Cryogenic Silicon Detectors and Analysis of Primakoff Contributions to the Reaction $\pi^{-}Pb \to \pi^- \pi^- \pi^+ Pb$ at COMPASS

The COMPASS experiment at CERN employs silicon microstrip detectors in the target region which are cooled with liquid nitrogen to 200 K since the beam time 2009. Various extensions around the previously existing setup were required, particularly concerning the stability of the detector modules and the cooling stability. The detector performance has been improved by 15-20 % with respect to the warm operation.
During the beam time 2004 with high-energetic pion beam on a lead target, one million exclusive three-pion events were recorded with smallest momentum transfer t'<10^-3 GeV²/c². Employing partial-wave analysis techniques, Primakoff-produced resonances and the interference between Primakoff and diffractive production have been observed. The absolute cross-section π^-γ→π^-π^-π⁺ was determined for the first time.