Charge-symmetry-breaking effects on displacement energies and charge radius differences in mirror nuclei

Abstract

We study the effect of charge symmetry breaking (CSB) energy density function (EDF) on the mirror displacement energies (MDEs) and charge radius differences of mirror nuclei within the self-consistent Hartree-Fock-Bogolyubov (HFB) model taking Skyrme EDFs, SLy4 and SkM* as the central part of nuclear potential. We introduce the volume and the derivative terms in CSB EDF and calibrate the strength adopting four reference mirror pairs $^{34}$Ar--$^{34}$S, $^{36}$Ca--$^{36}$S, $^{38}$Ca--$^{38}$Ar, and $^{54}$Ni--$^{54}$Fe, for which experimental data of both MDEs and mirror charge-radius differences are available. We introduce a sensitivity matrix which connects two CSB terms to residuals of MDEs and mirror charge-radius differences after subtracting the effect of Coulomb interaction. By using the sensitivity matrix, we found out that the derivative term is important to reproduce both observables in a good accuracy together with the volume term, especially for the residual of mirror charge radii. The optimized CSB EDF are further applied to predict the charge radius differences of mirror pairs, $^{40}$Ti--$^{40}$Ar, $^{42}$Ti--$^{42}$Ca, $^{46}$Cr--$^{46}$Ti, and $^{50}$Fe--$^{50}$Cr. We pointed out also that the CSB effects change neutron skins of mirror proton-rich nuclei at the $10^{-2}$ fm level so that the CSB contributions must be included before charge-radius differences between mirror nuclei are used to extract neutron-skin or symmetry-energy parameters.