Neutrino Flavor Transformations from New Short-Range Forces

We examine the commonly explored beyond-standard-model physics scenario of secret neutrino forces, and point out a model prediction that appears to have been overlooked: the generation of unique flavor-changing effects in experiments featuring decay-at-rest (DAR) neutrino sources. These flavor changes occur because the decay that drives neutrino and antineutrino production, $\mu^{+}\rightarrow e^+ +\bar{\nu}_{\mu}+\nu_{e}$, is unique in producing two neutrinos in the final state. Any non-flavor-universal force between the emerging neutrinos would thus induce a new oscillation phase as they escape from each-other's potential wells, an effect which is largely absent in experiments that primarily rely on meson decay-in-flight and nuclear decay. We calculate the magnitude of the associated observable and compare it to the anomalous neutrino flavor transformation seen by the LSND experiment, finding a wide but constrained allowed parameter space. We also evaluate existing limits from other experiments, and the testability of this new effect at the future DAR programs JSNS$^2$ and OscSNS.