CMS Tracker Upgrade

CMS Tracker Upgrade
Name	CMS Tracker Upgrade
Location	CERN
Start	2015
Completion	2024
Participants	Compact Muon Solenoid, Large Hadron Collider, European Organization for Nuclear Research, Fermi National Accelerator Laboratory, Deutsches Elektronen-Synchrotron, National Institute of Standards and Technology, INFN, CERN Medical Applications
Type	Particle detector upgrade

CMS Tracker Upgrade

The CMS Tracker Upgrade is a major modernization of the silicon tracking system of the Compact Muon Solenoid detector at CERN carried out for the High-Luminosity Large Hadron Collider era. The project replaced the original pixel and strip trackers with a new all-silicon system designed to operate in the higher radiation, higher pileup environment planned for the Large Hadron Collider luminosity upgrades. The upgrade was conceived and executed by an international collaboration including institutions such as Fermi National Accelerator Laboratory, Deutsches Elektronen-Synchrotron, INFN, and many universities that contribute to CMS subsystem construction and commissioning.

Overview

The upgraded tracker is a modular, radiation-hard, low-mass silicon system comprising an inner pixel detector and an outer tracker with novel sensor geometries; it interfaces with the CMS trigger and data acquisition systems and the accelerator complex at CERN. The new detector replaces legacy components to meet the performance requirements driven by the High-Luminosity Large Hadron Collider project and the extended physics program of the Large Hadron Collider. Key elements include hybrid pixel modules, macro-pixel and strip modules, high-speed optical links, and enhanced cooling and support structures coordinated with CMS subsystems such as the Electromagnetic Calorimeter and Hadron Calorimeter.

Motivation and Objectives

The principal motivation was to maintain tracking resolution and efficiency under conditions projected for the High-Luminosity Large Hadron Collider, where instantaneous luminosity increases from early Large Hadron Collider runs would produce far higher pileup and radiation fluence. Objectives included preserving vertexing and b-tagging performance crucial for analyses from the ATLAS–CMS precision electroweak program to exotic searches, enabling robust track reconstruction for triggers used by collaborations such as CMS Collaboration and interfacing with global projects like the European Strategy for Particle Physics. The upgrade targeted improvements in radiation tolerance to levels tested at facilities like CERN PS and Brookhaven National Laboratory, and aimed to reduce material budget to limit multiple scattering impacting measurements like the Higgs boson coupling determinations.

Design and Technology

Design choices emphasized radiation-hard silicon planar sensors, 3D sensor options for the innermost layers, CMOS readout ASICs compatible with the RD53 family, and high-bandwidth optical transmission using components qualified in test beams at CERN and Fermilab Test Beam Facility. Mechanical supports use carbon-fiber composites and lightweight cooling based on CO2 evaporative systems inspired by implementations in experiments such as LHCb and ALICE. The pixel system features fine pitch sensors bump-bonded to readout chips developed within collaborations tied to EPFL and University of California, Santa Cruz, while the outer tracker employs pT modules with on-board pattern recognition to provide seeds for the Level-1 trigger and to reduce data rates for the DAQ chain.

Construction and Installation

Construction followed distributed production across labs including Fermi National Accelerator Laboratory, Deutsches Elektronen-Synchrotron, Istituto Nazionale di Fisica Nucleare, and university groups at University of Bristol, MIT, University of California, Berkeley; components underwent acceptance testing at facilities such as CERN integration centers and regional test sites. Installation phases were synchronized with long shutdowns of the Large Hadron Collider; precise alignment used laser survey techniques and metrology tools common in projects at SLAC National Accelerator Laboratory and KEK. Integration required coordination with major CMS upgrades including calorimeter electronics and magnet services, and compliance with safety and cryogenic interfaces overseen by CERN engineering groups.

Performance and Commissioning

Commissioning comprised detector readout checkout, timing calibration, alignment with tracks from cosmic-ray campaigns, and first-beam synchronization during Run 3 and early High-Luminosity Large Hadron Collider commissioning periods. Performance metrics targeted spatial resolution, hit efficiency, fake-rate suppression, and radiation-induced noise levels validated against benchmarks from test beams at DESY and irradiation campaigns at TRIUMF. Results demonstrated improved track reconstruction in high-pileup simulations used by physics groups studying processes such as Top quark production, Supersymmetry searches, and precision Higgs boson measurements.

Operation and Maintenance

Operational procedures emphasize remote monitoring via centralized control rooms at CERN with diagnostic tools developed in collaboration with groups at Université de Genève and ETH Zurich. Maintenance strategies incorporate modular replacement of faulty components during scheduled shutdowns and use of in-situ annealing and calibration programs informed by studies at radiation facilities like Ion Beam Center Darmstadt. Spare-component inventories and quality-assurance databases are managed by consortium laboratories including Brookhaven National Laboratory and Lawrence Berkeley National Laboratory to ensure rapid intervention and minimal downtime.

Impact on Physics Program and Upgrades Timeline

The tracker upgrade is critical to realizing the physics goals of the High-Luminosity Large Hadron Collider and to maintaining CMS competitiveness with projects such as ATLAS and future colliders contemplated in the European Strategy for Particle Physics. By preserving tracking performance under extreme conditions, the upgrade enhances sensitivity to rare processes, precision measurements of the Higgs boson, flavor physics involving the Bottom quark, and searches for beyond-standard-model signatures investigated by international collaborations. The timeline aligned with LHC long shutdowns and the staged High-Luminosity Large Hadron Collider installation schedule to facilitate continuous operation of the Compact Muon Solenoid experiment.

Category:Particle detectors Category:Compact Muon Solenoid Category:CERN projects