The LHCb VELO detector: Design, operation and first results

Abstract

The LHCb experiment has been upgraded during the second long shutdown of the Large Hadron Collider at CERN, and a new set of refurbished detectors are currently operating at the LHC. The Vertex Locator (VELO) detector surrounds the proton beams interaction region, it is responsible of the reconstruction of the proton–proton collision (primary vertices) as well as the decay vertices of long-lived particles (secondary vertices). The VELO consists of 52 modules using hybrid silicon pixel detector technology. Compared to the previous detector, the new VELO encompass an enhanced track reconstruction speed and precision, even at the expected much higher occupancy conditions, thanks to its pixel geometry and the reduced distance to the LHC beams, with the first sensitive pixel being at just 5.1mm from the beam line. The sensors consist of 200μm thick n-on-p planar silicon sensors, read out via front-end ASICs. The detector contains 41 million 55μm×55μm pixels, read out by a custom developed front-end ASIC (VeloPix), cooled by evaporative CO2 circulating in 500μm thick silicon microchannel substrates. The VELO operates in an extreme environment, which poses significant challenges to its operation. During the lifetime of the detector, the sensors are foreseen to accumulate an integrated fluence of up to 8×10151MeVneqcm−2, roughly equivalent to a dose of 400MRad. Moreover, due to the geometry of the detector, the sensors will face a highly non-uniform irradiation, with fluences in the hottest regions expected to vary by a factor 400 within the same sensor. The highest occupancy ASICs foresee a maximum pixel hit rate of 900Mhit/s and an output data rate exceeding 15Gbit/s. The design, operation and early results evaluating the radiation damage and detector performance throughout the first year of operation will be presented in this paper.