Neutrinoless ββ decay and a new limit on the right-handed current

We have calculated the 0νββ and 2νββ decay rates of the transition 76 Ge(0 + 1 ) → 76 Se(0 + 1 ). We took into account a relativistic correction to the nuclear current including weak magnetism. The nuclear two-body transition operator for the 0νββ decay originating from this correction acquires a finite range due to the finite extension of the nucleon and the short-range NN correlations reduce its matrix element only moderately. The contribution from this “second-forbidden” transition plays a dramatic role in the 0νββ decay caused by a specific admixture of a right-handed leptonic current because of the high momentum of the virtual neutrino exchanged between nucleons and systematic cancellations in “allowed” and other “second-forbidden” 0νββ transitions. A new limit on the right-handed current coupling strength |〈 η 〉| −8 was obtained, which is more stringent by an order of magnitude than that obtained recently by the Osaka group.