(1) University of Glasgow, Kelvin Building, Glasgow G12 8QQ, Scotland.
LHCb is a dedicated experiment at the LHC to study CP violation and rare $b$ decays. The vertex locator (VELO) is a silicon strip detector designed to measure precisely the production and decay vertices of B-mesons. The detector is positioned at 8 mm of the LHC beams and will operate in an extremely harsh radiation environment. The VELO consists of two retractable detector halves with 21 silicon micro-strip tracking modules each. A module is composed of two n+-on-n 300 mu m thick half disc sensors with R and \Phi micro-strip geometry. The detectors are operated in vacuum and a bi-phase CO_2 cooling system is used. The full system has been operated since June 2008 and its commissioning experience will be reported. During the LHC synchronization tests in August and September 2008, and June 2009 the LHCb detectors measured secondary particles produced by the interaction of the LHC primary beam on a beam dump. About 50,000 tracks were reconstructed in the VELO and they were used to derive the relative timing alignment between the sensors and for the first evaluation of the spatial alignment. Using this track sample the VELO has been aligned to an accuracy of 5 $\mu$m. A single hit resolution of 10 mu m was obtained at the smallest pitch for tracks of perpendicular incidence. The design and the main components of the detector system are introduced. The commissioning of the detector is reported and the talk will focus on the results obtained using the first beam-induced reconstructed tracks.
11th ICATPP Conference on Astroparticle, Particle, Space Physics, Detectors and Medical Physics Applications
5-9 Oct. 2009, Villa Olmo, Como (Italy)