Tau Leptons at HERA

We review recent analyses at HERA dealing with a tau lepton in the final state of ep-collisions. At the ZEUS experiment, a search for events containing isolated tau leptons and missing transverse momentum in addition to that due to the tau decay was performed. The analysis, based on isolated tracks from hadronic tau decays and the exploitation of the internal jet structure, yields 3 tau candidates where 0.40−0.13+0.12 are expected from the Standard Model. With the H1 detector, the production of high-PT isolated tau leptons was studied using typical one-prong hadronic tau decays as identification. In addition, an analysis of tau lepton pairs in elastic photon-photon collisions was performed. Furthermore, a search for doubly charged Higgs bosons includes also like-sign tau lepton pairs in the final state. Finally, the tau lepton was studied by both experiments in searches for new physics as a decay product of a resonantly produced leptoquark violating the conservation of lepton flavor.