Massive Neutrinos and (Heterotic) String Theory

TL;DR

This paper tests whether the canonical, minimal see-saw mechanism for small neutrino masses can be realized in a concrete, top-down string construction—the BSL_A class of Z3 heterotic orbifolds. By systematically analyzing D-/F-flat directions and string selection rules, the authors search for Majorana mass operators and the necessary Dirac couplings, uncovering two promising patterns but finding that neither yields a fully viable minimal see-saw; extended see-saw scenarios and Dirac-dominated possibilities remain as alternatives. They also show that, within this class, there are only 20 inequivalent models, illustrating how ultraviolet-complete constraints can drastically constrain low-energy neutrino physics and guiding future model-building efforts across string vacua.

Abstract

String theories in principle address the origin and values of the quark and lepton masses. Perhaps the small values of neutrino masses could be explained generically in string theory even if it is more difficult to calculate individual values, or perhaps some string constructions could be favored by generating small neutrino masses. We examine this issue in the context of the well-known three-family standard-like Z_3 heterotic orbifolds, where the theory is well enough known to construct the corresponding operators allowed by string selection rules, and analyze the D- and F-flatness conditions. Surprisingly, we find that a simple see-saw mechanism does not arise. It is not clear whether this is a property of this construction, or of orbifolds more generally, or of string theory itself. Extended see-saw mechanisms may be allowed; more analysis will be needed to settle that issue. We briefly speculate on their form if allowed and on the possibility of alternatives, such as small Dirac masses and triplet see-saws. The smallness of neutrino masses may be a powerful probe of string constructions in general. We also find further evidence that there are only 20 inequivalent models in this class, which affects the counting of string vacua.