Proton-neutron correlations, particle number conservation and double beta decay

The detection of neutrinoless double beta (Oνββ) decay would indicate exotic and exciting physics beyond the standard model. To extract limits on the lepton number violating parameters from present experimental bounds on the Oνββ half-life time one must describe satisfactorily the details of the nuclear structure. The short-range particle-particle correlations should play an important role here and proton-neutron (p-n) correlations in particular, since the decay process involves transition of two neutrons into protons. Their importance is supported by the recent calculations that show quite big p-n pairing gaps for many double beta decaying parent-daughter pairs of nuclei. We have extended the usual Bardeen-Cooper-Schrieffer (BCS) quasiparticle concept to take p-n pairing interaction into account. (Author)