[Paper Review] Test beam results of the GE1/1 prototype for a future upgrade of the CMS high-$\eta$ muon system

D. Abbaneo, M. Abbrescia+56 more|arXiv (Cornell University)|Nov 21, 2011

Particle Detector Development and Performance5 references3 citations

TL;DR

This paper presents the test beam results of the GE1/1 II prototype, a large-area Triple-GEM detector designed for the high-eta region (1.6 < |η| < 2.4) of the CMS muon system upgrade. Using a 3/1/2/1 mm gap configuration and Ar/CO2/CF4 gas mixture, the prototype achieved 96.5% detection efficiency at a gain of 7,000, demonstrated sub-100 µm spatial resolution, and showed stable performance under a 3T magnetic field, validating GEMs as a viable technology for high-rate, high-precision forward muon tracking and triggering at the HL-LHC.

ABSTRACT

Gas Electron Multipliers (GEM) are an interesting technology under consideration for the future upgrade of the forward region of the CMS muon system, specifically in the 1.6 &lt; |eta| &lt; 2:4 endcap region. With a sufficiently fine segmentation GEMs can provide precision tracking as well as fast trigger information. The main objective is to contribute to the improvement of the CMS muon trigger. The construction of large-area GEM detectors is challenging both from the technological and production aspects. In view of the CMS upgrade we have designed and built the largest full-size Triple-GEM muon detector, which is able to meet the stringent requirements given the hostile environment at the high-luminosity LHC. Measurements were performed during several test beam campaigns at the CERN SPS in 2010 and 2011. The main issues under study are efficiency, spatial resolution and timing performance with different inter-electrode gap configurations and gas mixtures. In this paper results of the performance of the prototypes at the beam tests will be discussed.

Motivation & Objective

• To develop a large-area, high-precision GEM-based muon detector for the high-eta region (1.6 < |η| < 2.4) of the CMS experiment.
• To address the limitations of Resistive Plate Chambers (RPCs) in the hostile high-luminosity LHC environment by evaluating GEM technology as a radiation-hard, high-rate alternative.
• To optimize the construction and assembly of full-size Triple-GEM detectors with fine segmentation and minimal gaps (1 mm), ensuring mechanical stability and electrical performance.
• To validate detector performance under realistic conditions, including high magnetic fields (3T) and various gas mixtures, for future integration into the CMS forward muon system.
• To demonstrate the feasibility of using analog pulse height information for high-resolution track reconstruction in large-area GEM detectors.

Proposed method

• The GE1/1 II prototype is a full-size Triple-GEM detector with a 3/1/2/1 mm gap configuration (drift, transfer 1, transfer 2, induction) to enable fine segmentation and high spatial resolution.
• GEM foils were fabricated using a single-sided photolithographic mask process on 50 µm Kapton with 5 µm copper cladding, enabling precise hole patterning for large-area production.
• The detector features 35 high-voltage sectors and 384 readout strips per η partition, with variable strip pitch (0.6–1.0 mm) to optimize segmentation and reduce discharge probability.
• Beam tests were conducted at CERN SPS H8 and H4 muon beam lines using pion beams with a 2×2 mm² central spot to minimize divergence effects.
• Spatial resolution was measured using two small 10×10 cm² reference GEM detectors (TR5 and TR1) with known resolution, and the GE1/1 II resolution was derived via ∆x and ∆y distribution analysis.
• The detector was operated with VFAT2 and APV25 readout electronics, and performance was evaluated using pulse height information, HV scans, and timing measurements under 3T magnetic field.

Experimental results

Research questions

• RQ1 Can a large-area Triple-GEM detector with 1 mm transfer and induction gaps achieve high detection efficiency and spatial resolution under high-rate and high-magnetic-field conditions?
• RQ2 How does the performance of the GE1/1 II prototype compare to small-scale prototypes in terms of efficiency and resolution when operated with different gas mixtures?
• RQ3 To what extent does a 3T magnetic field affect signal cluster size and position in the GEM detector, and is this consistent with simulation predictions?
• RQ4 Can analog pulse height information from the full-scale GE1/1 II detector be used to achieve sub-100 µm spatial resolution, and what is the upper bound on resolution?
• RQ5 What is the achievable detection efficiency and gain stability of the full-size GEM prototype under realistic beam and magnetic field conditions?

Key findings

• The GE1/1 II prototype achieved 96.5% detection efficiency at a gain of 7,000, confirming excellent performance and stability in full-scale operation.
• The spatial resolution of the GE1/1 II prototype was bounded at ≤103 µm when using analog pulse height information, based on ∆x distribution analysis with reference detectors.
• The detector demonstrated stable performance under a 3T magnetic field, with cluster size unaffected and signal displacement consistent with GARFIELD simulations.
• Noise levels were negligible, with a VFAT2 threshold of 12 units (≈0.08 fC) and comparator current set to 40 µA, enabling low-noise operation across a gain range of 0–10⁴.
• The use of Ar/CO2/CF4 gas mixture (45:15:40) enabled optimal performance, with no significant degradation in efficiency or resolution.
• The prototype successfully demonstrated the capability to reconstruct tracks using APV25 readout electronics and full pulse height information, validating the scalability of the design for future CMS upgrades.