Neutron-antineutron oscillations: Theoretical status and experimental prospects

Abstract

The observation of neutrons turning into antineutrons would constitute a discovery of fundamental importance for particle physics and cosmology. Observing the n−n transition would show that baryon number (B) is violated by two units and that matter containing neutrons is 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 asymmetry of the universe. A demonstration of the violation of B − L by 2 units would have a profound impact on our understanding of phenomena beyond the Standard Model of particle physics.Slow neutrons have kinetic energies of a few meV. By exploiting new slow neutron sources and optics technology developed for materials research, an optimized search for oscillations using free neutrons from a slow neutron moderator could improve existing limits on the free oscillation probability by at least three orders of magnitude. Such an experiment would deliver a slow neutron beam through a magnetically-shielded vacuum chamber to a thin annihilation target surrounded by a low-background antineutron annihilation detector. Antineutron annihilation in a target downstream of a free neutron beam 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 neutron oscillation searches in nuclei using large underground detectors built mainly to search for proton decay and detect neutrinos. This paper summarizes the relevant theoretical developments, outlines some ideas to improve experimental searches for free neutron oscillations, and suggests avenues both for theoretical investigation and for future improvement in the experimental sensitivity.

Un-oriented quiver theories for Majorana neutrons

“…Such a mass term could induce neutron-antineutron transitions, violating Baryon number, contrary to the predictions of the Standard Model [36][37][38]. The next generation of experiments is expected to test P eV physics [39,40] by improving the limits on the oscillation time to τ n−n 10 10 s, two orders of magnitude higher than the current limits [41].…”

Search for neutron-antineutron oscillations at the Sudbury Neutrino Observatory

“…Two experimental scenarios exist in which the neutronantineutron oscillation process is potentially observable: (1) the oscillations of neutrons to antineutrons in bound nuclei and (2) the oscillations of a beam of cold neutrons incident on an annihilation target situated at an optimized distance [3]. This paper will concentrate on the former scenario, in which the antineutrons interact with the surrounding nucleons and produce a GeV-scale signature.…”

“…In intranuclear experiments, the rate reduction due to the suppression factor needs to be offset by the exposure to a large quantity of bound neutrons, requiring kiloton-scale experiments. The suppression factor varies for different nuclei and can be derived from theoretical models [3]. In fact, measurements of intranuclear oscillation and free neutronantineutron oscillations are complementary.…”

‘Exotic vector-like pair’ of color-triplet scalars

“…This limit is particularly loose with respect to other rare processes violating Baryon or Lepton numbers: τ nn > 3 yr for neutron-antineutron can be compared with τ p−decay ∼ 10 34÷35 yr for the Proton decays, τ 0νββ > 10 25 yr for neutrinoless double beta decays [6]. For these reasons, neutron-antineutron is becoming more and more an interesting challenge for model building [20][21][22][23][24][25][26], 1 also considering possibility in the next future to enhance best limit of a factor 100: τ nn > 10 10 s, testing 1000 TeV scale [7]. (For a recent review about phenomenology of Baryon and Lepton violations, see also [27]).…”