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

of the gas rigidity or discharge. Systematic investigations have been carried out; the most significant study is the
measurement of the discharge probability of a Triple-GEM detector in a high intensity, low energy beam at PSI, the
beam that best simulates the conditions expected at the LHC. It has been demonstrated that the detector does not
deteriorate after multiple discharges with large repetition rates, provided that the amplifiers of the GEM electronics
are properly protected 1) .
In the following feasibility study we have studied small and large size prototypes to demonstrate that the Triple-
GEM detector fulfils all requirements for forward muon tracking and triggering at high eta CMS.
1.7 High-η muon detectors proposed for LS2 (2017-2018)
In Fig.2, the nose region of the first endcap disk is visible. In this document, we propose the installation of the
stations called GE1/1 and GE2/1 as shown in Fig. 3. For the GE1/1 station the chambers will be installed on the
YE1 ’nose’. For the GE2/1 station, which sits exactly on the backside of YE1 ’nose’, there will be two sets of
chambers; one long chamber and one shortened chamber due to the neutron shielding in the2.1 < |η| < 2.4 region.
Since there is room for long chambers in z, the 2/1 station offers 4 measurements points from eta 1.6 < |η| < 2.1
and two measurements points from 2.1 < |η| < 2.4. The first station GE1/1 would have 10 degree chambers
while for the second station GE2/1, 20 degree chambers are proposed. In the following section we describe
the preliminary study of the impact of upgrading the CMS forward muon system with these detectors, on the
performance of the muon system. In Section 3 the case for micropattern detectors is made based on the past and
present experience. The intensive experimental and simulation studies with small prototypes are summarized. Full
size prototypes were constructed and tested during 2010-2011 and the results from laboratory and beam tests are
elaborated. The focus then shifts to the development of new technologies for fabrication and construction of full
scale CMS detectors. Results from measurements in the laboratory and at the beam, with and without magnetic
field upto 1.6 T are summarised.
From simulation studies we can conclude that the GEM detector stations would significantly improve the L1 muon
trigger performance over that of the originally planned RPC-only system in the forward direction |η| > 1.6.
Large scale production scenarios are discussed with the assembly procedure and work flow description. Studies on
integration and installation within the CMS high eta envelope are presented followed by the electronics system and
other services including gas and cooling. A tentative schedule is presented for construction of the first two stations
along with preliminary budget and resources needed. Finally the structure of the GEMs for CMS collaboration is
described towards installation in LS2 (2017/2018).
1) Whenever the total charge in the avalanche exceeds a value between 10 7 and 10 8 electron-ion pairs (Raethers limit), an
enhancement of the electric field in front of and behind the primary avalanche induces the fast growth of a long, filament-like
streamer. This discharge is measured by counting the number of alpha particles that produce a current greater than 1µA, the
current from nominal radiation being of the order of few tens of nA.
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