The Vertexing package and its application in the \tau-tagging

A package for the vertex reconstruction (Vertexing) has been developed in the ATLAS offline software. It consists of an Object Oriented interface to an existing fitting routine and of a set of test algorithms. The code is described in the first part of this note. In the second part, the Vertexing algorithms are applied to the identification of the hadronic tau decays. A 44.1% efficiency has been achieved in A -> tau tau -> hadrons (mA = 800 GeV/c^2) events. The jet-rejection has also been estimated in QCD events with high-pT jets.