Ab initio study of the photoabsorption of $^4$He

TL;DR

This study uses ab initio methods with realistic nuclear forces to accurately calculate the photoabsorption cross section of helium-4, resolving discrepancies in experimental data and highlighting the importance of cluster configurations and pion exchange effects.

Contribution

It presents a comprehensive ab initio calculation of helium-4 photoabsorption cross sections using two methods, incorporating complex interactions and decay channels for improved accuracy.

Findings

Cross section peaks at 26-27 MeV, matching experimental data.

Both methods yield consistent results, validating the approach.

Cluster configurations and pion exchange significantly influence the cross section.

Abstract

There are some discrepancies in the low energy data on the photoabsorption cross section of $^4$He. 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 in 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 both cross sections $^4$He($\gamma, p$)$^3$H and $^4$He($\gamma, n$)$^3$He are calculated below 40\,MeV. Both methods give virtually the same result. The cross section rises sharply from the $^3$H+$p$ 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 $^3$H+$p$ and $^3$He+$n$ cluster configurations on the cross section as well as the effect of the one-pion exchange potential on the photonuclear sum rule.