`Light U(1)s in Heterotic String Models'

Abstract:  
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 present a
recipe of how models with the required properties may be constructed in the
free fermionic formulation, via a Standard-like model.