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Performance of the LHCb Vertex Locator
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LHCb VELO Group (Including: Dossett, D., Gershon, T. J., Latham, Thomas, Reid, Matthew M., Silva Coutinho, R., Wallace, Charlotte and Whitehead, M. (Mark)). (2014) Performance of the LHCb Vertex Locator. Journal of Instrumentation, Volume 9 . Article number P09007 . doi:10.1088/1748-0221/9/09/P09007
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Official URL: http://dx.doi.org/10.1088/1748-0221/9/09/P09007
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 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 4µm 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 1µm for translations in the plane transverse to the beam. A primary vertex resolution of 13µm in the transverse plane and 71µm along the beam axis is achieved for vertices with 25 tracks. An impact parameter resolution of less than 35µm is achieved for particles with transverse momentum greater than 1GeV/c.
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