ATLAS Pixel Phase-I Upgrade

ATLAS Pixel Phase-I Upgrade
Name	ATLAS Pixel Phase-I Upgrade
Institution	CERN
Collaboration	ATLAS Collaboration
Project	LHC upgrade
Status	Completed
Start	2013
End	2018

ATLAS Pixel Phase-I Upgrade

The ATLAS Pixel Phase-I Upgrade was a substantial hardware and readout enhancement to the ATLAS inner detector at the LHC, executed during a Long Shutdown at CERN to prepare for higher luminosity running. It complemented broader upgrade programs at the LHC and involved institutions such as the University of Oxford, Lawrence Berkeley National Laboratory, University of Glasgow, and University of Geneva. Work on the upgrade intersected with coordination by the CERN Physics Department and input from experiments like CMS and projects such as HL-LHC studies.

Introduction

The ATLAS Pixel Phase-I Upgrade replaced and augmented parts of the original ATLAS pixel detector to improve tracking under increased pile-up expected in Run 2 and Run 3 of the LHC. The upgrade took place in the context of the LHC Long Shutdown 1 and Long Shutdown 2 planning and was driven by performance requirements from analyses like Higgs boson property measurements, searches for supersymmetry and rare processes studied by the ATLAS Collaboration. Institutions across Europe, North America, and Asia collaborated, including INFN, CEA Saclay, KEK, and TRIUMF.

Motivation and Objectives

The upgrade aimed to cope with higher instantaneous luminosity and pile-up projected by the LHC schedule, ensuring robust vertexing for signatures such as b-tagging in top quark studies, Higgs boson decays, and exotic searches pursued by ATLAS. Objectives included reducing dead time relative to the original pixel detector used during the LHC Run 1 era, increasing granularity to limit occupancy issues reported in Run 1, improving radiation tolerance following studies at facilities like CERN PS and TRIUMF, and enabling new trigger strategies coordinated with the ATLAS trigger and DAQ groups.

Design and Architecture

The Phase-I upgrade introduced new structural elements and service routing compatible with the ATLAS inner detector envelope and the Inner Detector mechanical interfaces managed by CERN engineering teams. Architecturally, it included additional pixel layers and a new readout architecture derived from developments at DESY, LBNL, and University of Bonn. The design leveraged front-end ASICs produced using CMOS processes studied at foundries linked to Europractice and tested under irradiation campaigns modelled after environments at the LHC beam pipe and CERN SPS facility. Thermal management strategies integrated microchannel cooling concepts investigated at Paul Scherrer Institute and CERN workshops.

Detector Modules and Sensors

The upgrade deployed advanced hybrid pixel modules using sensor technologies such as planar silicon sensors and three-dimensional (3D-silicon) architectures developed by groups at CNRS/IN2P3, University of Manchester, and MPI. Modules combined bump-bonded sensors with readout ASICs designed by collaborations including engineers from KEK and University of Bonn. Sensor layout and segmentation were optimized using simulations from GEANT4 studies and beam test campaigns at facilities like CERN SPS and DESY, and qualification included irradiation at test beams coordinated with RD50 and materials testing at EMMI laboratories.

Readout Electronics and DAQ

Readout electronics were updated with the new generation of front-end chips and high-speed serial links designed for robustness in the LHC radiation environment, an effort involving teams from SMU, University of Wisconsin–Madison, and Fermilab. The DAQ integration interfaced with the TDAQ system and adopted optical links and bandwidth strategies informed by developments in GBT project and Versatile Link programs. Firmware and software for readout borrowing toolkits from ROOT and Gaudi frameworks were developed and validated during commissioning campaigns coordinated with the ATLAS computing group.

Installation and Commissioning

Installation was performed during a scheduled LHC Long Shutdown with interventions at the ATLAS experimental cavern (UX15) under coordination by CERN safety and integration teams and participating institutes such as University of Liverpool and University of Bonn. Commissioning included detector cooling, power-up sequences, timing synchronization with the LHC clock, and calibration runs that used cosmic ray data and injected test pulses. Alignment procedures used data analyzed with alignment tools originating from Millepede and leveraged track reconstruction efforts shared with ATLAS Tracking Group and detector performance teams.

Performance and Results

Post-installation performance showed improved tracking efficiency, vertex resolution, and b-tagging performance at elevated pile-up consistent with simulations from GEANT4 and reconstruction improvements in ATHENA. Radiation hardness tests and in-situ monitoring by groups such as RD50 confirmed increased longevity relative to the original detector, enabling precise measurements for Higgs boson coupling studies and searches reported by the ATLAS Collaboration in conference notes and peer-reviewed publications coordinated with funding agencies like ERC and DOE. Operational stability metrics and downtime statistics were managed jointly by ATLAS Run Control teams and infrastructure partners.

Future Developments and Legacy

The Phase-I upgrade provided a technological and organizational foundation for subsequent efforts toward the HL-LHC era, informing the design of the Phase-II ATLAS Inner Tracker replacement and R&D programs at RD53 Collaboration, CERN microelectronics initiatives, and sensor developments at IMB-CNM. The project strengthened collaborations among institutions including University of Oxford, INFN, CEA Saclay, and Lawrence Berkeley National Laboratory, and its lessons influenced detector upgrade strategies for future collider proposals such as the Future Circular Collider and detector concepts evaluated by the European Strategy for Particle Physics.

Category:ATLAS experiment Category:Particle detectors Category:CERN projects