Chiral Two-body Currents and Neutrinoless Double Beta Decay

The nuclear matrix elements (NMEs) of neutrinoless double-beta (0νββ) decay are studied using weak currents derived in the framework of chiral effective field theory. Apart from the standard one-body (1b) currents, it is shown that two-body (2b) currents contribute to weak processes. The normal-ordered 1b part of 2b currents modifies the Gamow-Teller (GT) στ^- part of the 1b current, contributing to the well-known quenching of GT single-β decays. The momentum-transfer dependence of the quenching due to 2b currents is also predicted. Therefore, including 2b currents allows to address, microscopically, the problem of the axial weak coupling (g_A) value, which is the biggest uncertainty in the 0νββ decay NME calculations for all available methods.