# First search for 2$\varepsilon$ and $\varepsilon\beta^+$ processes in $^{168}$Yb

## TL;DR

This study conducted the first search for double-electron capture and electron capture with positron emission in $^{168}$Yb, setting new half-life limits and exploring rare decay processes using ultra-low-background germanium detectors.

## Contribution

It is the first experimental investigation of these decay modes in $^{168}$Yb, establishing new half-life limits and demonstrating the experimental feasibility at underground laboratories.

## Key findings

- No gamma signals observed for double beta processes in $^{168}$Yb.
- Set lower half-life limits on various decay modes, up to $10^{18}$ years.
- Established the first limits on resonant neutrinoless double-electron capture in $^{168}$Yb.

## Abstract

The double-electron capture and the electron capture with positron emission in $^{168}$Yb 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 cm$^3$ ultra-low-background high purity Germanium detector (HPGe). No gamma associated to double beta processes in $^{168}$Yb have been observed after 2074 h of data taking. This has allowed setting the half-life limits on the level of $\lim T_{1/2}\sim$ $10^{14}-10^{18}$ yr at 90\% C.L. Particularly, a lower half-life limit on a possible resonant neutrinoless double-electron capture in $^{168}$Yb to the $(2)^-$ 1403.7 keV excited state of $^{168}$Er is set as $T_{1/2}\geq1.9\times 10^{18}$ yr at 90\% C.L. Half-life limits $T_{1/2}^{2\nu(0\nu)}\geq 4.5(4.3)\times10^{16}$ yr were set on the $2\nu(0\nu)2\beta^-$ decay of $^{176}$Yb to the $2^+$ 84.3 keV first excited level of $^{176}$Hf.