Alignment and monitoring of ATLAS Muon Spectrometer

The ATLAS Muon Spectrometer is designed to measure muons from 14 TeV p-p interactions over a wide transverse momentum (pT ) range with accurate momentum resolution, taking into account the high background environment, the inhomogeneous magnetic field, and the large size of the apparatus. At high pT one of the main contributions to the resolution is the uncertainty on the Spectrometer alignment. The design resolution requires accurate control of the position and deformation of muon chambers, induced by mechanical stress and thermal gradients. Corrections provided by an optical alignment system, as well as track-based alignment procedures, are integrated in the software leading to track reconstruction and momentum measurement. We present here this optical system and the alignment procedures, along with recent results on muon reconstruction quality improvements obtained after feeding the software with alignment corrections. These results are obtained on muons from cosmic rays, which are currently being used to finalize detector commissioning. To that purpose, and also for future running of ATLAS with LHC collisions, tools to monitor the status of the Muon Spectrometer and determine the quality of the data are being developed. The goal is to spot problems during data taking and flag the data accordingly. Careful monitoring is important at the beginning of an experiment where the environment is new and requires some experience to fully comprehend; such tools can help determine problems quickly and solve them. The different steps of offline monitoring are presented here with examples.