Bounds on effective Majorana neutrino masses at HERA

The lepton-number violating process e±p→νe(νe)l±l′±X mediated by Majorana neutrinos is studied for the HERA collider for (ll′)=(eτ), (μτ), (μμ) and (ττ). Only the muonic decay of the τ is considered. The limit on the effective muon Majorana mass, 〈mμμ〉 is improved significantly to 4.0·103 GeV and for the first time limits on the analogous effective masses connected with the tau sector are given, namely 8.4·103 GeV for 〈meτ〉, 8.7·103 GeV for 〈mμτ〉 and 2.0·104 GeV for 〈mττ〉. We find that a more general analysis for an upgraded HERA could improve this values by a factor of up to 40.