Neutron Majorana mass from exotic instantons

Abstract

We show how a Majorana mass for the neutron could result from nonperturbative 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 δm 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), δm 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 τ nn > 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.

Un-oriented quiver theories for Majorana neutrons

“…This peculiar feature of exotic instantons can lead to interesting B-(or L-) and B − L violating physics testable in laboratories, such as a Majorana mass for the neutron and related n −n oscillations [1]. These effects can also dynamically propagate from the string scale to much lower energies, as shown in [1]. 2 The possibility of an effective Majorana mass term for the neutron was firstly proposed by Majorana himself in [31].…”

“…The main purpose of the present paper is to clarify aspects of the mechanism proposed in [1] and to identify quiver theories leading to the interesting phenomenology introduced in [1]. The paper is organized as follows: in section 2 we briefly review the main features of the models with Majorana mass terms for the neutron; in section 3, we discuss the phenomenology related to neutron-antineutron oscillations, reviewing and extending our previous considerations and constraining the allowed region in parameter space, with particular attention to possible signatures at LHC; in section 4 we briefly review the construction of (un-oriented) quiver theories and identify SM-like (un-oriented) quivers for a Majorana neutron; in section 5 we discuss possible quantum corrections to the Kähler potential and D-terms as well as the role of susy breaking; in section 6 we present our conclusions and a preliminary discussion of Pati-Salam extensions presented in [42].…”

“…The relevant E2-brane (exotic instanton) is transversely invariant under Ω and intersects the physical D6-branes, as discussed in [1]. The non-perturbative mass scale is M 0 ∼ M S e −S E2 with M S the string scale, S E2 the E2 instanton action, depending on the closed string moduli parameterizing the complexified size of the 3-cycle wrapped by the world-volume of E2.…”

“…Recently we have proposed the possibility that a Majorana mass term for the neutron could be indirectly generated by non-perturbative quantum gravity effects present in string theory: the exotic instantons [1,2]. 1 In theories with open and un-oriented strings, instantons have a simple geometrical interpretation: they are nothing but Euclidean D-branes (a.k.a.…”

“…1 In theories with open and un-oriented strings, instantons have a simple geometrical interpretation: they are nothing but Euclidean D-branes (a.k.a. E-branes) wrapping internal cycles of the compactification.…”