An Unexpected Effect in ALEPH: Long-Term Displacements of the Silicon Vertex Detector

The ALEPH Silicon Vertex Detector for LEP2 featured a laser survey system to monitor its mechanical stability. The analysis of laser system data from 1997 to 2000 showed that VDET suffered a time-dependent displacement. It resulted to be compatible with a deformation of the support structure that made the device to slowly rotate during the data-taking. A maximal local displacement of ~20 microns was observed, corresponding to a rotation of ~10E-4 rad. The implementation of a time-dependent correction on the alignment by using the laser system data led to sizeable improvements on the ALEPH data quality.