New Vector-Scalar Contributions to Neutrinoless Double Beta Decay and Constraints onR-Parity Violation

We show that in the minimal supersymmetric standard model (MSSM), with R-parity breaking as well as in the left-right symmetric model, there are new observable contributions to neutrinoless double beta decay $\left(\beta {\beta }_{0\nu }\right)$ arising from hitherto overlooked diagrams involving the exchange of one W boson and one scalar boson. In particular, in the case of MSSM, the present experimental bounds on $\beta {\beta }_{0\nu }$ lifetime improves the limits on certain R-parity violating couplings by about 2 orders of magnitude. It is shown that similar diagrams also lead to enhanced rates for ${\mu }^{-}\to e^{+}$ conversion in nuclei, which are in the range accessible to ongoing experiments.