The LHCb experiment at the LHC is designed to capture decays of b- and c-hadrons for the study of CP violation and rare decays. It has already had a transformative impact in the field of flavour physics as well as making many general purpose physics measurements in the forward region. At the end of Run-II, many of the LHCb measurements will remain statistically dominated. For this reason the experiment is being upgraded to run at higher luminosity after 2020. The trigger scheme, which currently has a 1 MHz lowest level hardware rate, will be transformed to a strategy whereby the entire experiment is read out at 40 MHz to a flexible software trigger. The increased luminosity and trigger efficiency anticipated at the upgrade will allow significant improvements in measurements across the flavour sector and beyond. In order to allow the triggerless readout the front end electronics of all subdetectors will be changed, and many subdetectors will be upgraded to cope with the increased occupancy and radiation levels anticipated at the upgrade.
The Vertex Locator (VELO) surrounding the interaction region, whose role is to reconstruct and trigger on the primary and secondary vertices of the events, is an example of a subdetector which will be completely changed. The current strip detector will be replaced by a hybrid pixel detector read out with the VeloPix ASIC. The detector operates just 5 mm away from the collision region, and will give optimum track reconstruction efficiency and projected precision at the vertex region.
The upgraded VELO is composed of 52 modules placed along the beam axis. The 26 modules in each half can be retracted during LHC filling and only close to the nominal position after stable beams have been declared. Each module is equipped with 4 silicon hybrid pixel tiles, each read out with by 3 VeloPix ASICs. The pixels have a pitch of 55 μm × 55 μm and the sensors are produced in 200 μm thick p-in-n type silicon. The sensors must withstand an integrated fluence of up to 8×10151~MeV neq/cm2, a roughly equivalent dose of 400 MRad, and it is anticipated that the bias voltage must be raised to 1000V by the end of lifetime of the detector. The highest occupancy ASICs will have pixel hit rates of 800 Mhits/s and produce an output data rate of over 15 Gbits/s, with a total rate of 1.6 Tbits/s anticipated for the whole detector.
The VELO upgrade modules are composed of the detector assemblies and electronics hybrid circuits mounted onto a cooling substrate, which is composed of thin silicon plates with embedded micro-channels that allow the circulation of liquid CO2. This technique was selected due to the excellent thermal efficiency, the absence of thermal expansion mismatch with silicon ASIC’s and sensors, radiation hardness of CO2, and very low and uniform contribution to the material budget. The front-end hybrids host the VeloPix ASICs and a GBTx ASIC for control and communication. The signals are routed to the electronics mounted outside the vacuum tank via 56~cm copper data tapes running at 5 Gb/s and custom vacuum feedthrough boards.
The secondary vacuum in which the modules are located is separated from the beam vacuum by a thin custom made foil. This foil is manufactured through a novel milling process and possibly thinned further by chemical etching.
The upgraded VELO is currently under construction and module pre-production is underway. The performance of the ASICs, bump bonded sensors and double sided electrical module will be described, along with the first results from the prototype modules in terms of electrical, mechanical and thermal performance, along with the status of the mechanical construction and preparation for assembly.