The LHCb VELO Upgrade

The LHCb experiment is a forward spectrometer experiment located at CERN, dedicated primarily to study CP violation and rare decays of beauty and charmed hadrons. The LHCb experiment will be upgraded during the current shutdown (2019-2020) to a trigger-less system reading out data at 40 MHz event rate. To cope with the higher data rates and increased occupancy, the LHCb sub-detectors including the VErtex LOcator (VELO) need to be upgraded. The VELO is a Si microstrip detector that surrounds the interaction region and performs highly precise track and vertex reconstruction. The microstrip sensors in VELO will be replaced by hybrid pixel sensors having $55 \times 55 \ \mu\rm{m}^2$ pixels, that are bump-bonded to VeloPix ASICs. Module will be cooled by circulating evaporative CO$_2$ coolant through microchannels etched in the Si substrate that provides mechanical support to the module components. The detector modules will be enclosed within 250 $\mu$m thick RF foil to separate them from beam vacuum. Electrically working module prototypes have been built and rigorously tested at the assembly sites and at CERN SPS testbeam. This paper discusses the module components and design, electronic architecture, recent results and the current status of the VELO upgrade.