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.
Highlights
The induced neutron-antineutron oscillations could take place with a time scale τnn > 108 s that could be tested by the generation of experiments
We have shown how exotic instantons can indirectly generate a Majorana mass for the neutron
An O(1) instanton (E2-brane) singly intersecting the relevant D6-branes generates a dynamical super potential mass term for D, C. Integrating these out, while taking into account their interactions with the standard MSSM super-fields, produces an effective Baryon number violating term that in turn leads to the desired highly-suppressed Majorana mass for the neutron
Summary
∆(B −L) = 0, as expected for a non-anomalous global symmetry, allowing for proton decay via p → π0e+ or p → K+ν etc. We could well envisage the possibility of generating dimension six-fermion operators of the type. Inducing a Majorana mass for the neutron
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