Developments in Silicon Detectors and their impact on LHCb Physics Measurements

The LHCb experiment is a high energy physics detector at the Large Hadron Collider (LHC) which will probe the current understanding of the Standard Model through precise measurements of CP violation and rare decays. The LHCb detector heavily depends on the silicon vertexing (VELO) sub-detector for excellent vertex and proper decay time resolutions. The VELO detector sits at a position of only 7 mm from the LHC proton beams. However, the proximity of the silicon sensors to the proton beams results in the detectors suffering radiation damage. Radiation damage results in three changes in the macroscopic properties of the silicon detector: an increase of the leakage current, a decrease in the charge collection efficiency, and changes in the operation voltage required to fully deplete the silicon detector of the free charge carriers. Due to this radiation damage, it is expected that a replacement or upgrade of the LHCb vertex detector will be required by 2010, only 3 years after the turn-on of the LHC. This thesis investigates possible scenarios for the VELO upgrade in 2010. There are two properties through which LHCb could gain from an upgraded VELO: increased statistics, and an improved proper time resolution. Increased statistics could be achieved by an increase in luminosity. Since the radiation damage is primarily caused by the primary interactions, an increase in luminosity would require silicon with a greater radiation tolerance. This thesis investigates Czochralski grown silicon as a possible radiation hard silicon. A detailed characterization has been performed on Czochralski silicon, including the use of the Transient Current Technique. The type inversion point of Czochralski silicon was investigated and no type inversion was measured in Czochralski silicon. The first test beam with a Czochralski strip detector read out with LHC speed electronics was performed and yielded promising results for the future application of Czochralski silicon. An increase in proper time resolution and impact parameter resolution could be achieved through the reduction of material in the detector, or through positioning the silicon closer to the proton beams. Research into repositioning the VELO is presented in this thesis and includes an investigation of possible designs that would position the first active silicon strip closer to the LHC beam. This is achieved by a redesign of the VELO sensor geometry with a reduced active inner radius. Results are then presented on the impact of this VELO design on the physics reach of the experiment.