New Limits on $n \rightarrow n'$ Transformation from HFIR Cold Neutron Beam

Abstract

Hypothetical neutron $n$ to sterile neutron $n'$ transformations would violate baryon number $\mathcal{B}$ and point to the nature of Dark Matter. We performed a new search for $n \rightarrow n'$ using an intense cold neutron beam from the High Flux Isotope Reactor at Oak Ridge National Laboratory. We used a theoretical model that describes the transformation $n \rightarrow n'$ with two parameters: a small mass difference $\Delta{m}$ between the interaction states $n$ and $n'$ and a mixing vacuum angle $\theta_0$. A thin absorbing cadmium wafer was used in the center of the superconducting 6.6 T magnet which provided a large gradient for the non-adiabatic $n \rightarrow n'$ transition. No signal was observed above background in the $^{3}\text{He}$ neutron detector 20 meters downstream of the magnet. This result gives an order of magnitude improvement in the lower limit for the probability $2\theta_0^2$ of the $n \rightarrow n'$ transformation in vacuum in the range of $\Delta{m}$ between $0.1$ neV and $1000$ neV.