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Beam Test of a Large-area GEM Detector Prototype for the Upgrade of the CMS Muon Endcap System V. Bhopatkar , M. Hohlmann, M. Phipps, J. Twigger, A. Zhang Dept. of Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA ( for the CMS GEM Collaboration).

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Abstract 5415114

Beam Test of a Large-area GEM Detector Prototype for the

Upgrade of the CMS Muon Endcap System

V. Bhopatkar, M. Hohlmann, M. Phipps, J. Twigger, A. Zhang

Dept. of Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA

(for the CMS GEM Collaboration)



Gas Electron Multiplier (GEM) technology is being considered for the forward muon upgrade of the CMS experiment in Phase 2 of the CERN LHC. The first such implementation is planned for the GE1/1 system in the 1.5<|η|<2.2 region of the muon endcap. With precise tracking and fast trigger information, this system can significantly improve the CMS muon trigger as shown previously in simulations. We assembled a 1m full-size prototype of a GE1/1 triple-GEM detector with 3,072 radial readout strips at Florida Tech and tested it in hadron beams at Fermilab in October 2013. Strip cluster parameters, detection efficiency, and spatial resolution for charged particles are studied with position and high voltage scans and at different inclination angles. Strip cluster sizes increase with high voltage. We find a plateau detection efficiency of (97.8 ± 0.2)%. All eight eta sectors of the prototype detector show similar high efficiencies. Results of response uniformity and spatial resolution studies using four GEM-based reference tracking detectors are presented. Preliminary results show a spatial resolution of 97 µrad or 21% of strip pitch.

  • During the phase II upgrade process, we are planning to install large-area GEM detectors in the forward muon region.

  • GEM technology will improve overall muon trigger efficiency by providing fast triggering and precise tracking information.

  • Florida Tech is planning to contribute to this upgrade project by producing approximately 40 detectors. We constructed a first large-area GEM detector and studied its characteristics at a test beam at Fermilab in Oct. 2013

Construction of GE1/1 Prototype III Large-Area GEM Detector (at Florida Tech)

Step III: Finished GEM detector w/ APV frontend hybrids connected

Internal gap configuration of the detector:


Step I:

GEM foils assembly with inner frames

Strip Pitch 0.67mm

Strip Pitch 1.05mm

Drift electrode



Stretched GEM foils with inner and outer frames


Step II:

Stretching GEM foils by providing tension





Anode (readout)

  • GEM foils produced by single mask etching technique at CERN

  • Active area: approximately

  • 99×(28-45)cm2

Length 1m

  • 1D readout board with 3,072 radial strips connected through 24 Panasonic connectors to 24 APV hybrids

FNAL Test Beam Oct 2013 - Setup and Measurements

Performance Characteristics from FNAL Test Beam Data

  • Gas mixture used in all detectors: Ar/CO2 70:30

  • Beam Energies: 32 GeV mixed hadrons;120 GeV p

  • 4 GEM trackers @ 4200V

  • DAQ with RD51 SRS

  • CMS detector tests:

  • 1. High voltage scan from

  • 2900V to 3300V

  • 2. Position Scan at 3250V

  • in 3 positions:

  • Upper row APV

  • Middle row APV

  • Lower row APV

Cluster Size vs. High Voltage

Cluster Charge Distribution

Cluster Size at 3250V

CMS GE1/1-III Detector

Cluster size increases with voltage


Cluster charge distribution fitted with Landau function at 3250V


Detection Efficiency with

Different Cuts on Pedestal Widths

  • At 3250V, most probable values for all 3 APV position are used to determine the charge uniformity across the detector:

  • Detection efficiency measured with this detector is 97.8% (with 5-sigma cut on pedestal width).

  • Average cluster size at operating voltage is 2.3 strips

Charge Uniformity

Reference Tracker and Correlations with CMS GE1/1 Detector

Detection Efficiency is 97.8%

Beam profiles

  • Tracking is done in three steps:

  • Step I - Alignment: By iterating the shift parameters in X and Y, we center all detector residuals on zero and then with respect to the first reference tracker, we rotate the remaining three trackers until the residual widths are minimized.

  • Step II - Conversion from (x,y) to (r,φ) coordinate system: Since we are dealing with radial readout strips in the GE1/1, it is more appropriate to use (r,φ) coordinates for tracking.

  • Step III - Calculate final residuals (inclusive and exclusive)

At 3250V

Preliminary Tracking Results for CMS GE1/1-III

120 GeV proton

32 GeV mixed-hadron

Exclusive Residual in Eta 5 Sector

Inclusive Residual in Eta 5 Sector

Residuals of Tracker 1


σ = 21µrad


σ = 75µrad

σ= 87 µrad in φ

σ= 109 µrad in φ



Reference detector resolutions in φ

Correlation of GE1/1 detector hits with hits in first tracking detector:

Tracker 1

  • This translates to residual widths of σ = 201 µm (exclusive) and σ = 160 µm (inclusive) in the azimuthal direction (at the center of the eta sector 5) using R=1850 mm.

  • Taking the geometric mean of exclusive and inclusive residual widths we find a resolution of σ = 97µrad (21% of strip pitch) which corresponds to σ = 179 µm in the center of eta sector 5 when using the pulse-height sensitive analog readout.

  • Conclusion: The performance of this GE1/1 prototype in a beam meets expectations.

Tracker 2

Tracker 3

Tracker 4

CMS Upgrade week, Karlsruhe (Germany) - April 2014