ATLAS Phase-II

ATLAS Phase-II
Name	ATLAS Phase-II
Facility	CERN
Collaboration	ATLAS experiment
Location	Meyrin
Status	Planned / Ongoing
Start	2015
Expected completion	2026
Type	Particle detector upgrade

ATLAS Phase-II ATLAS Phase-II is the major upgrade campaign for the ATLAS experiment at CERN's Large Hadron Collider aimed at preparing the detector for the High-Luminosity LHC era associated with the High-Luminosity Large Hadron Collider project. The programme coordinates hardware, firmware, and software improvements across subsystems to maintain performance in the face of increased instantaneous luminosity and integrated luminosity targets set by the European Strategy for Particle Physics and the Particle Physics Project Prioritization Panel. It involves international institutions including SLAC National Accelerator Laboratory, Brookhaven National Laboratory, Fermilab, DESY, and numerous universities.

Overview

Phase-II is an ensemble of upgrades to tracking, calorimetry, muon detection, and the trigger system to ensure the ATLAS experiment can record and analyze collisions at the projected High-Luminosity LHC peak luminosity of 5–7.5×10^34 cm^−2 s^−1. The work aligns with accelerator upgrades such as magnet and injector improvements at CERN and with global programs at LHCb, CMS experiment, and other facility projects. Funding, procurement, and construction are coordinated with national agencies like the European Commission, U.S. Department of Energy, and national research councils.

Upgrade Motivations and Objectives

Primary motivations derive from physics drivers: precision measurements of the Higgs boson properties, searches for supersymmetry, characterization of dark matter candidates, and sensitivity to rare processes predicted by extensions to the Standard Model (particle physics). Objectives include coping with up to 200 simultaneous proton–proton interactions per bunch crossing (pile-up), preserving tracking efficiency in the inner detector region, improving b‑tagging for heavy‑flavor identification, and maintaining muon and calorimeter resolution for electroweak and beyond‑Standard Model searches. The goals map to recommendations from reviews by CERN Council, the Particle Physics Community Planning Exercise, and advisory panels including the European Research Council.

Detector Upgrades

The centerpiece is a new all‑silicon inner tracker combining pixel and strip technologies, replacing the present Inner Detector to provide extended coverage and radiation tolerance. Pixel developments leverage 3D sensors and thin‑planar designs tested at facilities such as CERN SPS and DESY test beam. Strip detectors employ advanced module designs and radiation‑hard electronics developed in collaboration with institutes like Nikhef and INFN. Calorimeter readout electronics for the ATLAS Liquid Argon Calorimeter and tile calorimeter are being upgraded to fully digital systems to provide continuous waveform sampling and higher granularity. The muon spectrometer receives new small‑diameter muon chambers and replacements for detectors in the forward region influenced by studies at TRIUMF and KEK. Cooling, powering, and mechanical supports are being redesigned with contributions from Imperial College London, University of Chicago, and University of Oxford.

Trigger and Data Acquisition

A radical redesign of the trigger and data acquisition (TDAQ) chain moves to a two‑level trigger architecture with a hardware level‑0 and a high‑level software trigger running on commodity compute clusters. The upgrade expands bandwidth, allowing full‑granularity calorimeter and muon information at early trigger stages to reduce thresholds for photons, electrons, and taus. Firmware and firmware‑accelerated algorithms incorporate developments from ATLAS Tile Calorimeter R&D, collaborations with industrial partners, and algorithms benchmarked on processors from Intel Corporation and NVIDIA. Dataflow and storage strategies coordinate with the Worldwide LHC Computing Grid and national computing centers such as CERN IT and NDGF.

Commissioning and Installation

Installation follows a staged schedule to coincide with LHC long shutdowns, notably the Long Shutdown 3 interval. Pre‑installation commissioning uses test stands at CERN Meyrin site, beam tests at the CERN SPS and DESY II test beam, and system integration exercises involving teams from University of Manchester, University of Tokyo, and Massachusetts Institute of Technology. Logistics include cavern work in the CERN Meyrin complex, cryogenic interfaces, and strict radiation safety coordination with the International Atomic Energy Agency guidelines. Detector module production, shipping, and on‑site assembly are tracked through shared tools adopted from large projects like ITER and SKA Observatory.

Physics Impact and Performance Projections

Simulations incorporating upgraded detector elements and TDAQ predict substantial gains: improved mass resolution for Higgs boson decays to photons and Z bosons, enhanced sensitivity to heavy resonances predicted by Grand Unified Theories and extra dimensions scenarios, and extended reach for long‑lived particle searches advocated by theory groups at CERN Theory Division and university collaborations. Projections estimate precision measurements of Higgs couplings competitive with proposed future colliders such as the International Linear Collider and Compact Linear Collider, while enabling complementary discovery potential to experiments like Belle II and IceCube Neutrino Observatory.

Collaboration and Project Timeline

The upgrade is executed by the international ATLAS Collaboration comprising hundreds of institutions across Europe, the Americas, Asia, Africa, and Oceania. Governance integrates institutional boards, technical coordination from CERN management, and funding agency oversight from bodies including DOE Office of Science and national agencies such as STFC and CNRS. Milestones include design reviews, production ramps, integration tests, and installation windows aligned with LHC scheduling authorities. Community workshops and conferences—such as the International Conference on High Energy Physics and CHEP—serve to disseminate technical results and schedule updates.

Category:ATLAS experiment Category:High-Luminosity Large Hadron Collider