The LHCb VELO: Performance, radiation damage, and upgrade

LHCb is a dedicated experiment to study New Physics and CP violation in the decays of heavy hadrons at the Large Hadron Collider (LHC). Heavy hadrons are identified through their flight distance in the VELO, the retractable silicon-strip vertex detector surrounding the LHCb interaction point at only 7 mm from the beam during normal LHC operation. Two VELO halves comprise 21 silicon micro-strip modules each. The performance of the VELO in its three years of successful operation during the LHC physics runs is presented. Highlights include cluster finding efficiency, single hit resolution, and impact parameter and vertex resolutions. The VELO sensors receive a large and non-uniform radiation dose having inner and outer radii of only 7 and 50 mm, respectively. In this extreme and highly non-uniform radiation environment type-inversion of the inner part of the n⁺-on-n sensors has already been measured. Radiation damage is monitored and studied in three ways: (1) dependence of sensor currents with fluence, (2) evolution of the depletion voltage, and (3) cluster finding efficiency. Results will be presented in all three areas with updates based on recent results. Current research on upgrades to the VELO are outlined to adapt to a new 40MHz trigger. Main focus given to the silicon technology to be used in the upgrade. Other aspects such as the cooling and readout requirements are also given.