ar X iv : h ep - e x / 02 08 03 6 v 1 2 1 A ug 2 00 2 1 The use of the τ in new particle searches at DELPHI

Several new particle searches have been performed in the DELPHI experiment involving τ leptons in the resulting final state. The topology and special characteristics of the τ leptons have been used to discriminate the signal from the Standard Model background. Limits on new particles have been set, playing an important role the channels with τ leptons. τ leptons are not only a precision measurement tool, but also a sensitive probe for Physics beyond the Standard Model. In this note we review the importance of their contribution and the results obtained at the DELPHI experiment at LEP. Several extensions to the Standard Model predict the existence of new particles involving τ leptons in the resulting final state. Data collected at LEP2 at centre-of-mass energies from 130 GeV to 209 GeV were used in the different analyses, with a total integrated lumi-nosity of about 650 pb −1. 2. The DELPHI detector DELPHI was one of the four detectors operating at the LEP collider from 1989 to 2000. It was designed as a general purpose detector for e + e − physics with special emphasis on precise tracking and vertex determination and on powerful particle identification. A detailed description of the DELPHI detector can be found in [1] and the detector and trigger performance in [2,3]. Charged particle tracks are reconstructed by a system of tracking chambers inside the 1.2 T solenoidal magnetic field: the Vertex Detector (VD), the Inner Detector (ID), the Time Projection Chamber (TPC) and the Outer Detector (OD) in the barrel region; two sets of planar drift chambers aligned perpendicular to the beam axis (Forward Chambers A and B) measure tracks in * Now in the Massachusetts Institute of Technology the forward and backward directions. The VD consists of three cylindrical layers of silicon detectors, at radii 6.3 cm, 9.0 cm and 11.0 cm, and polar angle acceptance from 24 • to 156 •. All three layers measure coordinates in the plane transverse to the beam (xy), and at least two of the layers also measure z coordinates along the beam direction. The ID consists of a cylindrical drift chamber with inner radius 12 cm and outer radius 22 cm, surrounded by 5 layers of straw tubes, having a polar acceptance between 15 • and 165 •. The TPC, the principal tracking device of DELPHI, consists of a 2.7 m long cylinder of 30 cm inner radius and 122 …