Neutrinoless double beta decay and chiral SU(3)

TeV-scale lepton number violation can affect neutrinoless double beta decay through dimension-9 $\Delta L= \Delta I = 2$ operators involving two electrons and four quarks. Since the dominant effects within a nucleus are expected to arise from pion exchange, the $ \pi^- \to \pi^+ e e$ matrix elements of the dimension-9 operators are a key hadronic input. In this letter we provide estimates for the $\pi^- \to \pi^+ $ matrix elements of all Lorentz scalar $\Delta I = 2$ four-quark operators relevant to the study of TeV-scale lepton number violation. The analysis is based on chiral $SU(3)$ symmetry, which relates the $\pi^- \to \pi^+$ matrix elements of the $\Delta I = 2$ operators to the $K^0 \to \bar{K}^0$ and $K \to \pi \pi$ matrix elements of their $\Delta S = 2$ and $\Delta S = 1$ chiral partners, for which lattice QCD input is available. The inclusion of next-to-leading order chiral loop corrections to all symmetry relations used in the analysis makes our results robust at the $30\%$ level or better, depending on the operator.