Abstract

The LHCb experiment at the Large Hadron Collider (LHC) at CERN is a dedicated experiment designed to search for New Physics in the decays of beauty and charm hadrons. These hadrons are identified via their flight distance in the Vertex Locator (VELO), making the VELO extremely important for the physics performance of LHCb. The VELO is a silicon strip detector, located around the interaction region. It consists of two retractable halves with 21 modules each, which are open during injection and closed when 'stable beams' are declared, bringing the VELO sensors at a distance of 8 mm from the LHC beams. Each module has two n-on-n type silicon sensors mounted back to back. Due to its proximity to the beam it is subject to high doses of radiation, which are non-uniform across the sensors and along the z-axis. The VELO has been operated successfully during the first two years of data taking. Results for the hit resolution, primary vertex resolution and impact parameter resolution are presented. Some aspects of radiation damage are also discussed.

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

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Summary

Introduction

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

Commissioning results
Vacuum stability
Cooling performance
Low voltage and high voltage
Motion performance
Material description
Data acquisition system
Timing and gain
ADC sampling time
Timing to the beam
FPGA data processing algorithms
Pedestal subtraction
Mean common mode suppression
Zero suppression and clusterisation
Error identification
Single event upsets
Monitoring
Simulation
Signal size and noise rate
Resolution
Occupancy
Beam backgrounds and high multiplicity events
Efficiency and faulty channel analysis
Radiation damage studies
Current measurements
Effective doping concentration
Charge loss to second metal layer
Findings
Conclusions

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