Efficiency measurements of GEM GE1/1 chambers in the upgraded CMS Endcap Muon System using 2023 collision data at $\sqrt{s}=13.6$ TeV

Technical Summary: Efficiency Measurements of GEM GE1/1 Chambers in the Upgraded CMS Endcap Muon System

Problem and Context
The Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC) is undergoing upgrades to sustain performance under the High Luminosity LHC (HL-LHC) conditions, characterized by increased instantaneous luminosity and significant pile-up. The forward region of the muon system (pseudorapidity $1.55 < |\eta| < 2.4$) faces high particle fluxes and background rates that complicate muon reconstruction and trigger rates. To address this, the GE1/1 Gas Electron Multiplier (GEM) detector station was installed during Long Shutdown 2 (2018–2022) to complement the existing Cathode Strip Chambers (ME1/1). While the system was commissioned, a comprehensive evaluation of its detection efficiency using collision data from the HL-LHC era (Run 3) was required to validate its readiness for future operations.

Methodology
This study analyzes proton-proton collision data collected in 2023 at a center-of-mass energy of $\sqrt{s} = 13.6$ TeV, corresponding to an integrated luminosity of 17.8 fb$^{-1}$. The dataset was enriched with muons originating from Z boson decays.

The efficiency measurement employed a "back-propagation" technique to ensure the GE1/1 detectors were not used in the track reconstruction that defined the muon candidates, thereby avoiding circularity. The methodology proceeded as follows:

1. Muon Selection: High-purity global muons were selected using standard Run 3 reconstruction algorithms. Selection criteria included reconstruction as a global muon with a particle flow algorithm, tracker tracks with hits in at least six layers (including one pixel hit), segment matching in at least two muon stations, and tight impact parameter requirements relative to the primary vertex ($|d_{xy}| < 0.2$ cm, $|d_z| < 0.5$ cm).
2. Track Re-fitting: Selected global muon tracks were re-fitted after removing all GE1/1 hits.
3. Back-Propagation: These GE1/1-free tracks were propagated backward from the outermost muon stations through the magnetic field to predict their intersection points with the GE1/1 chambers.
4. Matching: A GE1/1 hit was considered a match if it fell within $\pm 4$ cm of the predicted position in the $R\Delta\phi$ coordinate.
5. Efficiency Calculation: Efficiency was defined as the ratio of tracks with matched hits to the total number of tracks. This was calculated for 137 operational chambers, individual VFAT3 readout chips, and subsets of chambers based on high voltage (HV) stability and the presence of electrical shorts.

Key Results

• Overall Efficiency: The average detection efficiency across 137 measured GE1/1 chambers is approximately 93.3%.
• Optimal Performance: A subset of 108 chambers operated at their nominal high voltage (HV) working point showed an average efficiency of ~96%. When further restricted to 78 chambers that had no electrical shorts and no mechanical defects (such as PCB warping), the average efficiency remained at ~96%, satisfying the performance benchmarks set in the GEM Technical Design Report.
• Impact of Short Circuits: Electrical shorts significantly degrade performance. Chambers with no shorts achieved ~96% efficiency, while those with one short dropped to ~89%, two shorts to ~79%, and three shorts to ~69%.
• Pile-Up Dependence: The study found that detection efficiency is independent of the number of reconstructed vertices per event (pile-up), demonstrating robustness against the high interaction rates expected in the HL-LHC era.
• Operational Issues: Seven chambers were turned off during data taking. Additionally, some regions showed reduced efficiency due to operational issues, primarily the outgassing of adhesive materials on opto-hybrid boards causing optical connection failures.

Significance and Claims
The paper presents the first comprehensive report on GE1/1 efficiency using Run 3 collision data, representing the first full-system evaluation since the 2018 slice test. The results validate the readiness of the GE1/1 system for HL-LHC operation, confirming that the detectors can maintain high efficiency and stability under high pile-up conditions. The study highlights that while the system performs well overall, specific issues such as electrical shorts and HV instabilities require mitigation. The authors note that to address inefficiencies caused by optical connection failures and other issues, all chambers are scheduled to be extracted and refurbished during Long Shutdown 3 (2026–2030). The work confirms that the GE1/1 station successfully aids in muon reconstruction and level-1 trigger management in the forward region.