The commissioning and first results on the performance of the CMS Preshower Detector

The Preshower is the first part of the CMS endcap electromagnetic calorimeter, as seen by the particles. It is composed of 2 layers of lead absorbers followed by silicon sensors of a total surface of 17 m2 the biggest ever built. The use of the silicon strip sensors improves the spatial accuracy of the incidence position of electromagnetic showers. This allows an additional rejection of background (e.g. neutral pions) to new physics, such as the two-photon SM Higgs decay. In this article, the results from first commissioning, the performance with first LHC beams and first in-situ calibration with charged particles are described.