Abstract
The Vertex Locator (VELO) is a silicon microstrip detector that surrounds the proton-proton interaction region in the LHCb experiment. The performance of the detector duringthe first years of its physics operation is reviewed. The system isoperated in vacuum, uses a bi-phase CO2 cooling system, and thesensors are moved to 7 mm from the LHC beam for physics data taking. The performance and stability of these characteristic features of the detector are described, and details of the material budget are given. The calibration of the timing and the data processing algorithms that are implemented in FPGAs are described.The system performance is fully characterised. The sensors have asignal to noise ratio of approximately 20 and a best hit resolution of4 μm is achieved at the optimal track angle. The typical detectoroccupancy for minimum bias events in standard operating conditions in2011 is around 0.5%, and the detector has less than 1% of faultystrips. The proximity of the detector to the beam means that the innerregions of the n+-on-n sensors have undergone space-charge signinversion due to radiation damage. The VELO performance parametersthat drive the experiment's physics sensitivity are also given. Thetrack finding efficiency of the VELO is typically above 98% and themodules have been aligned to a precision of 1 μm for translations inthe plane transverse to the beam. A primary vertex resolution of 13 μm in thetransverse plane and 71 μm along the beam axis is achieved forvertices with 25 tracks. An impact parameter resolution ofless than 35 μm is achieved for particles with transverse momentumgreater than 1 GeV/c.
Highlights
LHCb [1] is an experiment dedicated to heavy flavour physics at the LHC
The LHCb Vertex Locator [2] is a silicon microstrip detector positioned around the protonproton interaction region
The impact parameter resolution was optimised by positioning the Vertex Locator (VELO) sensors as close to the LHC beam as permitted by safety consideration, having a small inter-strip pitch at the inside of the sensors, and minimising the amount of material traversed by a particle before the first measured hits in the VELO
Summary
LHCb [1] is an experiment dedicated to heavy flavour physics at the LHC. Its primary aim is to discover new physics through precision studies of CP violation and rare decays of beauty and charm hadrons. The LHCb Vertex Locator [2] is a silicon microstrip detector positioned around the protonproton interaction region. The VELO is designed to cover the forward region, such that all tracks inside the nominal LHCb acceptance of 15–300 mrad cross at least three VELO stations In this way the detector fully reconstructs roughly 27% of bb production for 7 TeV protonproton centre-of-mass collisions, while covering just 1.8% of the solid angle [3, 4]. The impact parameter resolution was optimised by positioning the VELO sensors as close to the LHC beam as permitted by safety consideration, having a small inter-strip pitch at the inside of the sensors, and minimising the amount of material traversed by a particle before the first measured hits in the VELO. The section ends with information on the material budget of the VELO
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