The CMS tracker and expected performances

A robust tracking and a detailed vertex reconstruction are essential characteristics of each modern High Energy Physics experiment. This is particularly true in the LHC environment where thousands of tracks are expected to cross the inner part of the detector each bunch crossing (25 ns). In order to meet these requirements the Compact Muon Solenoid (CMS) collaboration has recently proposed to build a multi-layer full-silicon tracker with a pixel vertex detector close to the beam pipe. This layout is different from the one originally proposed in the CRIS Tracker TDR (Technical Design Report, CERN/LHCC 98-6) where a inner silicon tracker surrounded by an outer MSGC tracker was foreseen. In this paper the reasons which led to change the CMS baseline proposal are explained, and the layout and expected performances of the recently proposed all-silicon tracker are described. (4 refs).