3. THE CMS DETECTOR The CMS detector is designed to explore the full range of physics at LHC up to the highest luminosity. The detector is to be built around a high field solenoid (4 Tesla) which leads to a very compact design. Reliable and efficient identification and high energy resolution (of about 1% over the large momentum range) for muons, photons and electrons are emphasized in the design considerations. A schematic view of the CMS detector is shown in Fig.3.1 and Fig.3.2. Muons are identified at four separate stations, each consisting of several layers of drift chambers inserted in the barrel part of the solenoid return yoke. The endcap muon stations will consist of cathode strip chambers since they are robust at high particle rates. Each station will also include triggering planes of resistive plate chambers. The muon momentum in barrel region will thereby measured three times: in the return yoke, just after the coil and in central tracker, the first two measurements are being performed by stand-alone muon system and available at any luminosity. The inner tracker aims to reconstruct and match all reconstructed high $p$t electrons and muons produced at the rapidities < 2.6 with a momentum resolution of , and to recognize all tracks with $p$t > 2 GeV. Microstrip gas chambers, silicon and pixel detectors provide the granularity sufficient to operate at the highest LHC luminosity. The electromagnetic calorimeter (ECAL) will provide the precise measurements of isolated photon energy and electron identification. It must be able to measure photon direction with sufficient precision so as not to degrade the di-photon mass resolution. Two shower separation capability is essential to eliminate background from jets. Several calorimeter designs are considered. The CMS baseline is the scintillating crystals PWO. The hadron calorimeter (HCAL) is naturally divided in two parts: the central part is installed inside the coil and two very forward calorimeters which are placed at a distance of ±10 meters from the interaction point. For the central part, a scintillator tiles equipped with wavelength-shifter fibers are used as detecting elements with copper as an absorber. The very forward calorimeter (VF) has to operate in a region where high radiation doses are expected. The baseline technology is the parallel plate chambers with iron absorber and circulating gas as an active medium. The alternative design is the Cherenkov quartz fiber calorimeter. In comparison with typical LEP detector the general purpose LHC detector has to cover much wider pseudorapidity range in order to provide the high potential for new phenomena search. Namely muon system, tracker, electromagnetic calorimeter has to cover the range of and hadron calorimetry has to cover the range up to The subdetectors those will be placed in the so-called endcap region have to operate in the highest unprecident fluxes of particles with the highest radiation dose levels.