Beyond the Mssm *

Specific quasi-realistic superstring constructions often predict the existence of new TeV-scale physics which may be different in character from bottom-up motivated constructions. I describe examples of such beyond the MSSM physics, with special emphasis on heavy Z ′ gauge bosons and associated exotic particles. Implications of an extra U (1) ′ include a viable scienario for electroweak baryogenesis, a highly non-standard light Higgs sector, and Z ′-mediated contributions to rare B decays. Even if supersymmetry holds, the MSSM may not be the full story. The MSSM stabilizes but does not explain the hierarchy between the electroweak and Planck scales, and it gives a plausible first step towards the incorporation of quantum gravity. However, the MSSM does not address the other problems of standard model (SM). Moreover, the µ problem (i.e., that the supersymmetric masses of the Higgs multiplets and their partners should be comparable to typical supersymmetry breaking scales) is introduced, and some versions have difficulty in avoiding new flavor changing effects and too large contributions to electric dipole moments. One possibility is that the underlying new physics is at the Planck or a GUT scale, but even then there may be remnants surviving to the TeV scale. For example, specific string constructions often have extended gauge groups, especially U (1) ′ factors, and associated exotic chiral supermulti-plets. Such remnants might not solve or be directly motivated by bottom-up solutions to the problems of the SM or MSSM, but should instead be