Indications for an Extra Neutral Gauge Boson in Electroweak Precision Data

A new analysis of the hadronic peak cross section at LEP 1 implies a small amount of missing invisible width in $Z$ decays, while the effective weak charge in atomic parity violation has been determined recently to $0.6\%$ accuracy, indicating a significantly negative $S$ parameter. As a consequence, the data are described well if the presence of an extra $Z^{\prime }$ boson, such as predicted in grand unified theories, is assumed. Moreover, the data are now rich enough to study an arbitrary extra $Z^{\prime }$ boson and to determine its couplings in a model independent way. An excellent fit to the data is obtained in this case, suggestive of a family nonuniversal $Z^{\prime }$ similar to those predicted in a class of superstring theories.