Two-body currents at finite momentum transfer and applications to M1 transitions

Abstract

We explore the impact of two-body currents (2BCs) at finite momentum transfer with a focus on magnetic dipole properties in ${}^{48}$Ca and ${}^{48}$Ti. To this end, we derive a multipole decomposition of 2BCs to fully include the momentum-transfer dependence in $abinitio$ calculations. As application, we investigate the effects of 2BCs on the strong M1 transition at 10.23MeV in ${}^{48}$Ca using the valence-space in-medium similarity renormalization group (VS-IMSRG) with a set of non-implausible interactions as well as the 1.8/2.0 (EM) interaction. Experiments, such as $(e,e^{\prime })$ and $(\gamma ,n)$, disagree on the magnetic dipole strength $B$(M1) for this transition. Our VS-IMSRG results favor larger $B$(M1) values similar to recent coupled-cluster calculations. However, for this transition there are larger cancellations between the leading pion-in-flight and seagull 2BCs, so that future calculations including higher-order 2BCs are important. For validation of our results, we investigate additional observables in ${}^{48}$Ca as well as M1 transitions in ${}^{48}$Ti. For these, our results agree with experiment. Finally, our results show that for medium-mass nuclei 2BC contributions to M1 and Gamow-Teller transitions are, as expected, very different. Therefore, using similar quenching factors for both in phenomenological calculations is not supported from first principles.

Comments: 12 pages, 5 figures, corrected one point in Fig. 3, published version