CMS data acquisition system

CMS data acquisition system
Name	CMS data acquisition system
Established	2008
Location	CERN
Type	Particle physics instrumentation

CMS data acquisition system The CMS data acquisition system is the ensemble of hardware and software that records proton–proton collision data produced by the Large Hadron Collider and routed from the Compact Muon Solenoid detector into archival storage and analysis facilities. It integrates electronics, network fabrics, real-time selection, and control frameworks developed in collaboration with institutions such as CERN, Fermilab, DESY, and SLAC National Accelerator Laboratory to meet the throughput demands driven by high-luminosity running and upgrades like the High-Luminosity Large Hadron Collider. The system interfaces with worldwide computing grids including the Worldwide LHC Computing Grid and regional centers like CERN Tier-1 to distribute physics events for reconstruction and analysis.

Overview

The CMS data acquisition system receives analog and digital signals from subdetectors including the Silicon Tracker, Electromagnetic Calorimeter, Hadron Calorimeter, and Muon system and converts them into formatted event fragments via front-end electronics such as the APV25 and QIE readout chips. It must cope with bunch crossings from the Large Hadron Collider bunch structure and with trigger decisions derived from the Level-1 trigger and High-Level Trigger, while coordinating with timing systems like the Beam Synchronous Timing and White Rabbit where deployed. The design balances low-latency requirements with integration to offline processing chains used by experiments such as ATLAS experiment, LHCb experiment, and ALICE experiment.

Architecture and Components

Physical and logical architecture deploys crates, custom backplanes, and commercial network switches sourced from manufacturers used by CERN procurement. Key hardware includes custom PCIe-based cards, field-programmable gate arrays such as the Xilinx Virtex family, and processor farms built on servers like those from Dell Technologies and Hewlett Packard Enterprise. Readout uses standards including S-LINK, PCIe, and 10 Gigabit Ethernet to transport event fragments to builder units; slow control leverages EPICS and SCADA-style interfaces. Project partners and institutions involved in hardware and firmware development include University of California, Berkeley, Imperial College London, Université Paris-Sud, and INFN divisions.

Trigger System

The trigger system is hierarchical: a hardware-based Level-1 trigger implemented with custom electronics and look-up tables reduces the 40 MHz collision rate to an accept rate compatible with downstream processing, while a software-based High-Level Trigger farm performs complex reconstruction using algorithms adapted from offline frameworks like CMSSW. The Level-1 system uses inputs from drift tubes, Cathode Strip Chambers, and calorimeter trigger primitives; firmware teams reference technologies from MicroTCA and AdvancedTCA standards. HLT nodes run on processor architectures including Intel Xeon and utilize libraries such as ROOT for data handling, with trigger menus coordinated alongside physics groups like CMS Electroweak Group and CMS Higgs Group.

Data Flow and Event Building

Event fragments are aggregated by readout units into complete events using event builder networks and switch fabrics, employing protocols and topologies similar to those in high-performance computing centers like CERN Data Centre. Event building farms instantiate multiple stages: readout, assembly, filtering, and streaming to disk or tape systems such as CASTOR and EOS, before transfers to the Worldwide LHC Computing Grid. Flow control interacts with the Timing, Trigger and Control system to maintain synchronization and to avoid buffer overflows under pileup conditions resembling those studied by pileup analyses. Data quality monitoring integrates with services used by collaborations including ATLAS and experiments at facilities like Fermilab.

Performance and Scalability

Performance metrics include sustained throughput in gigabytes per second, per-event latency, and deadtime fraction; design targets have evolved with upgrades inspired by projects such as the High-Luminosity Large Hadron Collider upgrade program and lessons from Run 1 and Run 2 operations. Scalability is achieved via modular farms, rack-level expansion, and use of commodity switches from vendors employed by data centers like Google Data Centers and Amazon Web Services for prototype testing. Stress tests use emulators and test beams coordinated with institutions like CERN SPS and DESY test beam facilities to validate behavior under simulated conditions drawn from studies by CMS computing and trigger performance groups.

Software and Control Systems

Control and configuration use the Run Control framework, integrating with CMSSW for reconstruction workflows and with databases such as Oracle and CERN IT Database for conditions data. Monitoring stacks employ telemetry tools comparable to Prometheus and visualization via frameworks akin to Grafana. Code repositories and continuous integration practices draw on platforms like GitHub and GitLab, and software certification follows processes used by collaborations including CMS Collaboration and supported by institutions like FNAL. Calibration and alignment workflows interface with offline teams involved in physics analysis and legacy software like COBRA where relevant.

Operations and Maintenance

Operational responsibilities are distributed among shifts staffed by members from universities and labs such as Princeton University, University of Cambridge, RWTH Aachen University, and INFN Sezione di Roma. Maintenance cycles coordinate with accelerator operations by the LHC operations team and with maintenance windows scheduled by CERN management. Upgrades and obsolescence mitigation rely on consortiums including HL-LHC Collaboration and hardware replacement strategies modeled after large-scale experiments like ATLAS and major observatory projects. Training, documentation, and incident response use procedures aligned with safety and quality standards adopted by CERN and partner institutions.

Category:High-energy physics instrumentation