the one-pion exchange potential on the photonuclear sum rule.