BUILDING GUTS FROM STRINGS

We study in detail the structure of Grand Unified Theories derived as the low-energy limit of orbifold four-dimensional strings. To this aim , new techniques for building level-two symmetric orbifold theories ar e presented. New classes of GUTs in the context of symmetric orbifolds are then constructed. The method of permutation modding is further ex plored and SO(10) GUTs with both 45- or 54-plets are obtained. SU(5) m odels are also found through this method. It is shown that, in the con text of symmetric orbifold SO(10) GUTs, only a single GUT Higgs, eithe r a 54 or a 45, can be present and it always resides in an order-two u ntwisted sector, Very restrictive results also hold in the case of SU( 5), General properties and selection rules for string GUTs are describ ed. Some of these selection rules forbid the presence of some particul ar GUT-Higgs couplings which are sometimes used in SUSY-GUT model buil ding. Some semi-realistic string GUT examples are presented and their properties briefly discussed.