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

Therefore, the physics case for addition of forward muon chambers is straight forward. The foreseen upgrade
for the CSC system will restore the reduced muon acceptance at trigger level in the region 1.2 < |η| < 1.6.
The installation of new RPC chambers in the same region (RE4) will provide the finer timing and redundancy
for the corresponding CSC system. Upgrade in the 2.1 < |η| < 2.4 will improve the trigger capability in the
high luminosity regime, where the present ganging of some channels in these chambers results in an unacceptable
number of spurious tracks.
The Muon System provides muon trigger and identification capability. The original CMS designed was tailored
to setup a robust system through the redundancy. So far the experiment has achieved the design goals only up to
η = 1.6.
In CMS the high momentum resolution of muons is given by the very precise point resolution of the Tracker
system. The Muon system starts giving a sizable contribution in the high-η region, because of the reduced lever
arm of muons in the Tracker volume. In Fig. 4 is shown the momentum resolution as a function of the muon
momentum for the reconstruction in the inner Tracker system and the Tracker and Muon system[6]; muons with
pT > 200 GeV/c improve their resolution thanks to the addition of the Muon system information. For this reason,
in this η region is particularly interesting to design a Muon system redundant also on the tracking capability. An
additional detector system for the Muon system, with high point resolution, would also improve over the limited
pT resolution obtained in the Muon system only, shown in the same figure, for the benefit of the trigger L2
reconstruction in the High Level Trigger selection.
In order to study the impact of the upgrade Muon system on muon reconstruction performance, Monte Carlo single
muon samples with differentpT have been generated. The CMS detector was simulated with GEANT4 within the
existent CMS framework, where the proposed upgrade has been described by extending the existing geometry of
the RPC system, up to |η| < 2.1. The muons transverse momenta, with respect to the beam axis, chosen for this
simulation study are 0.2, 0.5, 1, 2, 5 TeV/c.
2.2 Multiple scattering
The spatial resolution lower limit, set by the electromagnetic processes that muons experience in traversing the
CMS material, has been estimated from simulation and the results are depicted in Fig. 5. The left plot indicates the
dominant role of the multiple scattering with respect to the effect of the magnetic field and other electromagnetic
processes such as bremsstrahlung; the results refer to the first station but the conclusion is valid for all stations.
The right plot shows the displacements as a function of the pT in the different endcap station. In the first endcap
station the rms of the displacement of 1 TeV muon is 100 µm, while it reaches 500 µm for the fourth and last
station.
m)
µ
Multiple Scattering (
3
10
2
10
-1
RE1/1
10 1 10
P
(TeV/c)
T
All e.m. processes
Multiple Scattering B=0T
All e.m. processes B=0T
Multiple Scattering B=4T
Figure 5: The rms of the distribution of the displacement as a function of the muon pT of the muon due to
electromagnetic processes. On the left the results obtained in the first station are shown for nominal magnetic
field (no magnetic field) for all e.m. processes in full circles (upward triangles) and for multiple scattering only in
downward triangles (full squares). The right plot describes the values for the different stations.
16