Alignment

The alignment procedure aims to determine the positions of the SCT modules. An X-ray survey at the end of assembly provides the initial alignment. During the operation of ATLAS, the alignment of the detector is monitored by a combination of a run-time alignment system and tracking data.

Runtime alignment

Frequency Scanning Interferometry (FSI) is an interferometric length measurement technique which has been developed at Oxford. In ATLAS, it will be used to make in-situ measurements of the distances between nodes of an alignment grid spanning the Inner Detector. This information will enable the 3-D positions of the nodes to be determined. The positions of the modules are then interpolated from the node positions. More information can be found on the Inner Detector Run-Time Alignment System page.

Track based alignment

During track reconstruction, helical tracks are fitted to the hits generated by pixel and SCT modules. By studying the deviations of hit positions away from the fitted track it is possible to identify module misalignments. Two approaches for performing alignment using tracks are being studied.

• Global chi-squared minimisation , which proceeds by forming a global chi-squared based on hit residuals from all Si modules, and then aims to minimise this by varying all of the degrees of freedom of the modules.
• The robust alignment approach is based on optimising hit and overlap residuals. A hit residual is the difference between the hit position and the position of the fitted track. An overlap residual is the difference between two residuals from overlapping modules. Hit residual and overlap residual distributions are centered around zero for a perfectly aligned detector. The optimisation for the robust alignment is based on correcting module positions such that the hit residual and the overlap residual distribution are centered around zero. Correlations between residuals and overlap residuals on different modules within a ring or a stave are taken into account. The robust alignment approach is an iterative method. It is designed to align the PIXEL and SCT detector.

Alignment using tracks will provide high precision alignment data and will complement the FSI alignment technique.

Contact: Dr T. Weidberg, Dr P. Bruckman, Dr M. Karagoz Unel, Dr S. Gibson and Mr O. Brandt