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 cha rge in atomic parity violation has been determined recently to 0.6% accurac y, indicating a significantly negative S parameter. As a consequence, the d ata are described well if the presence of an extra Z' boson, such as predic ted in grand unified theories, is assumed. Moreover, the data are now rich enough to study an arbitrary extra Z' boson and to determine its couplings in a model independent way. An excellent fit to the data is obtained in thi s case, suggestive of a family nonuniversal Z' similar to those predicted i n a class of superstring theories.