Quantum Monte Carlo calculations of weak transitions in $A$=6--10 nuclei

Abstract

Ab initio calculations of the Gamow-Teller (GT) matrix elements in the ${\hat{I}}^2$ decays of ${}^6$He and ${}^{10}$C and electron captures in ${}^7$Be are carried out using both variational and Green's function Monte Carlo wave functions obtained from the Argonne $v_{18}$ two-nucleon and Illinois-7 three-nucleon interactions, and axial many-body currents derived from either meson-exchange phenomenology or chiral effective field theory. The agreement with experimental data is excellent for the electron captures in ${}^7$Be, while theory overestimates the ${}^6$He and ${}^{10}$C data by $\sim 2\%$ and $\sim 10\%$, respectively. We show that for these systems correlations in the nuclear wave functions are crucial to explain the data, while many-body currents increase by $\sim 2$--$3\%$ the one-body GT contributions. These findings suggest that the longstanding $g_A$-problem, i.e., the systematic overprediction ($\sim 20\%$ in $A\le 18$ nuclei) of GT matrix elements in shell-model calculations, may be resolved, at least partially, by correlation effects.

6 pages, 2 figures