Spin-dipole nuclear matrix element for the double beta decay of 76Ge by the (3He, t) charge-exchange reaction

Nuclear matrix elements (NMEs) for double beta decays (DBDs) are crucial for studying the neutrino mass and other neutrino properties beyond the standard electro-weak model by measuring neutrino-less DBDs. The spin-dipole (SD) J π = 2− NME is one of the major components associated with the DBD NME. The SD NME for 76Ge was derived for the first time by using the 74,76 Ge (3He, t) at RCNP Osaka. The obtained SD NME for the 76Ge → 76As ground-state transition is $| {M}_{mathrm{EXP}}^{-}(mathrm{SD})| =1.5 imes {10}^{-3}$ in natural units. This is smaller by a coefficient around k ≈ 0.2 with respect to the quasi-particle model NME $| {M}_{mathrm{QP}}^{-}(mathrm{SD})| $. The impact of the reduced (quenched) SD NME on DBD neutrino studies is discussed.