Vanishing Str M^2 in the presence of anomalous U_A(1)

Abstract

We show that the presence of an anomalous ${\mathrm{U}}_{\mathrm{A}}(1)$ factor in the gauge group of string-derived models may have the new and important phenomenological consequence of allowing the vanishing of $\mathrm{Str}{\mathcal{M}}^2$ in the ``shifted" vacuum, that results in the process of cancelling the anomalous ${\mathrm{U}}_{\mathrm{A}}(1)$. The feasibility of this effect seems to be enhanced by a vanishing vacuum energy, and by a ``small" value of $\mathrm{Str}{\mathcal{M}}^2$ 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 $\mathrm{Str}{\mathcal{M}}^2>0$ in the original vacuum. A vanishing $\mathrm{Str}{\mathcal{M}}^2$ ameliorates the cosmological constant problem and is a necessary element in the stability of the no-scale mechanism.

Comments: 7 pages, no figures, LaTeX