First search for 2ɛ and ɛβ+ 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 lim ⁡T_1/2 ∼10¹⁴-10¹⁸ yr at 90% C.L. Particularly, a lower half-life limit on a possible resonant neutrinoless double-electron capture in ¹⁶⁸Yb to the ^{(2) -} 1403.7 keV excited state of ¹⁶⁸Er is set as T_1/2 ≥ 1.9 ×10¹⁸ yr at 90% C.L. Half-life limits T_1/2² ν (0 ν) ≥ 4.5 (4.3) ×10¹⁶ yr were set on the 2 ν (0 ν) 2β^- decay of ¹⁷⁶Yb to the 2⁺ 84.3 keV first excited level of ¹⁷⁶Hf.