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

¹ Institut Max von Laue - Paul Langevin, 71 avenue des Martyrs, Grenoble, France - 38000
² Department of Physics and Astronomy, University of South California, South California, USA - 29208
³ Laboratoire de Phisique Subatomique et de Cosmologie, UGA-CNRS/IN2P3, Grenoble, France - 38026
⁴ Department of Physics, Indiana University, 727 E. Third St., Bloomingston, Indiana, USA - 47405
⁵ Lebedev Physical Institute, 53 Leninsky prospect, Moscow, Russia - 119991

^* e-mail: nesvizhevsky@ill.eu

Published online: 31 October 2018

Abstract

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.