Searches for Flavour Changing Neutral Currents and Lepton Flavour Violation at HERA

Abstract

A summary of the recent searches for anomalous single top-quark production and lepton flavour violation in high energy ep collisions is presented. Single top-quark production via an anomalous flavour changing neutral current gamma-u-t coupling would lead to the reaction ep->etX, t->Wb. H1 observed an excess of events above the Standard Model prediction in the leptonic decay channel of the W boson, leading to an isolated lepton, large missing transverse momentum and a jet with large transverse momentum. Five of these events are compatible with single top-quark production, compared to 1.8 events expected from background processes. ZEUS observed no excess above the SM background expectation in the leptonic channel. Neither of the two experiments found a deviation from the SM background expectation in the 3-jet final state, which would result from the hadronic decay of the W boson. Searches for lepton flavour violating interactions of the type ep->mu X and ep->tau X have been performed. No evidence was found for lepton flavour violation and limits were derived on the production of leptoquarks and R_p violating squarks, which could mediate such interactions.