ATLAS Transition Radiation Tracker (TRT): Straw tubes for tracking and particle identification at the Large Hadron Collider

Bartosz Mindur

doi:10.1016/j.nima.2016.04.026

Abstract

The ATLAS Transition Radiation Tracker (TRT) is the outermost of the three inner detector tracking subsystems and consists of ∼300,000 thin-walled drift tubes (“straw tubes”) that are 4mm in diameter. The TRT system provides ∼30 space points with ∼130micron resolution for charged tracks with |η|<2 and pT>0.5GeV/c. The TRT also provides electron identification capability by detecting transition radiation (TR) X-ray photons in an Xe-based working gas mixture.Compared to Run 1, the LHC beams now provide a higher centre of mass energy (13TeV), more bunches with a reduced spacing (25ns), and more particles in each bunch leading to very challenging, higher occupancies in the TRT. Significant modifications of the TRT detector have been made for LHC Run 2 mainly to improve response to the expected much higher rate of hits and to mitigate leaks of the Xe-based active gas mixture. The higher rates required changes to the data acquisition system and introduction of validity gate to reject out-of-time hits. Many gas leaks were repaired and the gas system was modified to use a cheaper Ar-based gas mixture in some channels. A likelihood method was introduced to optimise the TRT electron identification.

Highlights

The ATLAS Inner Detector (ID) is the innermost part of the ATLAS experiment [1] and has three main subcomponents: the silicon based Pixel and SemiConductor Tracker (SCT) detectors, and the Transition Radiation Tracker (TRT) based on gaseous type of detectors called straws [2]
Compared to Run 1, the LHC beams provide a higher centre of mass energy (13 TeV), more bunches with a reduced spacing (25 ns), and more particles in each bunch leading to very challenging, higher occupancies in the TRT
The TRT offers particle identification by separating electrons from charged pions using the fraction of high threshold (HT) hits on track

Summary

Introduction

The ATLAS Inner Detector (ID) is the innermost part of the ATLAS experiment [1] and has three main subcomponents: the silicon based Pixel and SemiConductor Tracker (SCT) detectors, and the Transition Radiation Tracker (TRT) based on gaseous type of detectors called straws [2]. Data acquisition and tracking conditions become very challenging when the detector runs at the high occupancy levels due to the increasing numbers of protons per bunch and the reduction in bunch spacing provided by the LHC accelerator. Of the gas, electron drift velocity, energy fluctuations, signal propagation, shaping and discrimination has been developed to make a precise simulation and comparison with measured data. Tracking properties For any tracking detector finding and measuring tracks in the dense environment at high occupancy from pile-up events is very challenging. The residual is the difference between the drift radius extracted from the drift time and the track radial position, measured from the track fit to all hits on the track except the one in the straw being considered

TRT gas leaks and argon based gas mixture studies

Particle identification

Findings

Conclusion