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Register a new userAlignment procedure of the LHCb Vertex Detector
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LHCb is one of the four main experiments of the Large Hadron Collider (LHC) project, which will start at CERN in 2008. The experiment is primarily dedicated to B-Physics and hence requires precise vertex reconstruction. The silicon vertex locator (VELO) has a single hit precision of better than 10 micron and is used both off-line and in the trigger. These requirements place strict constraints on its alignment. Additional challenges for the alignment arise from the detector being retracted between each fill of the LHC and from its unique circular disc r/phi strip geometry. This paper describes the track based software alignment procedure developed for the VELO. The procedure is primarily based on a non-iterative method using a matrix inversion technique. The procedure is demonstrated with simulated events to be fast, robust and to achieve a suitable alignment precision.
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LHC will offer the opportunity of probing the mass scale of the electro-weak symmetry breaking. Thus we expect to uncover direct manifestations of physics beyond the Standard Model, which will raise new questions that may be elucidated by precision measurements of beauty and charm decays. The LHCb experiment is poised to pursue this ambitious program as soon as LHC turns on. An upgrade to enhance its physics sensitivity by at least one order of magnitude is critical to the completion of this study, as new physics effects may be subtle. A new vertex detector is a crucial element of this project. Important requirements are a radiation resistance up to a fluence of about 10$^{16} n_{eq} {rm cm}^{-2}$, and a front end electronics capable of delivering its event information to the back end receiver boards synchronously with the beam interactions, at 40 MHz.
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 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 a signal to noise ratio of approximately 20 and a best hit resolution of 4 microns 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 experiments 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 micron for translations in the plane transverse to the beam. A primary vertex resolution of 13 microns in the transverse plane and 71 microns along the beam axis is achieved for vertices with 25 tracks. An impact parameter resolution of less than 35 microns is achieved for particles with transverse momentum greater than 1 GeV/c.
The extreme radiation dose received by vertex detectors at the Large Hadron Collider dictates stringent requirements on their cooling systems. To be robust against radiation damage, sensors should be maintained below -20 degree C and at the same time, the considerable heat load generated in the readout chips and the sensors must be removed. Evaporative CO2 cooling using microchannels etched in a silicon plane in thermal contact with the readout chips is an attractive option. In this paper, we present the first results of microchannel prototypes with circulating, two-phase CO2 and compare them to simulations. We also discuss a practical design of upgraded VELO detector for the LHCb experiment employing this approach.
The LHCb Vertex Locator (VELO) is a silicon strip detector designed to reconstruct charged particle trajectories and vertices produced at the LHCb interaction region. During the first two years of data collection, the 84 VELO sensors have been exposed to a range of fluences up to a maximum value of approximately $rm{45 times 10^{12},1,MeV}$ neutron equivalent ($rm{1,MeV,n_{eq}}$). At the operational sensor temperature of approximately $-7,^{circ}rm{C}$, the average rate of sensor current increase is $18,upmurm{A}$ per $rm{fb^{-1}}$, in excellent agreement with predictions. The silicon effective bandgap has been determined using current versus temperature scan data after irradiation, with an average value of $E_{g}=1.16pm0.03pm0.04,rm{eV}$ obtained. The first observation of n-on-n sensor type inversion at the LHC has been made, occurring at a fluence of around $15 times 10 ^{12}$ of $1,rm{MeV,n_{eq}}$. The only n-on-p sensors in use at the LHC have also been studied. With an initial fluence of approximately $rm{3 times 10^{12},1,MeV,n_{eq}}$, a decrease in the Effective Depletion Voltage (EDV) of around 25,V is observed, attributed to oxygen induced removal of boron interstitial sites. Following this initial decrease, the EDV increases at a comparable rate to the type inverted n-on-n type sensors, with rates of $(1.43pm 0.16) times 10 ^{-12},rm{V} / , 1 , rm{MeV,n_{eq}}$ and $(1.35pm 0.25) times 10 ^{-12},rm{V} / , 1 , rm{MeV,n_{eq}}$ measured for n-on-p and n-on-n type sensors, respectively. A reduction in the charge collection efficiency due to an unexpected effect involving the second metal layer readout lines is observed.
The LHCb experiment has been taking data at the Large Hadron Collider (LHC) at CERN since the end of 2009. One of its key detector components is the Ring-Imaging Cherenkov (RICH) system. This provides charged particle identification over a wide momentum range, from 2-100 GeV/c. The operation and control software, and online monitoring of the RICH system are described. The particle identification performance is presented, as measured using data from the LHC. Excellent separation of hadronic particle types (pion, kaon and proton) is achieved.
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