Nuclear matrix elements and the neutrinoless double beta decay

Systematics of the nuclear matrix elements related to the neutrinoless double beta (0νββ) decay are discussed. The calculation of the related nuclear matrix elements in the framework of the proton-neutron quasiparticle random-phase approximation (pnQRPA) is analyzed. In particular, the extraction of a proper value of the proton-neutron particle-particle interaction parameter in this theory is addressed. The relevance of different multipoles in the multipole decomposition of the 0νββ matrix element is also discussed.