Pionic Matrix Elements in Neutrinoless Double-Beta Decay
  Physics beyond the standard model of electroweak interactions that violates lepton number will, in general, induce operators involving the quark and lepton fields that violate lepton number. I point out that some matrix elements of particular operators are known. In the limit of exact SU(3) flavor symmetry the operator overline{u} gamma_mu (1-gamma_5)d overline{u} gamma_mu (1-gamma_5)d is part of the ${\bf 27}$ representation responsible for Delta I=3/2 strangeness changing decays in the standard model. At leading order in the chiral expansion there is only one coupling constant that describes the weak interactions of the pseudo-Goldstone bosons induced by the 27 representation. The observed branching fraction for K+ to pi+ pi0 determines this coupling constant. Contributions of the form m_s log m_s are used to estimate SU(3) breaking corrections to the relation between K+ to pi+ pi0 and the pi- to pi+ e- e- interaction. 
Martin J. Savage