Limits on spatial extent of entangled nuclear and neutrino wavepackets using 7-beryllium electron capture decays in the BeEST experiment*

Measuring the wavepackets of nuclear decay products can be used as a fundamental test of quantum mechanics. The inherent uncertainty in measurements of nuclear decay products is expected to depend heavily on the scale of localizing interactions within the environment. The BeEST experiment uses superconducting tunnel junction (STJ) sensors to measure the low-energy 7-lithium nuclear recoil produced by the electron capture decay of 7-beryllium embedded within the sensor itself, giving direct access to environmental localization scales. The precision of the BeEST energy measurement and the Heisenberg uncertainty principle have been used to place limits on the size of the wavepackets of 7-lithium nuclei and the entangled neutrinos. The BeEST limits are important for the interpretation of neutrino oscillation data within eV-scale sterile neutrino models. Improvements to this technique can potentially allow it to be used to distinguish between competing models of quantum localization in weak nuclear decays.