Spurious Isospin Symmetry breaking in the In-Medium Similarity Renormalization Group

Superallowed Fermi beta decays provide a high-precision test of the Standard Model, but they require high-precision theoretical calculations, including corrections for isospin-symmetry-breaking effects. Ab initio methods, like the in-medium similarity renormalization group (IMSRG), can provide a systematic framework to consistently evaluate these corrections with a quantified theoretical error bar. This requires a comprehensive understanding of the various contributions to the theoretical error. In particular we have found that the truncations made in standard IMSRG calculations lead to spurious isospin breaking, even in the presence of an isospin-conserving interaction. I will describe our understanding of how this comes about, and lay out some strategies to mitigate this effect.