The CMS ME0 Upgrade: Enhancing Forward Muon Reconstruction at the HL-LHC

Abstract

The CMS muon system is undergoing substantial upgrades to meet the challenges of the High-Luminosity LHC (HL-LHC), including the installation of the new Muon Endcap 0 (ME0) detector. Large-scale production started in 2024. ME0 is a six-layer station designed to extend pseudo-rapidity coverage to |η| = 2.8 from the previous maximum of |η| = 2.4, enhancing sensitivity to forward physics processes. Each endcap will host 18 ME0 stacks, with each stack comprising six triple-layer gas electron multiplier (GEM) chambers. The system adds up to six additional hits per track, which significantly improves muon identification, spatial resolution, and robust track reconstruction at the first trigger level. Chamber production and quality control across multiple international sites ensure scalability and timely delivery. The ME0 design incorporates lessons learned from earlier GEM deployments, with improvements in electronics robustness, grounding, and segmentation to withstand high background rates and minimize damage from discharges. This contribution provides a comprehensive overview of the ME0 detector concept, assembly strategy, quality assurance procedures, current production status, and its pivotal role in strengthening CMS muon reconstruction during HL-LHC operations.

Figures (4)

• Figure 1: Layout of the CMS muon endcap region showing GE1/1, GE2/1, and ME0 stations, extending muon coverage up to $|\eta| \approx 2.8$.
• Figure 2: Quality control results for ME0 module production. (Left) Gas leak measurements for two ME0 modules, showing the exponential decrease of the over-pressure modeled as $P(t) = P_{0} e^{-t/\tau}$. The leak-rate parameter $\tau$ quantifies the pressure-decay timescale. The two modules shown have passed the test with measured values of $\tau = 4.36$ h and $11.14$ h and (Right) High-voltage divider test showing linearity between applied voltage and divider current with nominal and measured resistance values of the HV divider, respectively $R_{n} = 5.0~\mathrm{M\Omega}$ and $R_{m} = 4.95~\mathrm{M\Omega}$.
• Figure 3: ME0 performance results: (Left) Efficiency as a function of hit rate measured at GIF++, showing stable performance above 97% with fitted $\tau$ between 98--182 ns DP119 performed in the ABS (Aging and Beam Studies) setup and (Right) Gain stability during aging measurements, where the integrated charge corresponds to the expected exposure in the hottest $\eta$ region GEMPublic.
• Figure 4: Timing resolution of single ME0 layers and full stack measured with cosmic tests DP089.