Neutrinoless double-electron capture

Direct determination of the neutrino mass is at the present time one of the most important aims of experimental and theoretical research in nuclear and particle physics. A possible way of detection is through neutrinoless double-electron capture, 0vECEC. This process can only occur when the energy of the initial state matches precisely that of the final state. We present here a calculation of prefactors (PFs) and nuclear matrix elements (NMEs) within the framework of the microscopic interacting boson model (IBM-2) for Xe-124, Gd-152, Dy-156, Er-164, and W-180. From the PF and NME we calculate the expected half-lives and obtain results that are of the same order as those of 0v beta(+)beta(+) decay, but considerably longer than those of 0v beta(-)beta(-) decay.