Progress on the development of the Compact Muon Solenoid barrel minimum ionizing particle timing detector

This paper presents a detailed overview of the Barrel Timing Layer (BTL), the subsystem of the Minimum Ionizing Particle Timing Detector (MTD) in the Compact Muon Solenoid (CMS) experiment at CERN, focusing on its physics motivation, detector design, performance verification, and construction progress for the High-Luminosity Large Hadron Collider (HL-LHC) era. The HL-LHC, scheduled to start in 2030, will deliver an integrated luminosity of approximately 3000 fb−1 over a decade, with peak instantaneous luminosity leading to 140–200 proton–proton interactions per bunch crossing, creating extremely challenging pileup conditions for traditional event reconstruction algorithms. To address these challenges and maintain high-precision reconstruction performance, CMS introduced the MTD in the Phase-II upgrade, adding a precise timing dimension to the conventional spatial measurements of tracks and vertices from each collision. MTD aims to provide time-of-flight (TOF) measurements for minimum ionizing particles (MIPs) with a time resolution of 30–60 ps, enabling pileup discrimination utilizing the time information and the four-dimensional vertex and particle-flow reconstruction. The MTD consists of the BTL covering |η|η|2 cooling together with Thermo-Electric Coolers (TECs) on the SiPMs to maintain the detector at approximately −45°C during operation, and at 60°C for periodic thermal annealing during technical stops to partially restore radiation damage. Beam tests at the CERN SPS H8 line, including both unirradiated and irradiated modules, validated the final BTL design, showing an initial time resolution of ~25 ps, degrading to ~55 ps after lifetime-equivalent irradiation, consistent with model predictions. Systematic studies of SiPM cell size, LYSO:Ce crystal thickness, operating voltage, and incident angle dependence confirmed the final design choices and demonstrated that the timing performance meets design expectations across the entire coverage. BTL has now entered large-scale production and assembly at multiple international centers, using standardized tooling, procedures, and QA/QC to guarantee high quality, interchangeability, and traceability. The first full trays have been delivered and passed a variety of acceptance tests performed at CERN. In summary, MTD BTL is critical for maintaining high-precision event reconstruction under extreme HL-LHC pileup conditions, and its mature timing performance, radiation tolerance, and large-scale construction readiness provide strong assurance for physics operations in the coming decade while offering valuable experience for next-generation high-precision timing detectors.