The LHCb Vertex Locator upgrade

The upgrade of the LHCb experiment, planned for 2018, will transform the experiment to a trigger-less system reading out the full detector at 40 MHz event rate with all data reduction algorithms executed in high level software on a computing farm. The Vertex Locator (VELO) detector which surrounds the interaction region is used to reconstruct primary and secondary vertices and measure the flight distance of long-lived particles. The upgraded VELO will be a hybrid pixel detector read out by the VeloPix ASIC with on-chip zero-suppression. The hottest region will have peak pixel hit rates of 900 Mhits/s yielding a total data rate more than 3 Tbit/s for the upgraded VELO. The data will be read out on high-speed electrical and optical serial links. The detector modules are located in a secondary vacuum, separated from the beam vacuum by a thin custom made foil. The detector halves are retracted during injection of the LHCb beams and closed when the beams are declared stable. In the closed position the innermost sensitive pixels are at 5.1 mm from the beams. The material budget will be minimised by the use of evaporative CO2 coolant circulating in micro channels within 400 μm thick silicon substrates. Microchannel cooling brings many advantages: very efficient heat transfer with almost no temperature gradients across the module, no CTE mismatch with silicon components, and low material contribution. The current status of the VELO upgrade will be described together with a presentation of recent test results.