A new design using GEM-based technology for the CMS experiment

The muon system of the Compact Muon Solenoid (CMS) experiment at the LHC is currently not instrumented for pseudorapidity higher than |η|> 2.4. The main challenges to the installation of a detector in that position are the high particle flux to be sustained, a high level of radiation, and the ability to accomodate a multilevel detector into the small available space (less than 30 cm). A new back-to-back configuration of a Gas Electron Multiplier (GEM) detector is presented with the aim of developing a compact, multi-layer GEM detector. It is composed of two independent stacked triple-GEM detectors, positioned with the anodes toward the outside and sharing the same cathode plane, which is located at the center of the chamber, to reduce the total detector's thickness. A first prototype has been produced and tested with an X-Ray source and muon beam. First results on its performance are presented.

The ME0 station is proposed for the installation in 2024 in the region 2.0 < |η| < 2.8, behind the upgraded, shorter hadron calorimeter, which will replace the existing HCAL calorimeter. A slot less than 30 cm thick will become available in front of the muon system endcap. This station will partially overlap in eta coverage with CSCs, restoring redundancy in the region 2.0 < |η| < 2.4, but its major goal will be the extension of the muon system acceptance up to |η| = 2.8. It will provide an improved trigger and improved tagging of high-eta muons. The current configuration of the muon system, along with the stations proposed for its upgrade, are shown in figure 1.
The high-eta position of the ME0 station requires a detector with high rate capability ∼MHz/cm 2 , good time (few ns) and spatial resolution ∼100 µm for triggering and tracking. In addition, the ME0 station needs to provide at least three hits for muon track reconstruction with high efficiency [1]. For this reason a multilayer structure with at least 6 layers of detectors is necessary.
Gas Electron Multiplier (GEM) detectors have already been chosen and widely studied for another high-eta upgrade of the muon system endcap, the GE1/1 station [2,3]. Consequently, this technology provides an appealing choice for the ME0 location as well.

The back to back GEM detector
In order to fit a multilayer structure into the small space available for the ME0 station, a new design of a GEM-based technology detector has been developed, referred to as a back-to-back (B2B) GEM detector, whose structure is shown in figure 2. It is composed of two independent triple-GEM detectors positioned with the anodes toward the outside and sharing the same cathode at the center of the system. This design allows us to reduce the total thickness occupied by two triple-GEM detectors. We report on the studies performed on a prototype with 10 × 10 cm 2 active area operated with Ar:CO 2 70:30 and Ar:CO 2 :CF 4 45:15:40 gas mixtures. Each triple GEM detector has a 3/1/2/1 mm spacing and is supplied through a voltage divider using the same resistor values used in the GE1/1 station [3]. The two dividers are supplied in parallel by the same high voltage supply. The electric fields of drift and amplification stages can be determined from the value of the resistors, the spacing between the foils and their thickness.
Each GEM detector has its own readout plane with 128 parallel strips and 800 µm pitch. One readout plane has strips along the x direction, the other one along the y direction. For this reason each GEM detector will be identified as X-axis GEM or Y-axis GEM (reading the x or y coordinate respectively). The readout is made through VFAT2 chips [4]; each GEM detector (or axis) is read by two VFAT2 ASICs. They have been numbered from 0 to 3, such that VFAT2-0 and VFAT2-1 read the x coordinate, VFAT2-2 and VFAT2-3 read the y coordinate. The total detector thickness is 2.64 cm.

Detector performance
The detector performance results are presented as a function of the voltage divider current. First studies have been carried out operating the detector with Ar:CO 2 gas mixture. A 109 Cd source was placed on the readout plane of the triple-GEM detector under study. The logical OR of all the strips was read. First, the net rate of observed hits is measured while varying the voltage applied to the detector; then the net current I net is measured at the same voltage. At high voltage (divider current greater than about 675 µA and 685 µA for the two gas mixtures) the rate curve reaches a plateau, corresponding to the operational range in which the detector is working at high efficiency. The average rate at the plateau R plateau is used to calculate the detector gain G at different voltage applied using the formula G = I net /(N p q 0 R plateau ); where q 0 is the elementary charge and N p is the number of electrons produced by primary ionization, calculated assuming that the ionizing radiation is the luminescence of the copper inside the detector. The rate and gain curves are shown in figure 3. The gain reached at the efficiency plateau is of the order of 10 4 .

