Drift Tube based Pseudorapidity Assignment of the Level-1 Muon Trigger for the CMS Experiment at CERN

CMS, the Compact Muon Solenoid experiment under construction at the CERN Large Hadron Collider, will explore new physics at high energies. Proton-proton and heavy-ion collisions will be studied. Muons with large transverse momenta are expected to be among the decay products of many new particles. Their identification and selection is the task of the trigger system. Specifically, the Level-1 Muon Trigger has to search for muon candidates and to determine their parameters at a rate of 40 MHz, corresponding to a beam crossing interval of 25 ns. The precise knowledge of the spatial parameters of a track allows to fully make use of the possibility to select topological trigger conditions already at Level-1 in CMS. Track segments measured in orthogonal layers of drift tube chambers arecombined to form a muon candidate. Its transverse momentum is calculated from the track curvature in the (r/ph i)-projection caused by a magnetic field along the beam direction z. The azimuthal angle phi in the plane transverse to the beam is also determined. The pseudorapidity eta is a function of the track angle theta relative to the beam axis. Using the information from the bending plane projection only allows a coarse assignment of eta in the central region of CMS by determining which chambers were crossed by the track. A method to assign eta-values of much greater precision is presented. It relies on track finding performed in the non-bending plane (r/z) and on matching the found tracks with those of the (r/phi)-projection. The requirements, the chosen algorithm, its simulated performance and the feasibility for a hardware implementation are described.