A GEM Detector System for an Upgrade of the CMS Muon Endcaps

1 Endcap muon system
1.1 Introduction
The CMS muon system[1] is designed to provide robust, redundant and fast identification of the muons traversing
the system, in addition to trigger capabilities and momentum measurement. One of the main goals during the upgrade
phases will be to have a detector capable of fully exploiting the increased LHC performance, and sustaining
reliable operation for at least ten years.
Here a big challenge arises, coming from the higher luminosity and more hostile conditions, in particular in terms
of expected rates and backgrounds in near future. The high instantaneous luminosity can confuse the CMS Level-1
trigger. To keep up with the 40 MHz rate, this uses partial data from events in each beam crossing and dedicated,
custom, hardware. At very high luminosity, with 20-40 interactions superimposed, and with only some of the event
information available the trigger performance will degrade. Upgrades to the muon system (and also to the hadron
calorimeters) aim to preserve the Level-1 trigger capability by providing it with more and higher quality inputs.
For the initial phase of CMS, three types of gaseous detection technologies (see Fig. 1) have been chosen, according
to the different background rates and magnetic field the detectors have to withstand. Where neutron background is
relatively small and in the 1-10 Hz/cm 2 range, Drift Tube Chambers (DTs) are used. In the endcap regions where
the background rate is higher and around 100-200 Hz/cm 2 and the magnetic field is more intense than in the barrel,
cathode strip chambers (CSCs) are selected.
DTs and CSCs, both with self-trigger capabilities, can be used to measure the momentum and time of crossing of
the muon. In addition, to assure an unambiguous bunch crossing (bx) identification and to build up a robust and
redundant system, Resistive Plate Chambers (RPCs) both in the barrel and endcaps are used, providing a fast and
accurate time measurement. The muon momentum resolution is extracted by combining information of the muon
track from the muon system, modulated by the Global Muon Trigger with an efficiency > 96%, complemented
with the more precise tracker measurement (including the vertex constraint). A momentum resolution down to 2 %
is obtained forpT < 100 GeV (at all pseudorapidities) and 18 % for the most forward 1 TeV muon measurable by
the CMS Muon spectrometer.
Figure 1: CMS transverse section showing the existing gaseous detector for muon tracking and triggering namely
RPCs, DTs and CSCs and the locations of the proposed GEM detectors GE1/1 and GE2/1.
1.2 CSC (Cathode Strip Chambers)
CSCs are multiwire proportional chambers comprising anode wire planes interleaved with cathode strip panels
where strips run radially according to CMS coordinates. By interpolating charges induced in the strips from
the avalanche of positive ions near a wire, a precise localization accuracy 75µm-150µm, from low to high-η is
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