Readiness of the ATLAS Liquid Argon Calorimeter for LHC Collisions

TL;DR

<3-5 sentence high-level summary> The paper documents the readiness of the ATLAS Liquid Argon (LAr) calorimeter for LHC collisions by presenting in-situ performance measurements from calibration triggers, cosmic muons, and beam splash events. It details hardware status, data-taking conditions, and precise electronic performance, including pedestal/noise/gain stability, timing alignment, and the energy-reconstruction workflow at both cell and trigger-tower levels. It demonstrates good agreement between data and Geant4-based MC simulations for uniformity in the EM barrel and for electromagnetic shower shapes, supporting an anticipated global energy-resolution constant term around 0.5–0.7% for the EM calorimeter. Overall, the results establish the calorimeter as robust, well-calibrated, and ready for first LHC collisions, with MC validation bolstering confidence in electron/photon and ETmiss measurements.

Abstract

The ATLAS liquid argon calorimeter has been operating continuously since August 2006. At this time, only part of the calorimeter was readout, but since the beginning of 2008, all calorimeter cells have been connected to the ATLAS readout system in preparation for LHC collisions. This paper gives an overview of the liquid argon calorimeter performance measured in situ with random triggers, calibration data, cosmic muons, and LHC beam splash events. Results on the detector operation, timing performance, electronics noise, and gain stability are presented. High energy deposits from radiative cosmic muons and beam splash events allow to check the intrinsic constant term of the energy resolution. The uniformity of the electromagnetic barrel calorimeter response along eta (averaged over phi) is measured at the percent level using minimum ionizing cosmic muons. Finally, studies of electromagnetic showers from radiative muons have been used to cross-check the Monte Carlo simulation. The performance results obtained using the ATLAS readout, data acquisition, and reconstruction software indicate that the liquid argon calorimeter is well-prepared for collisions at the dawn of the LHC era.

Figures (23)

• Figure 1: Cut-away view of the LAr calorimeter, 17 m long (barrel + endcaps) and 4 m of diameter.
• Figure 2: Distribution of barrel cryostat probe temperatures averaged over a period of ten days.
• Figure 3: High voltage correction factors for all LAr cells at the end of September 2009.
• Figure 4: L1 transverse energy ($E_{\rm T}^{\rm L1}$) computed with the receiver gains extracted from calibration runs versus the sum of cell transverse energies in the same trigger tower ($E_{\rm T}^{\rm LAr}$).
• Figure 5: Relative difference of $E_{\rm T}^{\rm L1}$ and $E_{\rm T}^{\rm LAr}$ (L1 Calorimeter $E_{\rm T}$ resolution) as a function of $E_{\rm T}^{\rm LAr}$. Strict projectivity cuts for the track pointing to the EM shower are applied. Horizontal error bars reflect the RMS of $E_{\rm T}^{\rm LAr}$ in each bin.