Performance of the LHCb Vertex Locator

R. Aaij*, A. Affolder, K. Akiba, M. Alexander, S. Ali, R.B. Appleby, M. Artuso, A. Bates, A. Bay, O. Behrendt, J. Benton, M. Van Beuzekom, P.M. Bjornstad, G. Bogdanova, S. Borghi, A. Borgia, T.J.V. Bowcock, J. Van den Brand, H. Brown, J. BuytaertO. Callot, J. Carroll, G. Casse, P. Collins, S. De Capua, M. Doets, S. Donleavy, D. Dossett, R. Dumps, D. Eckstein, L. Eklund, C. Farinelli, S. Farry, M. Ferro-Luzzi, R. Frei, J. Garofoli, M. Gersabeck, T. Gershon, A. Gong, H. Gong, H. Gordon, G. Haefeli, J. Harrison, V. Heijne, K. Hennessy, W. Hulsbergen, T. Huse, D. Hutchcroft, A. Jaeger, P. Jalocha

*Corresponding author for this work

Research output: Contribution to journalArticleAcademicpeer-review


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 during the first years of its physics operation is reviewed. The system is operated in vacuum, uses a bi-phase CO2 cooling system, and the sensors are moved to 7mm 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 a signal to noise ratio of approximately 20 and a best hit resolution of 4 mu m is achieved at the optimal track angle. The typical detector occupancy for minimum bias events in standard operating conditions in 2011 is around 0.5%, and the detector has less than 1% of faulty strips. The proximity of the detector to the beam means that the inner regions of the n(+)-on-n sensors have undergone space-charge sign inversion due to radiation damage. The VELO performance parameters that drive the experiment''s physics sensitivity are also given. The track finding efficiency of the VELO is typically above 98% and the modules have been aligned to a precision of 1 mu m for translations in the plane transverse to the beam. A primary vertex resolution of 13 mu m in the transverse plane and 71 mu m along the beam axis is achieved for vertices with 25 tracks. An impact parameter resolution of less than 35 mu m is achieved for particles with transverse momentum greater than 1GeV/c.
Original languageEnglish
Article numberP09007
Number of pages52
JournalJournal of Instrumentation
Publication statusPublished - 1 Sept 2014
Externally publishedYes


  • Solid state detectors
  • Performance of High Energy Physics Detectors
  • Particle tracking detectors (Solid-state detectors)
  • Radiation-hard detectors

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