Development of a novel alignment system for the ATLAS Inner Detector and an investigation of the effect of alignment inaccuracies on tracker performance
The Large Hadron Collider at CERN will offer an unparallelled opportunity to probe fundamental physics at an energy scale well beyond that reached by current experiments. The ATLAS detector is being designed to fully exploit the potential of the LHC for revealing new aspects of the fundamental structure of nature. In order to meet the stringent tracking requirements of ATLAS, it will be necessary to determine the positions of over 100 million tracking elements to very high precision during operation of the detector. The principles of the alignment and survey techniques used to do this are introduced and the current activities concerning the development of an alignment strategy for the ATLAS Inner Detector are presented. After consideration of the motivation and requirements, descriptions of several of the candidate technologies are given, together with explanations of how they might be applied in the various stages of the alignment process. A fast remote measurement system known as Frequency Scanned Interferometry (FSI) which is capable of making precise measurements of absolute lengths has been developed. This novel technique is likely to be used as the basis of a run-time survey system for the ATLAS Inner Detector. The basic principles are explained and a detailed design and laboratory test results are presented. An element common to all types of survey system is the need to combine of a number of measurements to form a three-dimensional picture of the positions of all the detector elements. An introduction to measurement combination using geodetic networks is given, and the results of a study of networks suitable for use with the FSI measurement technique are presented. As part of the process of deriving a detailed set of requirements for the survey system, a full .tvionte Carlo simulation study has been performed to investigate how the Inner Detector track fitting resolutions vary as a function of the alignment precisions of the SCT barrel. A physical signal which is important for defining the required momentum resolution is the forward-backward asymmetry of a heavy analogue of the charged electroweak gauge boson. The sensitivity to this signal is investigated as a function of alignment precision. Finally, work done during the development of the current layout of the ATLAS Inner Detector is presented
