Abstract
The LHC data-taking will resume in 2015 with energy of 13–14 TeV and luminosity of 2÷5 × 1034 cm−2 s−1. At those energies, a considerable fraction of the particles produced propagate in the high pseudo-rapidity regions. The proposal for the upgrade of the CMS muon forward system involves Gas Electron Multiplier (GEM) chambers to be installed during the second LHC Long Shutdown (LS2) covering the pseudorapidity range 1.5 < |η| < 2.2. This detector is able to handle the extreme particle rates expected in this region when the LHC will be running at higher luminosity. The GEM is an excellent choice, as its high spatial resolution (order of 100 μm) allows to combine tracking and triggering capabilities, which will improve the CMS muon High Level Trigger, the muon identification and the track reconstruction. Intense R&D has been going on since 2009 and it has lead to the development of several GEM prototypes and associated detector electronics. These GEM prototypes have been subjected to extensive tests in the laboratory and in test beams at the CERN Super Proton Synchrotron (SPS). This contribution will review the status of the CMS upgrade project with GEMs, discussing also the trigger performance.
Highlights
The installation of the Gas Electron Multiplier (GEM) is aimed at maintain the existing forward muon trigger coverage of |η| < 2.4 reducing the trigger rate in the region which currently suffering from the highest background rates and a non-uniform magnetic field
Trigger rate reduction is possible with the improved momentum resolution deriving from precision measurements of the bending angle performed measuring the lever arm between the existing CSC chambers in stations ME1/1 and the triple GEM detectors in new GE1/1 station [5]
Results are compared with the standard CMS configuration used in 2012 modified to account for the improved ME1/1 detector electronics to be installed in LS1
Summary
The installation of the GEMs is aimed at maintain the existing forward muon trigger coverage of |η| < 2.4 reducing the trigger rate in the region which currently suffering from the highest background rates and a non-uniform magnetic field. Trigger rate reduction is possible with the improved momentum resolution deriving from precision measurements of the bending angle (see figure 2, left) performed measuring the lever arm between the existing CSC chambers in stations ME1/1 and the triple GEM detectors in new GE1/1 station (see figure 2, right) [5]. The trigger rate reduction in the region 1.6 < |η| < 2.1 covered by the new station GE1/1 is shown in figure 3. The trigger rate reduction is primarily due to better discrimination of high transverse momentum muon candidates obtained by measuring the bending angle with pairs of GEM and CSC chambers. In the two simplified scenarios shown, the tracks satisfy the requirement of having reconstructed trigger hits in at least two (figure 3, left) or three (figure 3, right) stations out of four possible. The plots demonstrate the increased dynamic range available for optimizing the muon trigger due to the addition of the bending angle
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