Limits on the spatial width of the electron-neutrino wave packet using 7-beryllium electron capture decay

The coherence length of a neutrino source is proportional to the spatial width of neutrino wave packets at production. A sufficiently small width of the wave packet could affect current and future neutrino oscillation data. Theoretical predictions for the spatial width of the neutrino wave packet vary by several orders of magnitude and are only loosely constrained by current oscillation data. The BeEST experiment uses superconducting sensors to measure the kinetic energy of the recoiling 7Li atom in 7Be electron capture decay final state which is entangled with the neutrino at its creation. The high energy resolution of the BeEST recoil spectrum combined with knowledge of the spatial localization within a single pixel are converted to lower-limit and upper-limits on the spatial width of the entangled electron-neutrino wave packets. In this talk, I will present this concept and our preliminary results to stimulate discussion within the community on this technique.