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| ALICE Inner Tracking System | |
|---|---|
| Name | ALICE Inner Tracking System |
| Location | CERN, Geneva |
| Institution | European Organization for Nuclear Research |
| Detector | ALICE |
| Established | 2008 |
| Upgrade | 2019–2021 (ITS2), ongoing R&D (ITS3) |
ALICE Inner Tracking System
The ALICE Inner Tracking System is the innermost silicon tracker of the ALICE experiment at the Large Hadron Collider, designed to provide high-precision tracking, vertexing, and particle identification for heavy-ion and proton collisions. It sits around the beam pipe at the CERN Geneva site and operates in concert with other ALICE subdetectors such as the Time Projection Chamber, Electromagnetic Calorimeter, and Time-Of-Flight detector to reconstruct short-lived hadrons and heavy-flavor production. The system has undergone a staged evolution including the original ITS, the ITS2 upgrade installed during the Long Shutdown 2, and development activities toward ITS3 to meet increasing luminosity and physics demands.
The Inner Tracking System provides primary and secondary vertex reconstruction crucial for measurements of heavy-flavor hadrons, open heavy-flavor electrons, and light-flavor hadrons, connecting to analyses by collaborations like the ALICE Collaboration and complementary measurements from ATLAS, CMS, and LHCb. It occupies the region nearest the interaction point between the beam pipe and the Time Projection Chamber and is optimized for low transverse momentum tracking, low material budget, and high readout rate compatible with the LHC Run 3 and Run 4 operational scenarios. The ITS supports physics programs that include studies related to the Quark–Gluon Plasma created in lead–lead collisions, as well as precision studies in proton–proton collisions and proton–lead collisions.
The ITS2 design is a six-layer cylindrical arrangement built from monolithic active pixel sensors using CMOS technology developed with institutes such as CERN microelectronics groups and universities including University of Padua and Università di Torino. Layers are numbered from the innermost Layer 0 to outermost Layer 5 and include an inner barrel and outer barrel layout physically integrated with services provided by collaborations like HOMER and institutes from the INFN network. Key mechanical components include a lightweight beampipe support, carbon-fiber staves, cooling pipes, and low-mass electrical services developed with industrial partners and national laboratories such as GSI Helmholtz Centre for Heavy Ion Research and Czech Technical University groups. The sensor readout chain connects to front-end electronics, FPGA-based readout units, and the ALICE Detector Control System for configuration and monitoring.
The ITS achieves impact-parameter resolution improvements over its predecessor, enabling separation of primary and secondary vertices for charm and beauty hadron decays with high precision. Pointing resolution benefits from the small pixel pitch and reduced material budget per layer, influenced by simulations validated against test beams at facilities like CERN SPS and DESY. Momentum resolution at low transverse momentum is enhanced through synergy with the Time Projection Chamber and central solenoidal field from the ALICE magnet, while spatial resolution and detection efficiency are characterized using tracks from cosmic ray runs and collision data during LHC commissioning. Radiation tolerance targets were driven by expectations from LHC Run 3 and extrapolations toward high-luminosity scenarios.
ITS2 replaced the original ITS during the Long Shutdown 2 with monolithic pixel sensors to increase readout rate and reduce material budget, a project coordinated by the ALICE Collaboration and executed with contributions from institutions including CERN, INFN, Technical University of Munich, and IPHC Strasbourg. Development toward ITS3 explores curved wafer-scale bent monolithic sensors to further reduce the material budget and improve tracking at small radii, an R&D program involving microelectronics groups at CERN and national laboratories such as CEA Saclay and Rutherford Appleton Laboratory. ITS3 aims to integrate novel silicon bending, ultra-thin supports, and advanced cooling concepts to enable enhanced vertexing performance for future High-Luminosity LHC conditions.
The ITS is mechanically and electronically integrated with the ALICE central barrel, interfacing to the Time Projection Chamber, the Transition Radiation Detector, and the Time-Of-Flight detector for combined tracking and particle identification. Synchronization with the ALICE Trigger and interaction with the Data Acquisition System allow global event reconstruction and calibration workflows executed by the ALICE Offline and ALICE Online teams. Services integration required coordination with the CERN Accelerator operations group for beam pipe clearances and with detector alignment efforts tied to the ALICE Alignment Group and international calibration teams.
Operational procedures include continuous sensor monitoring, cold commissioning with evaporative or two-phase cooling loops, and periodic calibration runs to correct time-zero and gain variations, managed through the Detector Control System and operations shifts staffed by the ALICE Collaboration institutes. Data from the ITS are digitized on front-end boards, aggregated by readout units, and streamed to the ALICE High-Level Trigger and O^2 processing framework for prompt reconstruction and compression. Online monitoring and data-quality assessment are performed by shift crews and calibration experts drawn from participating universities and laboratories.
ITS contributions enable reconstruction of D mesons, B hadrons, Λc baryons, and hypernuclei, supporting measurements of nuclear modification factors, elliptic flow, and heavy-quark diffusion coefficients in studies led by the ALICE Collaboration. Precision vertexing from the ITS has impacted heavy-flavor cross-section determinations, charm baryon enhancement studies relative to results from CMS and ATLAS, and differential flow measurements that inform theoretical models from groups studying the Quark–Gluon Plasma and transport coefficients. ITS performance also supports rare probe searches, femtoscopy analyses, and constraints on hadronization mechanisms cited by experimental and theoretical collaborations across the LHC community.