2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/46108In the context of the minimal supersymmetric standard model (MSSM), we discuss the introduction of exotic matter below the string scale $M_X$ in order to achieve gauge unification at $M_X$ (a constraint of a large class of string models). The possible types of exotic matter that can realise this are investigated and its effect on the top quark mass $m_t$ is presented. The implementation of a theory of fermion masses which utilises the exotic matter is briefly discussed.7 pages, 1 Postscript figure, uses epsf.styHigh Energy Physics - Phenomenologytext