Inner Detector (ATLAS)

Inner Detector (ATLAS) is the central tracking system of the ATLAS experiment at the Large Hadron Collider at CERN. It provides precision charged-particle tracking, vertex reconstruction, and particle identification inside the ATLAS magnet system and interfaces with the calorimeter and muon spectrometer. The detector played a pivotal role in the discovery of the Higgs boson and ongoing searches for supersymmetry, dark matter, and beyond-Standard-Model phenomena.

Overview

The Inner Detector occupies the innermost region of the ATLAS experiment detector assembly within the solenoidal field produced by the ATLAS solenoid magnet. Designed to measure trajectories of charged particles from proton–proton collisions at the Large Hadron Collider, it delivers inputs to the ATLAS trigger system, track reconstruction, and vertexing algorithms used by collaborations such as ATLAS Collaboration for analyses including top quark measurements, electroweak studies, and searches like diboson resonance. The subsystem integrates with services from CERN engineering groups and interfaces with international institutes including Oxford University, Lawrence Berkeley National Laboratory, KEK, INFN, and DESY.

Design and Components

The Inner Detector combines three principal technologies: pixel sensors, silicon microstrip detectors, and a transition radiation tracker. The innermost Pixel detector layer, including the Insertable B-Layer upgrade, provides high-resolution primary vertex reconstruction and heavy-flavor tagging used in b-tagging analyses such as those by the ATLAS Collaboration on Higgs boson decays to b quarks. Surrounding the pixels, the Semiconductor Tracker (SCT) uses silicon microstrips for precise momentum determination; SCT modules were developed in collaborations involving University of Cambridge, University of Michigan, and CERN. The outermost component, the Transition Radiation Tracker, employs straw-tube detectors and transition-radiation identification to assist electron identification in studies like W boson and Z boson measurements. Mechanical support structures, cooling systems, and readout electronics were engineered with contributions from Fermilab, Max Planck Society, and Institute of High Energy Physics (IHEP) teams. Radiation-hard electronics conform to standards developed with CERN microelectronics groups and are deployed alongside power-distribution systems inspired by designs used at DESY and SLAC National Accelerator Laboratory.

Performance and Calibration

Performance metrics for the Inner Detector—spatial resolution, track efficiency, momentum resolution, and impact-parameter resolution—are characterized using control samples from J/ψ meson decays, Z boson events, and minimum-bias collisions recorded during LHC Run 1 and LHC Run 2. Alignment procedures rely on survey data from metrology groups and track-based alignment algorithms developed with software frameworks influenced by ROOT and Geant4 simulation, with comparisons to reference datasets from ATLAS Combined Performance Group. Calibration of time, charge collection, and Lorentz-angle correction draws on expertise from CERN electronics laboratories and detector-specific teams at University of Chicago, University of Oxford, and University of Tokyo. The Inner Detector's performance is monitored in near real time by ATLAS operations centers at CERN and remote centers at partner institutions including Brookhaven National Laboratory and TRIUMF.

Operational History

Commissioned prior to LHC Run 1, the Inner Detector recorded first collision tracks in tandem with ATLAS commissioning runs and contributed to early measurements such as the observation of high-multiplicity events and initial top quark cross-section determinations. During the discovery of the Higgs boson in 2012, Inner Detector data were crucial for reconstructing decay vertices and identifying b-jets in channels like H→bb̄ and H→ττ. Maintenance campaigns during long shutdowns involved collaborative interventions by teams from INFN, CERN, and national laboratories; notable upgrades included installation of the Insertable B-Layer between LHC Long Shutdown 1 and LHC Run 2. The subsystem has coped with progressive radiation damage characterized by fluence studies performed with beam tests at facilities such as CERN PS and DESY test beam campaigns.

Upgrades and Future Plans

Planned upgrades align with the ATLAS Phase‑I and Phase‑II projects in preparation for the High-Luminosity Large Hadron Collider upgrade. The replacement of current Inner Detector elements with an all-silicon Inner Tracker is coordinated with international consortia including groups from CERN, LPNHE, University of Bonn, and University of Glasgow. R&D focuses on radiation-hard sensors, low-mass support structures, and high-bandwidth readout compatible with expected pile-up conditions in HL-LHC operations and physics programs targeting precision measurements of the Higgs boson self-coupling and rare processes. Future plans include integration tests at facilities like CERN Integration Centre and performance validation using simulated datasets in software stacks influenced by Athena (software), Geant4, and ROOT, with governance by the ATLAS Collaboration upgrade boards and funding agencies such as European Commission research instruments and national research councils.

Category:ATLAS experiment Category:Particle detectors