Neutrinoless double beta decay with scalar bilinears
  One possible probe to physics beyond the standard model is to look for scalar bilinears, which couple to two fermions of the standard model. We point out that the scalar bilinears allow new diagrams contributing to the neutrinoless double beta decay. The upper bound on the neutrinoless double beta decay lifetime would then give new constraints on the ratio of the masses of these scalars to their couplings to the fermions. 
H.V. Klapdor-Kleingrothaus and U. Sarkar