Neutron-antineutron oscillations: Theoretical status and experimental prospects

Abstract The observation of neutronsturning into antineutronswould constitute a discovery of fundamental importance for particle physicsand cosmology. Observing the n – n transition would show that baryon number( B ) is violated by two units and that matter containing neutronsis unstable. It would provide a clue to how the matter in our universe might have evolved from the B = 0 early universe. If seen at rates observable in foreseeable next-generation experiments, it might well help us understand the observed baryon asymmetryof the universe. A demonstration of the violation of B– Lby 2 units would have a profound impact on our understanding of phenomena beyond the Standard Model of particle physics. Slow neutronshave kinetic energies of a few meV. By exploiting new slow neutronsources and optics technology developed for materials research, an optimized search for oscillations using free neutronsfrom a slow neutron moderatorcould improve existing limits on the free oscillation probability by at least three orders of magnitude. Such an experiment would deliver a slow neutronbeam through a magnetically-shielded vacuum chamber to a thin annihilation target surrounded by a low-background antineutronannihilation detector. Antineutronannihilation in a target downstream of a free neutronbeam is such a spectacular experimental signature that an essentially background-free search is possible. An authentic positive signal can be extinguished by a very small change in the ambient magnetic field in such an experiment. It is also possible to improve the sensitivity of neutronoscillation searches in nuclei using large underground detectors built mainly to search for proton decayand detect neutrinos. This paper summarizes the relevant theoretical developments, outlines some ideas to improve experimental searches for free neutronoscillations, and suggests avenues both for theoretical investigation and for future improvement in the experimental sensitivity.