Time calibration of the ATLAS tile calorimeter using an integrated laser system

Abstract

The ATLAS hadronic Tile Calorimeter is ready for data taking with proton-proton collisions provided by the Large Hadron Collider (LHC). The Tile Calorimeter is a sampling calorimeter with iron absorbers and scintillators as active medium. The scintillators are read out by wave length shifting fibers (WLS) and photomultipliers (PMTs). The LHC provides collisions every 25 ns, putting very stringent requirements on the synchronization of the ATLAS triggering systems and the read out of the on-detector electronics. More than 99% of the read out channels of the Tile Calorimeter have been time calibrated using laser pulses sent directly to the PMTs. Timing calibration constants can be calculated after corrections for differences in laser light paths to the different parts of the calorimeter. The calibration consists of two parts: programmable corrections implemented in the on-detector electronics, and residual deviations from perfect timing stored in a database used during the offline reconstruction of the Tile Calorimeter data. Data taken during long ATLAS cosmic runs and during LHC beam time in September 2008 has confirmed a timing uniformity of 2 ns in each of the four calorimeter sections. The remaining offsets between the four calorimeter sections, have been measured in two ways. First by using the laser pulses interleaved with cosmic triggers inside a global ATLAS run. The second method uses the real LHC events acquired during the 2008 beam time. Both methods give consistent results. The main limitations on the precision of the time calibration are presented.