Pion-exchange currents in neutrinoless double-beta decay and limits on supersymmetry

The pion-exchange mode of the neutrinoless double-beta decay (0vββ) induced by the R-parity-violating quark-lepton operators of the supersymmetric (SUSY) extensions of the standard model of the electroweak interactions is examined. The corresponding nuclear matrix elements are evaluated within the renormalized quasiparticle random-phase approximation, which takes into account both proton-neutron pairing and the Pauli effect of fermion pairs and which does not collapse for a physical value of the nuclear force. It is argued that the pion-exchange mode of 0vββ decay dominates over the conventional two-nucleon mode in the case of the SUSY mechanism. As a result, the sensitivity of 0vββ decay to the SUSY contribution proves to be significantly higher than what was previously expected from the calculations for the two-nucleon mode. An upper limit on the coupling constant λ'₁₁₁ violating R parity is derived from nonobservation of neutrinoless double beta decay. This limit is much more stringent than what is expected from the near-future accelerator experiments.