2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/202377A 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 e^- and five events with a mu^+- 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.18 pages, 5 figures, 2 tables, submitted to Eur. Phys. JHigh Energy Physics - Experimenttext