New Limits on $n\to n^{\prime }$ Transformation from HFIR Cold Neutron Beam

Abstract

Hypothetical neutron $n$ to sterile neutron $n^{\prime }$ transformations would violate baryon number $\mathcal{B}$ and point to the nature of Dark Matter. We performed a new search for $n\to n^{\prime }$ 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 $n\to n^{\prime }$ with two parameters: a small mass difference $\Delta m$ between the interaction states $n$ and $n^{\prime }$ 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 $n\to n^{\prime }$ 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 $n\to n^{\prime }$ transformation in vacuum in the range of $\Delta m$ between $0.1$ neV and $1000$ neV.