Experimental Approach to Search for Free Neutron-Antineutron Oscillations Based on Coherent Neutron and Antineutron Mirror Reflection

An observation of neutron-antineutron oscillations (n-n[over ¯]), which violate both B and B-L conservation, would constitute a scientific discovery of fundamental importance to physics and cosmology. A stringent upper bound on its transition rate would make an important contribution to our understanding of the baryon asymmetry of the Universe by eliminating the postsphaleron baryogenesis scenario in the light quark sector. We show that one can design an experiment using slow neutrons that in principle can reach the required sensitivity of τ_{n-n[over ¯]}∼10^{10}  s in the oscillation time, an improvement of ∼10^{4} in the oscillation probability relative to the existing limit for free neutrons. The improved statistical accuracy needed to reach this sensitivity can be achieved by allowing both the neutron and antineutron components of the developing superposition state to coherently reflect from mirrors. We present a quantitative analysis of this scenario and show that, for sufficiently small transverse momenta of n/n[over ¯] and for certain choices of nuclei for the n/n[over ¯] guide material, the relative phase shift of the n and n[over ¯] components upon reflection and the n[over ¯] annihilation rate can be small enough to maintain sufficient coherence to benefit from the greater phase space acceptance the mirror provides.

A new operating mode in experiments searching for free neutron-antineutron oscillations based on coherent neutron and antineutron mirror reflections

An observation of neutron-antineutron oscillations (n - n¯), which violate both B and B - L by 2 units, would constitute a fundamental discovery and contribute to our understanding of the baryon asymmetry of the universe. A sufficiently stringent upper constraint on this process would also make a major contribution by ruling out the possibility of post-sphaleron baryogenesis (PSB) involving first-generation quarks, which would mean that sphaleron transitions at the electroweak scale are essential for baryogenesis within the Sakharov paradigm. We show that one can design an experiment with free n using existing or projected neutron sources that can reach the sensitivity needed to rule out PSB if one allows the n and n¯, with sufficiently small tangential velocity, to coherently reflect from n/n¯ mirrors composed of certain nuclei. We show that the sensitivity of a future experiment can be greatly improved, and a more compact and less expensive apparatus can be realized. A sensitivity gain of ~ 104 in the oscillation probability relative to the existing free-n limit can be reached if one is willing to adopt a long flight path with a horizontal guide viewing a cold neutron source, or a significantly shorter flight path with a vertical guide viewing a very cold neutron source.