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

Nesvizhevsky, V. V. (Institut Max von Laue—Paul Langevin, 71 avenue des Martyrs, Grenoble, France 38042) ; Gudkov, V. (Department of Physics and Astronomy, University of South Carolina, Columbia, South Carolina 29208, USA) ; Protasov, K. V. (Laboratoire de Physique Subatomique et de Cosmologie, UGA-CNRS/IN2P3, Grenoble, France 38026) ; Snow, W. M. (Department of Physics, Indiana University, 727 E. Third St., Bloomington, Indiana 47405, USA) ; Voronin, A. Yu. (P.N. Lebedev Physical Institute, 53 Leninsky prospect, Moscow, Russia 119991)

07 June 2019

Abstract: An observation of neutron-antineutron oscillations ($n-\overline{n}$), 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 ${\tau }_{n-\overline{n}}\sim {10}^{10}\text{}\text{}s$ in the oscillation time, an improvement of $\sim {10}^{4}$ 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 $n/\overline{n}$ and for certain choices of nuclei for the $n/\overline{n}$ guide material, the relative phase shift of the $n$ and $\overline{n}$ components upon reflection and the $\overline{n}$ annihilation rate can be small enough to maintain sufficient coherence to benefit from the greater phase space acceptance the mirror provides.

Published in: Physical Review Letters 122 (2019)