Position Reconstruction and Charge Distribution in LHCb VELO Silicon Sensors

In 2006, a partially equipped LHCb VELO detector half was characterised in a test beam experiment (Alignment Challenge and Detector Commissioning, ACDC3). The position reconstruction and resolution for 2-strip R-sensor clusters was studied as a function of strip pitch and track inclination on the sensor surface. The Charge Density Distribution (CDD) is derived from the weighted charge distribution. It becomes asymmetric for tracks non-perpendicular to the strip surface. It is shown that the asymmetric broadening of the CDD around the track intercept position results in a linear eta-function at higher angles (>6 degrees). The sensor spatial resolution is determined both using a linear weighted mean of strip charges, as well as a third-order polynomial approximation via a eta-correction. The experimental results are in agreement with previous simulations. Future studies are underway to determine the angle and pitch dependent parameters which will be implemented in the LHCb VELO cluster position software tools.