Helium-4 gravitational form factors: exchange currents

Abstract

We evaluate the leading exchange corrections to the Helium-4 gravitational form factors (GFFs) upto momenta of the order of the nucleon mass. We use both the K-harmonic method with simple pair nucleon potential, and a Jastrow trial function using the Argonne $v_{14}$ potential, to evaluate the Helium-4 GFFs. The exchange current contributions include the pair interaction, plus the seagull and the pion exchange interactions, modulo the recoil corrections. To estimate the off-shellness of the pion nucleon coupling in this momenta range, we discuss the results using either the pseudo-scalar (PS) or pseudo-vector (PV) pion-nucleon couplings. When the PV coupling is used, the pair diagram contribution is higher order in the non relativistic expansion. The results for the Helium-4 A-GFF are comparable to those given by the impulse approximation, especially for the PS coupling using both the K-Harmonic method and variational method. The exchange current contributions with the PS coupling for the charge form factor of Helium-4, yield better agreement with the existing data over a broad range of momenta, especially when the Argonne $v_{14}$ potential including the D-wave admixture is used.