First search for 2ε, εβ+ processes in 168Yb

The double-electron capture and the electron capture with positron emission in 168Yb have been investigated for the first time at the STELLA facility of the Gran Sasso underground laboratory (Italy) measuring 371 g of highly purified ytterbium oxide placed on the end-cap of a 465 cm3 ultra-low-background high purity Germanium detector (HPGe). No gamma associated to double beta processes in 168Yb have been observed after 2074 h of data taking. This has allowed setting the half-life limits on the level of T_1/2 circa 10^14 -10^18 yr at 90% C.L. Particularly, a lower half-life limit on a possible resonant neutrinoless double-electron capture in 168Yb to the 1403.7 keV excited state of 168Er is set as T_1/2 higher/equal 1.9 x 10^18 yr at 90% C.L. Half-life limits T_1/2^2nu(0nu) higher/equal 4.5(4.3) x 10^16 yr were set on the 2 neutrino(0 neutrino) 2beta^- decay of 176Yb to the 84.3 keV first excited level of 176Hf.