Neutron-antineutron oscillations: theory

We review the problematics connected with the occurrence of n (n) over bar oscillations both in free space and inside nuclei, a process entailing baryon number violation and leading to a Delta B = 2 nuclear instability mode. A few experimental lower bounds have been till now established, both in attempts to measure directly the n (n) over bar oscillation time in a beam of free neutrons, and in setting indirect limits for specific channels of nuclear instability. The analysis of indirect measurements relies on theoretical assumptions: providing the same mechanism is active as for the free neutron conversion, the influence of the nuclear medium turns out to be crucial in determining the global nuclear lifetime. The newly formed antineutron state feels a considerably different mean field, with an imaginary absorptive part which accounts for the annihilation, As a result, the n (n) over bar conversion is substantially suppressed, in the nuclear medium, with respect to the free process. Various theoretical approaches are revisited here and the corresponding estimates for the suppression factor are confronted, finding rather stable results. Thus both direct and indirect measurements can be employed in the search for n (n) over bar oscillations.