LHCb VELO upgrade technical design report

The upgraded LHCb VELO silicon vertex detector is a lightweight hybrid pixel detector capable of 40 MHz readout at a luminosity of $2 \times 10^{33} cm^{-2} s^{-1}$. The track reconstruction speed and precision is enhanced relative to the current VELO detector even at the high occupancy conditions of the upgrade, due to the pixel geometry and a closest distance of approach to the LHC beams of just 5.1 mm for the first sensitive pixel. Cooling is provided by evaporative $CO_2$ circulating in microchannel cooling substrates. The detector contains 41 million 55$\mu \times 55 \mu$ pixels, read out by the custom developed VeloPix front end ASIC. The detector will start operation together with the rest of the upgraded LHCb experiment after the LHC LS2 shutdown, currently scheduled to end in 2019. This Technical Design Report describes the upgraded VELO system, planned construction and installation, and gives an overview of the expected detector performance.