New Leptoquark Mechanism of Neutrinoless Double Beta Decay
  A new mechanism for neutrinoless double beta ($\znbb$) decay based on leptoquark exchange is discussed. Due to the specific helicity structure of the effective four-fermion interaction this contribution is strongly enhanced compared to the well-known mass mechanism of $\znbb$ decay. As a result the corresponding leptoquark parameters are severely constrained from non-observation of $\znbb$-decay. These constraints are more stringent than those derived from other experiments. 
M. Hirsch, H.V. Klapdor-Kleingrothaus and S.G. Kovalenko