Constraints for Rare Electron-Capture Decays Mimicking Detection of Dark-Matter Particles in Nuclear Transitions.

We give for the first time theoretical estimates of unknown rare electron-capture (EC) decay branchings of ^{44}Ti, ^{57}Co, and ^{139}Ce, relevant for searches of (exotic) dark-matter particles. The nuclear-structure calculations have been done exploiting the nuclear shell model with well-established Hamiltonians and an advanced theory of β decay. In the absence of experimental measurements of these rare branches, these estimates are of utmost importance for terrestrial searches of dark-matter particles, such as axionic dark matter in the form of axionlike particles, anapole dark matter, and dark photons in nuclear transitions. Predictions are made for EC-decay rates of second-forbidden unique and second-forbidden nonunique EC transitions that can potentially mimic dark-matter-particle detection in dedicated underground experiments designed to observe the absence of the corresponding nuclear electromagnetic transitions.