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

distribution measured with these two tracker detectors:
σ 2 ∆y = σ 2 y5 +σ 2 y1 = 2σ 2 y
σy = σ∆y
√2 = 53µm (2)
Hit positions in x and y are computed from the mean (or “center-of-gravity”) of the corresponding strip cluster
using the analog pulse height information for each strip. It is a reasonable assumption that σx5 ≈ 53 µm if
σy5 = 53 µm since in TR5 strips in x and y have the same pitch. Consequently, the following upper bound on the
spatial resolution of the GE1/1-II prototype can be established in the chamber section with smallest strip pitch at
the high-η end when analog pulse height information is used:
σxGE11 ≤
σ2 ∆x −σ2 x5 = 103 µm (3)
Equ. (3) gives an upper bound value, as any remaining beam divergences in x still contribute to the width of the
∆x distribution in Fig. 29.
Fig. 30 shows the measured shift in the average strip cluster position due to the presence of a magnetic field for the
GE1/1-II prototype operated inside the M1 magnet. Measurements were performed in June and July 2011 using
the SPS H2 150 GeV muon beam, with the detector flowing an Ar/CO2/CF4 (45:15:40) gas mixture. The angle
between the GEM electric field and the external magnetic field could be varied by rotating the detector inside the
M1 magnet. The data are in good agreement with a Garfield/Magboltz [16] simulation performed for a 90 ◦ angle
between the magnetic field and the GEM electric field. The measurement at 1.5 T was performed for an angle of
30 ◦ , which accounts for the observed small deviation between data and simulation for that particular point.
More details on test beam results obtained with the two large-area prototypes can be found in [17].
Figure 30: Left: GE1/1-II cluster sizes inside a strong magnetic field. Right: Comparison of measured and
simulated strip cluster position displacement in GE1/1-II due to an external magnetic field.
3.5.4 Conclusion from measurements with full-size prototypes
Beam tests of both full-size prototypes produced very positive and promising results. The performance of the
large-area prototypes with single-mask GEMs was quite similar to that of the small double-mask “Timing GEM”
prototype. A significant advance was achieved with the construction and assembly of the second full-scale detector
GE1/1-II even though it was technically challenging to construct a large-area GEM with transfer and induction gap
sizes as small as 1 mm. Good detector performances regarding detection efficiency and spatial resolution were
demonstrated and the second detector behaved as expected when operated in a magnetic field.
3.6 Present status and improved detector assembly technique
With a GEM detector geometry similar to the RPCs in the CMS endcap disks, the smallest active GEM detector
area required by CMS is of the order of 100 cm × 50 cm. Depending on how many endcap disks would be
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