Data reconstruction with the LHCb VELO: Hit processing, tracking, vertexing and luminosity monitoring

The LHCb experiment is dedicated to performing a detailed study of CP symmetry violation and rare decays of B and D mesons. In order to reach these physics goals the LHCb spectrometer must provide excellent vertexing and tracking performance both off-line and on-line in the trigger. The LHCb VELO (VErtex LOcator) is the silicon microstrip detector which surrounds the collision point and hence is critical to these aims. Its hit processing and zero suppression is performed in a series of algorithms implemented on FPGAs. The tuning of the parameters of these algorithms is performed using a bit-perfect emulation of these algorithms integrated in to the full off-line software of the experiment. Tracking and vertexing is then performed using the clusters produced. These algorithms are described and the results for primary and secondary vertex resolutions are given. Finally, a novel technique for measuring the absolute luminosity using gas injection in the VELO is described

Velo Event Model

This note presents the Velo Event Model, it describes the classes and data flow used in the Velo software for all stages up to and including the clusters. The description is provided for the classes used for both the real data and the simulation. This description includes the data classes used during the standard running and simulation of the experiment, and all classes defined for calibration and commissioning

Description of the Vetra Project and its Application for the VELO Detector

Vetra is the LHCb data reconstruction project which emulates the performance of the TELL1 readout board processing algorithms. This project is required for monitoring and commissioning the LHCb silicon detectors. A bit-perfect emulation of the TELL1 processing algorithms is performed. This project allows raw data (non-zero suppressed) to be processed to produce the standard zero suppressed cluster data, used by the LHCb reconstruction project Brunel. The Vetra framework is used by the VELO and ST detectors in LHCb. This note provides a general description of Vetra but concentrates on the VELO usage. Vetra is used to monitor the performance of the detector and the data acquisition board algorithms. The parameters that control the data acquisition boards are determined and optimised using Vetra. The project is used widely in the VELO and is used for testbeam and laboratory tests, including production testing for the modules

Testbeam studies of pre-prototype silicon strip sensors for the LHCb UT upgrade project

The LHCb experiment is preparing for a major upgrade in 2018-2019. One of the key components in the upgrade is a new silicon tracker situated upstream of the analysis magnet of the experiment. The Upstream Tracker (UT) will consist of four planes of silicon strip detectors, with each plane covering an area of about 2 m$^2$. An important consideration of these detectors is their performance after they have been exposed to a large radiation dose. In this article we present test beam results of pre-prototype n-in-p and p-in-n sensors that have been irradiated with fluences up to $4.0\times10^{14}$ $n_{\rm eq}$ cm$^{-2}$.Comment: 25 pages, 20 figure

Performance of the LHCb Vertex Detector Alignment Algorithm determined with Beam Test Data

LHCb is the dedicated heavy flavour experiment at the Large Hadron Collider at CERN. The partially assembled silicon vertex locator (VELO) of the LHCb experiment has been tested in a beam test. The data from this beam test have been used to determine the performance of the VELO alignment algorithm. The relative alignment of the two silicon sensors in a module and the relative alignment of the modules has been extracted. This alignment is shown to be accurate at a level of approximately 2 micron and 0.1 mrad for translations and rotations, respectively in the plane of the sensors. A single hit precision at normal track incidence of about 10 micron is obtained for the sensors. The alignment of the system is shown to be stable at better than the 10 micron level under air to vacuum pressure changes and mechanical movements of the assembled system.Comment: accepted for publication in NIM

Radiation damage in the LHCb vertex locator

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 45 × 1012 1 MeV neutron equivalent (1 MeV neq). At the operational sensor temperature of approximately −7 °C, the average rate of sensor current increase is 18 μA per 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 Eg = 1.16±0.03±0.04 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 × 1012 of 1 MeV neq. The only n+-on-p sensors in use at the LHC have also been studied. With an initial fluence of approximately 3 × 1012 1 MeV neq, a decrease in the Effective Depletion Voltage (EDV) of around 25 V is observed. Following this initial decrease, the EDV increases at a comparable rate to the type inverted n+-on-n type sensors, with rates of (1.43±0.16) × 10−12 V/ 1 MeV neq and (1.35±0.25) × 10−12 V/ 1 MeV neq 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

Performance of the LHCb vertex locator

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 μm 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 μm for translations in the plane transverse to the beam. A primary vertex resolution of 13 μm in the transverse plane and 71 μm along the beam axis is achieved for vertices with 25 tracks. An impact parameter resolution of less than 35 μm is achieved for particles with transverse momentum greater than 1 GeV/c