Observation of events with an isolated high energy lepton and missing transverse momentum at HERA

A search for events with an imbalance in transverse momentum and with isolated high energy leptons has been carried out at the positron-proton collider HERA. One event with an and five events with a are found together with evidence for undetected particles carrying transverse momentum. Within the Standard Model the dominant origin of events with this kind of topology is the production of W bosons with subsequent leptonic decay. Three of the six events are within measurement errors found in a region of phase space likely to be populated by this process, while the remaining events show kinematic properties which are atypical of all Standard Model processes considered. Received: 9 June 1998 / Published online: 26 August 1998