Vanishing Str M2 in the presence of anomalous UA(1)

We show that the presence of an anomalous UA(1) factor in the gauge group of string-derived models may have the new and important phenomenological consequence of allowing the vanishing of Str M2 in the “shifted” vacuum that results in the process of cancelling the anomalous UA(1). The feasibility of this effect seems to be enhanced by a vanishing vacuum energy, and by a “small” value of Str M2 in the original vacuum. In the class of free-fermionic models with vanishing vacuum energy that we focus on, a necessary condition for this mechanism to be effective is that Str M2 > 0 in the original vacuum. A vanishing Str M2 ameliorates the cosmological constant problem and is a necessary element in the stability of the no-scale mechanism.