Efficiency
The efficiency is evaluated by testing the B2B detector in the H8 muon test beam line at CERN SPS (muon momentum ∼ 150 GeV/c and intensity up to ∼ 10 4 muons/spill). The setup used three scintillators with approximately the same active area as the B2B detector to provide a beam trigger and two triple GEM detectors (trackers) with 10 × 10 cm 2 active area, operated with an Ar:CO 2 gas mixture, having 3/2/2/2 mm spacing and bidimensional readout (256 strips both in x and y direction with 400 µm pitch) for tracking. The trackers are read out by the VFAT2 chips. They provide a reconstructed muon particle track with x and y hit coordinates. An extrapolation from the reconstructed muon particle track to the B2B detector provides the expected location of the x and y hits in the B2B detector.
The efficiency of each triple-GEM is measured, along with the efficiency of their logical AND. For the efficiency calculation it is necessary to align the B2B GEM detector and the two trackers in software through a χ 2 minimization to correct the measured hits both for translational and rotational misalignment between the detectors. Then the distribution of residual distances, i.e. the distance between the predicted hit positions on B2B detector and the measured hit positions, is built both for the X and Y-axis GEM. Such distributions are fitted with a gaussian function, whose standard deviations -σ x and σ y -have been used to evaluate the maximum tolerated distance of measured hits on B2B detector from the position predicted by trackers. For the efficiency calculation events are accepted only if the residual distances x res < 3σ x and y res < 3σ y . For the calculation of the efficiency of the logic AND of the two triple-GEM detectors both conditions are required. The results for the two considered gas mixtures are shown in figure 4. A high detection efficiency has been observed in both cases: between 96.5% and 98.1% for a single triple GEM and about 94% for their logical AND.

Time resolution
The time resolution has been measured in the muon beam with the setup described in section 3.2. For the timing measurements the first signal after the trigger pulse satisfying a predefined logic condition has been used. Because four VFAT2 chips (0, 1, 2, 3) are reading the B2B detector (see section 2), the conditions 0 ∨ 1, 2 ∨ 3 and (0 ∧ 2) ∨ (1 ∧ 2) ∨ (0 ∧ 3) ∨ (1 ∧ 3) have been used. 1 The first condition uses only signals from the X-axis GEM, the second condition from the Y-axis GEM. Dividing the surface of the detector in four quadrants, the third logic is an AND between X and Y in each sector, plus an OR of the four quadrants. The three logic conditions are designated in the plots as VFATX, VFATY and all VFATs respectively. The VFAT2 have a 40 MHz signal sampling that broadens the time distribution of signals. This has been taken into account by fitting the measured data to a gaussian distribution convolved with a rectangle function (25 ns width). The detector's time resolution has been taken as the standard deviation of the gaussian evaluated through the fit. As shown in figure 5, time resolutions of the order of 7 ns and 6 ns are measured with Ar:CO 2 and Ar:CO 2 :CF 4 gas mixtures respectively.

Summary.
The Back-to-Back (B2B) GEM detector was studied for a potential upgrade of the CMS muon system at high pseudorapidity, in a region that is not currently instrumented where a small space less than 30 cm thick will become available. A detector with multilayer structure, high rate capability, good time and spatial resolution is necessary.
The B2B detector is composed of two triple-GEM detectors sharing the same cathode, with anodes towards the outside.
A gain up to ∼ 10 4 has been measured with a 109 Cd source. Time resolutions up to 6 ns (Ar:CO 2 :CF 4 gas mixture) and 7 ns (Ar:CO 2 ), and efficiency between 96.5% and 98.1% for each triple-GEM were measured with muon beam.