CMS Phase-1 Upgrade

CMS Phase-1 Upgrade
Name	CMS Phase-1 Upgrade
Location	CERN
Start	2013
Completed	2017
Operator	European Organization for Nuclear Research
Project type	High-energy physics detector upgrade

CMS Phase-1 Upgrade

The CMS Phase-1 Upgrade was a major modernization of the Compact Muon Solenoid detector at the Large Hadron Collider, carried out between Long Shutdown 1 and Run 2 operations. It replaced and augmented critical subsystems to cope with increased instantaneous luminosity delivered by the Large Hadron Collider and to maintain the physics reach of CMS in searches tied to the Standard Model, Higgs boson, and beyond-the-Standard-Model scenarios such as supersymmetry and dark matter. The project involved collaborations among the European Organization for Nuclear Research, national laboratories such as Fermi National Accelerator Laboratory and DESY, and universities worldwide including MIT, University of California, San Diego, and Imperial College London.

Introduction

The upgrade targeted CMS subsystems that were most affected by pileup and radiation during higher-luminosity runs at the Large Hadron Collider. CMS has a long heritage connected to experiments like ATLAS (experiment), ALICE (A Large Ion Collider Experiment), and LHCb, and its Phase-1 program built on lessons from Run 1 operations and the CMS Silicon Tracker commissioning. The work spanned hardware, firmware, and software contributions from institutions such as CERN Experimental Physics Department, Brookhaven National Laboratory, and Lawrence Berkeley National Laboratory.

Motivation and Objectives

The primary motivation was to ensure CMS could operate at pileup conditions exceeding those at the Higgs boson discovery while preserving trigger efficiency and reconstruction performance needed for precision measurements tied to the Cabibbo–Kobayashi–Maskawa matrix and searches for phenomena predicted by grand unified theory-inspired models. Objectives included replacement of radiation-sensitive components, extension of acceptance in forward regions relevant for processes studied at ATLAS, improved timing and granularity useful for analyses like top quark production and B physics at LHCb, and upgrades to the trigger architecture to match developments in Field-programmable gate array technology deployed in projects such as HL-LHC planning.

Detector Upgrades

Key hardware changes included a complete redesign of the CMS pixel detector with a new 4-layer silicon detector barrel and 3-disk endcap geometry to replace the original 3-layer system, improving vertexing for analyses like H → γγ and ttbar reconstruction. The new pixel system integrated radiation-hard readout chips derived from work at Paul Scherrer Institute and CERN Microelectronics Group, and cooling innovations influenced by ATLAS Insertable B-Layer studies. The forward muon system saw additions of Gas Electron Multiplier detectors developed in collaboration with groups such as University of Maryland and INFN, while the hadron calorimeter received upgraded photodetectors and electronics inspired by NOvA and DUNE developments. The ECAL and Silicon Tracker firmware were revised to exploit firmware frameworks used at Fermilab test stands.

Trigger and Data Acquisition Enhancements

Trigger upgrades included expansion of the Level-1 trigger capabilities and replacement of the Global Trigger electronics to handle higher input rates compatible with the LHC luminosity profile. The new readout architecture used modern Field-programmable gate array platforms and high-speed optical links similar to systems developed for Belle II and ATLAS IBL upgrades, enabling greater regional granularity and improved selection for signatures like missing transverse energy and displaced vertices relevant to supersymmetry. The Data Acquisition (DAQ) chain was reworked with commodity computing and switched-fabric networks, integrating software frameworks from projects at CERN IT and high-throughput designs tested at GridPP and Open Science Grid sites.

Commissioning and Integration

Commissioning spanned laboratory tests at institutions such as CERN, Brookhaven National Laboratory, and DESY followed by installation during Long Shutdown 1 and early commissioning with cosmic-ray runs and beam splash events from the LHC injector complex. Integration required alignment with the CMS offline software framework developed by collaborations including FNAL and University of California, Berkeley, and synchronization with LHC timing systems maintained by CERN Accelerator Physics teams. Detector control and safety systems were validated using procedures influenced by ATLAS Detector Control System practices.

Performance and Early Results

Post-upgrade performance demonstrated improved vertex resolution, higher tracking efficiency at high pileup, and better muon identification in forward regions, directly benefiting measurements of processes such as Higgs boson coupling fits and searches for vector-like quarks. Early Run 2 results showed sustained trigger rates with acceptable latency, enabling studies published by collaborations including CMS Collaboration and joint analyses with ATLAS Collaboration. Radiation tolerance and operational stability were confirmed in comparisons to Monte Carlo predictions used by analysis groups at CERN Theory Division and external institutes.

Project Management and Timeline

The Phase-1 Upgrade followed a phased schedule coordinated across institutions including CERN, Fermilab, INFN, and national funding bodies such as National Science Foundation (United States) and European Commission. Milestones included technical design reviews, production runs at centers like DESY and University of Kansas, installation during Long Shutdown 1, and commissioning entering Run 2 operations. Governance combined the CMS Collaboration management board, subsystem project leaders, and review committees modeled after practices at CERN Research Board, ensuring delivery within the constraints set by accelerator upgrades and physics priorities.

Category:Compact Muon Solenoid upgrades