On the anomalous U(1) in free fermionic superstring models

The realistic free fermionic models have had an intriguing success in expla ining different properties of the observed particle spectrum. In this paper we discuss in some detail the anomalous U(1) symmetry which exists in thes e models. We study the properties of the anomalous U(1) in both the more re alistic NAHE-based free fermionic models and those in a general NAHE class. Appearance of an anomalous U(1) in the more realistic NAHE models is shown to be an effect of reduction of world sheet supersymmetry from (2, 2) to ( 2, 0). We show, however, that in more general (2, 1) and (2, 0) models, all U(1) can remain anomaly-free under certain conditions. Several phenomenolo gical issues related to the anomalous U(1) are discussed. In particular, we note that in some examples the anomalous U(1) arises from the breaking E-6 --> SO (10) x U(1)(A), resulting in U(1)(A) being family universal.