ATLAS Phase-II Upgrade

ATLAS Phase-II Upgrade
Name	ATLAS Phase-II Upgrade
Date	2024–
Location	CERN, Meyrin
Type	Particle detector upgrade
Participants	CERN; ATLAS Experiment collaboration; European Organization for Nuclear Research; United States Department of Energy; National Science Foundation (United States); Deutsches Elektronen-Synchrotron; Institute of High Energy Physics (Beijing); KEK; INFN; CNRS; STFC; TRIUMF

ATLAS Phase-II Upgrade is the comprehensive program to prepare the ATLAS Experiment at the Large Hadron Collider for operation at the High-Luminosity LHC era. The upgrade replaces and augments detector subsystems, electronics, and computing to cope with higher instantaneous luminosity, increased pile-up, and harsher radiation environments anticipated after the High-Luminosity Large Hadron Collider project. It is coordinated across multidisciplinary teams from national laboratories and universities including major partners such as CERN, Fermilab, and DESY.

Overview and Motivation

The upgrade responds to projections from the High-Luminosity Large Hadron Collider program and studies by the European Strategy for Particle Physics and the Particle Physics Project Prioritization Panel. Design drivers include sustaining physics goals like precision measurements of the Higgs boson, searches for supersymmetry, and sensitivity to dark matter signatures under >140 pile-up conditions. Requirements derive from simulation frameworks developed at CERN and validated using test beams at facilities such as CERN SPS, SLAC National Accelerator Laboratory, and KEK, while governance aligns with frameworks from the International Committee for Future Accelerators and oversight from funding agencies including the European Commission and U.S. Department of Energy.

Detector Upgrades

Major hardware changes span tracking, calorimetry, muon systems, and forward detectors. The new silicon tracker replaces the present inner detector and integrates innovations from 3D silicon sensors R&D collaborations and tests at DESY and MIPAS. The calorimeter front-end electronics are upgraded building on designs tested at CERN TTC and integrating radiation-hard ASICs from partnerships with IRFU and Brookhaven National Laboratory. The muon spectrometer receives improvements including new small-strip chambers inspired by developments at SLAC and NSCL; forward proton tagging and timing systems are enhanced drawing on expertise from TOTEM and AFP. Cooling, power distribution, and mechanical supports adopt techniques from ATLAS IBL trials and lessons from CMS Phase-1 work, while radiation monitoring uses sensors developed in collaboration with IN2P3 and IHEP.

Trigger and Data Acquisition

Trigger and DAQ are redesigned to manage a 1 MHz or higher Level-0 accept rate and an event-builder capable of multi-TB/s throughput. The architecture integrates high-speed links, FPGA-based preprocessing boards developed in cooperation with Xilinx partners, and commodity computing racks modeled after systems used by LHCb and ALICE. The real-time selection strategy adds hardware-based tracking triggers inspired by prototypes from ATLAS Fast TracKer initiatives and leverages machine learning primitives tested on Google TPU and high-performance clusters at NERSC. Synchronization, timing, and clock distribution draw on protocols from White Rabbit and timing systems validated at CERN BE-OP.

Physics Performance and Projected Reach

Upgraded detector performance enables substantial improvements in precision and discovery potential. Higgs coupling measurements to top quark and tau lepton channels, rare decay searches such as H→μμ, and differential cross-section determinations benefit from enhanced b-tagging and tracking resolution. Sensitivity to beyond Standard Model scenarios including supersymmetry, extra dimensions, and heavy resonances is extended by higher integrated luminosity and improved forward coverage; projections parallel studies by the European Strategy Group and phenomenology groups at CERN TH, Fermilab Theory Division, and DESY Theory. Flavor and electroweak measurements using vector-boson scattering and diboson channels also gain from upgraded muon chambers and timing detectors, matching analyses frameworks used in publications by collaborations at Princeton University, MIT, and University of Oxford.

Project Management and Timeline

The program follows staged milestones aligned with the HL-LHC accelerator upgrade schedule and governance models similar to large international projects like ITER and SKA. Technical design reports and milestone reviews are overseen by panels including representatives from CERN Council, national funding agencies, and external reviewers from DOE Office of Science. Integration and installation phases are planned for long shutdown windows such as Long Shutdown 3, with commissioning coordinated with accelerator teams from CERN Accelerator groups and machine protection experts. Risk management, quality assurance, and safety draw on standards employed by European XFEL and industrial partners across Switzerland and Germany.

Funding, Collaboration, and Infrastructure

Funding and in-kind contributions come from a broad consortium of national agencies and laboratories including DOE, NSF, INFN, CEA, Rutherford Appleton Laboratory, and others. Collaboration governance uses memoranda of understanding modeled after multi-institution projects like LIGO and IceCube, while computing demands are met by grid and cloud resources coordinated via WLCG and national centers such as CERN IT, GridPP, and NorduGrid. Civil engineering, cryogenics, and electrical infrastructure rely on CERN host-facilities and partnerships with regional utilities and contractors experienced in projects like LEP and PS. The upgrade thus represents a multinational enterprise spanning advanced detector R&D, accelerator coordination, and global resource mobilization.

Category:ATLAS