R-Sensor sectors and Strip Pitch

Four different designs for the layout of the R-sensors of the VELO are compared for their performance in signal to noise (S/N), the Pile-up Veto, the L1-tracking, the off-line tracking and the impact parameter resolution. The design as presented in the VELO-TDR was optimised solely for the impact parameter resolution, but a design where all R-strips cover a 45$^\circ$ sector, rather than strips at larger radii covering 90$^\circ$ sectors, is favoured for the expected S/N, the reconstruction time, and the number of ghost tracks, while the loss in impact parameter resolution is small. Hence it is proposed to change to a new design, with 45$^\circ$ sectors, and a variable strip pitch keeping the same number of channels per sensors as proposed in the VELO-TDR.