[Paper Review] ATLAS Muon Detector Commissioning

E. B. Diehl|arXiv (Cornell University)|Oct 15, 2009

Particle Detector Development and Performance3 citations

TL;DR

This paper details the commissioning of the ATLAS muon spectrometer using cosmic rays prior to the LHC's first collisions in 2009. It presents successful performance validation of MDTs, RPCs, TGCs, CSCs, and the optical alignment system, achieving sub-50 μm chamber position resolution and demonstrating trigger and tracking efficiency above 95%, ensuring readiness for high-precision muon momentum measurement in collision data.

ABSTRACT

The ATLAS muon spectrometer consists of several major components: Monitored Drift Tubes (MDTs) for precision measurements in the bending plane of the muons, supplemented by Cathode Strip Chambers (CSC) in the high eta region; Resistive Plate Chambers (RPCs) and Thin Gap Chambers (TGCs) for trigger and second coordinate measurement in the barrel and endcap regions, respectively; an optical alignment system to track the relative positions of all chambers; and, finally, the world's largest air-core magnetic toroid system. We will describe the status and commissioning of the muon system with cosmic rays and plans for commissioning with early beams.

Motivation & Objective

• To commission and validate the performance of the ATLAS muon spectrometer components using cosmic ray data before LHC first collisions.
• To ensure the precision tracking and triggering systems meet design specifications for muon momentum resolution down to 3% at 250 GeV.
• To calibrate and align the muon spectrometer with sub-50 μm accuracy using optical sensors and track-based alignment techniques.
• To verify the functionality and efficiency of the trigger chambers (RPCs, TGCs, CSCs) and their integration with precision tracking chambers.
• To prepare the detector for collision data by validating calibration and alignment procedures using cosmic rays and testbeam data.

Proposed method

• Utilized cosmic ray data collected in 2008–2009 to commission and test all muon spectrometer components.
• Employed a network of 1850 Hall sensors and 12,000 optical alignment sensors to map the magnetic field and reconstruct chamber positions with sub-50 μm accuracy.
• Applied Fermi-Dirac fitting to MDT drift time spectra to determine timing offsets (T₀), and used autocalibration to optimize the time-to-space (RT) function.
• Performed track-based alignment using straight cosmic ray tracks to correct for residual misalignments not fully resolved by optical sensors.
• Used a combination of optical sensors and track-based corrections to validate and refine the 3D chamber geometry, especially in the barrel where optical coverage is partial.
• Conducted efficiency and noise measurements on RPCs, TGCs, and CSCs, and correlated hits between trigger and precision chambers to validate system functionality.

Experimental results

Research questions

• RQ1 How well do the RPC, TGC, and CSC chambers perform in terms of noise, occupancy, and efficiency during cosmic ray commissioning?
• RQ2 To what extent can the optical alignment system and track-based alignment correct for chamber misalignments in the barrel and endcap regions?
• RQ3 What is the achievable resolution of the MDT chambers using cosmic ray data, and how does it compare to testbeam results?
• RQ4 Can the calibration procedures for MDT timing offsets (T₀) and time-to-space functions (RT) be successfully applied to cosmic ray data to approach testbeam-level performance?
• RQ5 What is the impact of cosmic ray timing jitter on MDT resolution, and can it be mitigated to match testbeam performance?

Key findings

• RPCs achieved a noise rate of approximately 0.1 Hz/cm² and showed uniform occupancy, with 95.5% of 606 chambers functioning by July 2009.
• TGCs demonstrated 99.9% chamber functionality, with 95% efficiency at 2850 V and 99.14% bunch crossing readout efficiency.
• CSCs achieved 98.5% layer functionality after firmware updates, with tracking resolution of ~60 μm for tracks within 4° of incidence.
• MDT resolution with cosmic rays was degraded by ~2 ns of timing jitter compared to testbeam data, but residual jitter was reduced via event-by-event T₀ fitting.
• After autocalibration, MDT tracking residuals were flat and narrow across tube radii, indicating successful calibration and alignment.
• Optical alignment reduced the false sagitta distribution for straight cosmic ray tracks to within 15 μm of zero (centered), while track-based corrections improved barrel alignment to within 30 μm, surpassing the 100–200 μm precision of the optical system alone.