ATLAS Inner Detector Upgrade

ATLAS Inner Detector Upgrade
Name	ATLAS Inner Detector Upgrade
Location	CERN, Geneva
Affiliation	CERN, University of Oxford, University of Birmingham, Lawrence Berkeley National Laboratory
Established	Phase-II (High-Luminosity LHC) upgrade period

ATLAS Inner Detector Upgrade is the comprehensive replacement and enhancement program for the inner tracking system of the ATLAS experiment at CERN, driven by the High-Luminosity LHC Large Hadron Collider upgrade schedule. The program integrates advances in silicon sensor technology, radiation-hard electronics, and mechanical engineering to meet the physics goals of Higgs boson Higgs boson coupling measurements, searches for beyond-Standard-Model signatures such as Supersymmetry and Dark matter, and precision flavour and electroweak measurements at higher pile-up. The upgrade effort is a multinational collaboration involving institutions such as CERN, the CERN member states, the United States Department of Energy, the NSF, and major university groups from United Kingdom, Germany, France, Italy, United States, and Japan.

Background and Motivation

The High-Luminosity LHC upgrade driven by the High-Luminosity Large Hadron Collider project increases instantaneous luminosity, creating higher occupancy, radiation damage, and data rates that exceed the capabilities of the original ATLAS Inner Detector designed for the Large Hadron Collider baseline. Physics priorities from the European Strategy for Particle Physics, the Particle Physics Project Prioritization Panel, and the Physics Briefing Book demand improved tracking resolution for precision measurements of the Higgs boson, rare decays studied at experiments like LHCb and detectors such as CMS, and enhanced heavy-flavour tagging used in analyses shared with collaborations like Belle II and ATLAS. Operational constraints and lessons from previous upgrades, including the Phase-0 Upgrade and Phase-I Upgrade projects, inform requirements to maintain performance under conditions set by the HL-LHC programme.

Design Requirements and Performance Goals

Design requirements are guided by physics acceptance goals including efficient vertexing for b quark identification, robust track reconstruction in high pile-up comparable to projections from the High-Luminosity LHC, and survival under fluences benchmarked against irradiation studies from facilities such as CERN Irradiation Facility and Los Alamos National Laboratory. Performance goals specify transverse momentum resolution competitive with previous ATLAS inner tracking and improved impact parameter resolution essential for measurements of the Higgs boson self-coupling, rare decay channels, and searches for long-lived particle signatures comparable to efforts at CMS and LHCb. Requirements include material budget minimization influenced by studies from GEANT4 simulation campaigns, timing resolution limits informed by Time-of-Flight detector R&D, and integration constraints aligned with ATLAS subdetectors such as the Calorimeter and Muon Spectrometer.

Detector Technologies and Components

The ATLAS inner tracker upgrade adopts all-silicon technologies including pixel sensors and silicon strip modules similar in concept to developments at CMS and ALICE, using planar silicon sensor technologies and 3D sensors developed in collaboration with institutes such as Fondazione Bruno Kessler and FBK. Readout ASICs and front-end electronics are implemented in radiation-hard CMOS processes following R&D trajectories analogous to RD53 developments, with module hybrids and stave structures derived from prototypes evaluated at beamlines like the CERN SPS and test facilities at DESY and SLAC National Accelerator Laboratory. Cooling employs evaporative CO2 systems comparable to those used in LHCb and CMS Phase-1 Upgrade, while power distribution leverages serial powering and DC-DC conversion concepts tested in projects at University of Manchester and Lawrence Berkeley National Laboratory. Support structures and services are optimized to minimize material using carbon-fiber composites and precision machining techniques pioneered in collaborations with European XFEL and Diamond Light Source partners.

Mechanical and Services Infrastructure

Mechanical integration builds on precision alignment strategies established by the ATLAS collaboration, with support structures, service routing, and access provisions coordinated with the ATLAS Detector Control System and experiment cavern infrastructure at Point 1. Service environments including cooling manifolds, low-mass power cables, and fiber-optic routing are designed in collaboration with industrial partners and national laboratories such as CERN, STFC Rutherford Appleton Laboratory, and Brookhaven National Laboratory. Installation tooling and handling procedures reflect lessons from previous major interventions like the ATLAS Pixel Detector insertion and maintenance campaigns, with cleanroom assembly protocols consistent with standards practiced at CERN Meyrin and partner laboratory facilities.

Readout Electronics and Data Acquisition

Front-end readout chains use radiation-tolerant ASICs and high-speed serial links developed in synergy with the RD53 collaboration and industry partners in the silicon photonics sector. Data acquisition architecture interfaces with the ATLAS Trigger and Data Acquisition upgrades, employing high-throughput optical links compatible with standards such as the GigaBit Transceiver and networking stacks influenced by architectures at ATLAS TDAQ and CMS DAQ systems. Firmware and software integration are validated through testbeds mirroring ATLAS Run 3 and HL-LHC operating modes, with event-building, zero-suppression, and lossless compression strategies coordinated with computing centres like CERN Tier-0, GridPP, and the Open Science Grid.

Integration, Installation and Commissioning

Integration follows staged module qualification campaigns, system-level integration tests, and combined commissioning with other ATLAS subsystems such as the Calorimeter and Muon Spectrometer to ensure interoperability with the ATLAS Trigger. Installation phases are scheduled around LHC long shutdowns, using procedures proven during the LS2 and LS3 maintenance periods, and require coordination with machine experts from CERN Accelerator groups and cryogenics teams. Commissioning employs cosmic-ray runs and beam-splash tests analogous to those used in commissioning of ATLAS and CMS detectors, with alignment strategies leveraging track-based algorithms from the Millepede family and optical survey techniques practiced at partner institutions.

Simulation, Calibration and Performance Validation

Simulation campaigns utilize detailed models in GEANT4 and reconstruction frameworks maintained by the ATLAS software group, cross-checked with testbeam results from facilities like CERN SPS and DESY. Calibration procedures exploit laser systems, charge-injection tests, and in-situ alignment using collision data comparable to methods used by CMS and LHCb, with performance validation against benchmarks for tracking efficiency, fake-rate, and resolution used in physics analyses by collaborations such as ATLAS and external reviewers. Ongoing detector performance studies inform upgrades to firmware, alignment constants, and operating parameters under oversight from project boards including representatives from CERN, national funding agencies, and major university consortia.

Category:Particle detectors