The Rpc System for the Cms Experiment at Lhc 1 Cms Detector 2 Cms Muon Trigger

The CMS experiment designed for Large Hadron Collider is brieey described. An idea of a muon trigger based on RPC is presented. This talk focuses on the requirements on RPC as the trigger detectors for the CMS. We discuss the required chamber's parformance and segmentation. Current design of the system fulllling these requirements is presented. Its expected trigger performances are given. The abbreviation \CMS" stands for Compact Muon Solenoid 1]. It is a general purpose particle detector to be operated at Large Hadron Collider at CERN in Geneva. Its main parts are an inner tracker, calorimeters and a muon system (Fig. 1). The inner tracker consists of silicon pixel, silicon microstrips detectors and microstrip gas chambers (MSGC). The electromagnetic calorimeter is a matrix of PbWO 4 crystals. The hadronic calorimeter is a copper/scintillator sandwich up to jj = 3. At higher jj it is completed with a very forward calorimeter made of iron with quartz bers as sensitive elements. The characteristic feature of the CMS detector is that the inner tracker and both calorimeters are contained within the large superconductive solenoid, 6 m in diameter and 13 m long. The coil creates 4 T magnetic eld. Outside the coil the magnetic ux is returned by an iron yoke. The yoke is interleaved with 4 muon stations. Each barrel muon station consists of drift tubes (DT) and RPC's. Endcap muon stations are equipped with cathode strip chambers (CSC) and RPC's as well. At the highest LHC luminosities about 20-30 pp interaction occure every 25 ns. Basic goal of the CMS rst level muon trigger is to reduce this rate (1 GHz) down to a level acceptable for the second level trigger. CMS have chosen a solution where the second and the third level trigger algorithms are performed by a farm of commercial processors. The farm is designed to accept 100 kHz input rate. The output rate of the rst level is, however, assumed to be only 20{30 kHz, leaving a large safety margin. After distributing this bandwidth among various calorimeter and muon triggers, about 6 kHz is left for a rst level single muon trigger. In order to achieve this huge rejection factor (1 GHz ! 6 kHz) it is not enough to recognize a muon. The trigger system should measure the muon momentum quite precisely in order to enable relatively sharp p t cut.