Natural Four-Generation Mass Textures in MSSM Brane Worlds

TL;DR

This paper proposes a string-inspired model with four fermion generations in the MSSM framework, naturally explaining mass hierarchies, gauge coupling unification, and hidden sector confinement.

Contribution

It introduces a intersecting D6 brane model that naturally produces four generations with realistic mass textures and unification properties.

Findings

Third and fourth generations split in mass without mixing at leading order

First two generations acquire lighter masses via higher-order couplings

Gauge couplings unify at the string scale and hidden sectors confine

Abstract

A fourth generation of Standard Model (SM) fermions is usually considered unlikely due to constraints from direct searches, electroweak precision measurements, and perturbative unitarity. We show that fermion mass textures consistent with all constraints may be obtained naturally in a model with four generations constructed from intersecting D6 branes on a T^6/(Z_2 x Z_2) orientifold. The Yukawa matrices of the model are rank 2, so that only the third- and fourth-generation fermions obtain masses at the trilinear level. The first two generations obtain masses via higher-order couplings and are therefore naturally lighter. In addition, we find that the third and fourth generation automatically split in mass, but do not mix at leading order. Furthermore, the SM gauge couplings automatically unify at the string scale, and all the hidden-sector gauge groups become confining in the range 10^{13}--10^{16} GeV, so that the model becomes effectively a four-generation MSSM at low energies.