Multi-lepton Production at High Transverse Momentum in electron-proton   collisions at HERA
  Multi-electron and multi-muon production have been measured at high transverse momentum in electron-proton collisions at HERA.Good overall agreement is found with the Standard Model predictions, dominated by photon-photon interactions. Events are observed with a di-electron mass above 100 GeV, a domain where the Standard Model prediction is low. 
Cristinel Diaconu (Centre de Physique des Particules de Marseille   CPPM/IN2P3/CNRS) (for the H1 Collaboration and the ZEUS ...