Speaker
Description
The CMS Electromagnetic Calorimeter (ECAL) barrel is being upgraded for HL-LHC operation while preserving its existing PbWO₄ crystals and APDs. The readout electronics have been fully redesigned to sustain trigger rates up to 750 kHz, increased latency, higher radiation levels, and improved timing performance. New front-end ASICs provide dual-gain amplification, 12-bit digitization at 160 MS/s, gain selection, and lossless data compression, while trigger processing is moved off-detector to FPGA-based back-end boards. Additional upgrades include thermal operation near 8°C and renewed services and safety systems. The status of electronics production, quality assurance, system integration, and LS3 installation plans will be presented.
Summary (500 words)
The CMS Electromagnetic Calorimeter (ECAL) barrel is being upgraded to maintain its precision energy and to improve timing performance during High-Luminosity LHC operation. Rather than replacing the detector crystals and photodetectors, the project preserves the existing 61,200 PbWO₄ crystals and avalanche photodiodes (APDs), while completely redesigning the readout, trigger, services, and environmental control systems. This approach minimizes detector risk while delivering the bandwidth, latency, and radiation tolerance required for operation through the HL-LHC era.
The principal performance drivers are the increase of Level-1 trigger accept rate from 100 kHz to 750 kHz, trigger latency growth to 12.5 μs, and operation in pileup conditions approaching 200 interactions per bunch crossing. These constraints require continuous high-rate digitization, larger buffering, and significantly improved rejection of anomalous APD signals (“spikes”).
The new front-end architecture is based on custom radiation-tolerant ASICs. A dual-gain transimpedance amplifier provides low-noise signal conditioning over a wide dynamic range, followed by a dual-channel 12-bit ADC operating at 160 MS/s. On-chip gain selection and lossless data compression reduce the output bandwidth to match optical transmission constraints while preserving physics performance. Trigger primitive generation and buffering are transferred off-detector to FPGA-based back-end processors, enabling more sophisticated and upgradable algorithms than in the legacy system.
The upgraded electronics also target improved time resolution. Beam-test measurements have demonstrated timing performance at the level of a few tens of picoseconds for energetic electromagnetic showers, providing an additional handle for pileup mitigation and object identification.
Beyond electronics, detector longevity imposes important environmental upgrades. Increased APD dark current and crystal transparency loss motivate operation at lower temperature, around 8°C instead of the present ~18°C. This requires refurbishment of cooling, humidity monitoring, interlocks, and safety systems, together with reliable operation below the cavern dew point.
The full electronics replacement must be performed during Long Shutdown 3 within a constrained installation schedule inside the CMS cavern. Thirty-six barrel supermodules, each weighing several tons and containing fragile crystals, must be extracted, refurbished, tested, and reinstalled with minimal risk. Mass production quality assurance, radiation qualification, long-distance signal integrity, clock distribution, and system synchronization are central technical topics.
The upgrade combines custom ASIC development, high-speed optical links, FPGA processing, embedded control, detector safety, and large-scale integration. It is therefore a representative example of how modern particle-physics instrumentation evolves an existing precision detector into a higher-rate, timing-capable system without replacing its active material. Presenting the current status and remaining challenges will be of broad interest to the TWEPP community.