Alignment of the CMS Preshower Detector

The Preshower detector, as part of the CMS endcap electromagnetic calorimeter, is designed to identify $\pi{}^{0}$ in the endcaps within a fiducial region $1.653<\mid\eta\mid<2.6$. It is based on 4288 silicon sensors, each with active area $61 \times 61 ~\rm{mm}^{2}$ segmented into 32 strips with $1.9 ~\rm{mm}$ pitch. For minimum ionizing particles the intrinsic spatial precision is $1.76 ~\rm{cm}$ parallel to the strip and $550 \mu\rm{m}$ perpendicular to the strips. Exploiting its spatial resolution relies upon excellent alignment with the neighbouring detectors, especially the Tracker. The alignment is based on the extrapolation of tracks from the Tracker to the Preshower and gives an alignment precision of a few millimeters for translation and a few milliradians for rotation. The method for Preshower alignment is discussed and the results of the alignment with respect to the Tracker, using LHC collision data collected by CMS in 2011, are given.