Neutron Majorana mass from exotic instantons

TL;DR

This paper proposes a string theory-based mechanism where exotic instantons induce a Majorana mass for the neutron, potentially leading to observable neutron-antineutron oscillations and providing a test for quantum gravity effects.

Contribution

It introduces a novel string theory model where exotic instantons generate neutron Majorana masses, linking quantum gravity effects to observable phenomena.

Findings

Neutron-antineutron oscillation time scale > 10^8 seconds.

Majorana mass for neutron can be as low as 10^{-24} eV.

Proton decay and FCNCs remain suppressed and within experimental limits.

Abstract

We show how a Majorana mass for the Neutron could result from non-perturbative quantum gravity effects peculiar to string theory. In particular, "exotic instantons" in un-oriented string compactifications with D-branes extending the (supersymmetric) standard model could indirectly produce an effective operator delta{m} n^t n+h.c. In a specific model with an extra vector-like pair of `quarks', acquiring a large mass proportional to the string mass scale (exponentially suppressed by a function of the string moduli fields), delta{m} can turn out to be as low as 10^{-24}-10^{-25} eV. The induced neutron-antineutron oscillations could take place with a time scale tau_{n\bar{n}} > 10^8 s, that could be tested by the next generation of experiments. On the other hand, proton decay and FCNC's are automatically strongly suppressed and are compatible with the current experimental limits. Depending on the number of brane intersections, the model may also lead to the generation of Majorana masses for R-handed neutrini. Our proposal could also suggest neutron-neutralino or neutron-axino oscillations, with implications in UCN, Dark Matter Direct Detection, UHECR and Neutron-Antineutron oscillations. This suggests to improve the limits on neutron-antineutron oscillations, as a possible test of string theory and quantum gravity.