Getting just the Standard Model at Intersecting Branes

TL;DR

This paper constructs specific string theory models using intersecting D-branes that precisely reproduce the Standard Model with three generations, no extra particles, and natural neutrino masses, ensuring proton stability.

Contribution

First explicit string D-brane models yielding the exact Standard Model spectrum with three generations and no additional fermions or U(1)s, with natural neutrino masses and proton stability.

Findings

Standard Model with three generations derived from string theory

Proton stability guaranteed by gauged baryon and lepton symmetries

Neutrino masses are naturally small Dirac types

Abstract

We present what we believe are the first specific string (D-brane) constructions whose low-energy limit yields just a three generation $SU(3)\times SU(2)\times U(1)$ standard model with no extra fermions nor U(1)'s (without any further effective field theory assumption). In these constructions the number of generations is given by the number of colours. The Baryon, Lepton and Peccei-Quinn symmetries are necessarily gauged and their anomalies cancelled by a generalized Green-Schwarz mechanism. The corresponding gauge bosons become massive but their presence guarantees automatically proton stability. There are necessarily three right-handed neutrinos and neutrino masses can only be of Dirac type. They are naturally small as a consequence of a PQ-like symmetry. There is a Higgs sector which is somewhat similar to that of the MSSM and the scalar potential parameters have a geometric interpretation in terms of brane distances and intersection angles. Some other physical implications of these constructions are discussed.