This work is supported in part by German BMBF under project 06HD197D tiplicity environment of up to 8000 particles per unit of rapidity. The main detector systems for these tasks are the Inner Tracking System, the Time Projection Chamber, the Transition Radiation Detector and the Time of Flight array. These systems cover the full azimuthal angle within the pseudorapidity range -0.9 < ฮท < 0.9 and are de scribed below. Additional detectors with partial coverage of the central barrel are a PHOton Spectrometer (PHOS), an electromagnetic calorimeter (EMCAL) and a High-Momentum Particle Identification Detector (HMPID).
2.1. The Inner Tracking System
The Inner Tracking System (ITS) consists of six cylindrical layers of silicon detectors at radii from 4 cm to 44 cm. The minimum radius is determined by the beam pipe dimensions whereas the maximum radius chosen is determined by the necessity of efficient track matching with the outer detectors in the central barrel. The innermost layer extends over the range -2 < ฮท < 2 such that there is continuous overlap with event classification detectors outside of the central barrel. Due to the high particle density of up to 80 particles/cm 2 and in order to achieve the required tracking resolution, pixel detectors have been chosen for the first two layers. Silicon drift detectors are located in the middle two layers whereas double sided silicon strip detectors are in the outer two layers.