Vertex evaluation in the Delphi experiment

At the e+e- storage ring (LEP) at CERN, reactions with complex decays of heavy particles will be of particular interest. The more stable ones (like B and D mesons, τ lepton) have lifetimes greater than 10-13 sec; evaluation of their decay vertices thus requires a spatial resolution of ca. 10m μm. For this aim, the DELPHI spectrometer contains a vertex detector (silicon microstrips). Also needed is optimal utilization of all measurement information and correct treatment of multiple scattering for the geometrical reconstruction of tracks and vertices. Since the straightforward method of least-squares fitting leads to prohibitive computing times, more sophisticated algorithms (which are equivalent to the Kalman filter) must be used. First results from Monte-Carlo studies on the precision of track and vertex fitting confirm the usefulness of these methods