New Interactions in Neutral Current Processes

Ever since the discovery of weak neutral currents (NC) at Gargamelle 1 25 years ago, their study has been a proving ground for the Standard Model (SM). During this period, the precision with which the SM has been confirmed has been improved dramatically, now reaching the 0.1% level in Z-pole precision experiments. Despite these successes of the SM, NC processes remain an excellent tool to search for, and perhaps discover, signals for new interactions and, hence, physics beyond the SM. The observation, two years ago at HERA , of an apparent excess of events in deep inelastic scattering at high Q is a case in point, even though the excess has not been confirmed by subsequent observations. 5 This possible signal for new physics has triggered much research, which has lead to a better understanding of the experimental constraints on new interactions in the NC sector. This talk tries to summarize these constraints in a model independent way. Precision experiments on NC interactions can largely be understood as four-fermion scattering, and among these llqq amplitudes are particularly important. In Section 2 we start out by describing these four fermion NC amplitudes in the SM, and discuss the general form of deviations due to new interactions. Two such deviations, from very different sources, are discussed in some detail: extra Z bosons are considered in Section 3. Section 4 deals