Dark sectors in neutron-shining-through-a-wall and nuclear-absorption signals
Matheus Hostert, David McKeen, Maxim Pospelov, Nirmal Raj
Abstract
We propose new searches for ${n}^{\ensuremath{'}}$, a dark baryon that can mix with the Standard Model neutron. We show that IsoDAR, a proposal to place an intense cyclotron near a large-volume neutrino detector deep underground, can look for $n\ensuremath{\rightarrow}{n}^{\ensuremath{'}}\ensuremath{\rightarrow}n$ transitions with much lower backgrounds than surface experiments. This opportune neutron-shining-through-a-wall search would be possible without any modifications to the primary goals of the experiment and would provide the strongest laboratory constraints on the $n\ensuremath{-}{n}^{\ensuremath{'}}$ mixing for a wide range of mass splitting. We also consider dark neutrons as dark matter and show that their nuclear absorption at deep-underground detectors such as SNO and Borexino places some of the strongest limits in parameter space. Finally, we describe other ${n}^{\ensuremath{'}}$ signatures, such as neutrons shining through walls at spallation sources, reactors, and the disappearance of ultracold neutrons.