Phenomenological Constraints on Light U(1)s in Heterotic String Models
Viraf Mehta (Liverpool University)

Although additional low scale U(1)s have been discussed extensively in GUTs and string models, having a viable light, extra U(1) in worldsheet heterotic string constructions has proven to be a challenge. Here, we present the construction of heterotic string models using the free fermionic formulation and focus on how viable U(1)s may arise. We motivate an example as an appealing proposition to explain the suppression of proton decay mediating operators. This forbids the undesired operators, and thus they may only arise after the U(1) is broken. We give details of a left-right symmetric model that may accommodate such a symmetry and consider constraints on such string-derived U(1)s focussing on those imposed by gauge coupling unification and proton stability. In two contrasting cases we show that the gauge data at the electroweak scale necessitate that the Z' charges are embeddable in E6. We finally discuss how models with the required properties may be constructed in the free fermionic formulation.