Resolution studies and performance evaluation of the LHCb VELO upgrade

Hynds, Daniel Peter McFarlane (2014) Resolution studies and performance evaluation of the LHCb VELO upgrade. PhD thesis, University of Glasgow.

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The LHCb detector at CERN is scheduled to undergo an upgrade during the second long shutdown of the LHC. As part of this upgrade, the vertex detector (VELO) will be replaced with a new hybrid pixel detector, based on an evolution of the Timepix ASIC. The performance of this detector should improve upon that achieved by the current VELO, in addition to facilitating the complete detector readout at 40 MHz.

As part of the preparation for this upgrade, this thesis presents the results of studies carried out on the single hit resolution of silicon hybrid pixel detectors. The development of a particle beam telescope has been carried out to allow these studies, shown to operate with track rates in excess of 45 kHz and with a pointing resolution at the device under test of less than 2 μm. A wide range of sensor types, thicknesses and resistivities have then been tested under different operating conditions and the results presented, with single hit resolutions varying between 4 μm and 12 μm depending on the conditions and incident angle. The resistivity of the devices is observed to have a significant effect on the single hit resolution, with high resistivity substrates allowing operation at lower bias voltages. This facilitates increased charge sharing, and the corresponding improvement in resolution. At sufficiently large incident angles however, the resolution becomes independent of the electric field, being instead dominated by the sensor geometry and variations in the charge deposited along the track length. No significant differences were found between the various detector technologies (n-on-n, n-on-p and p-on-n) though a difference in performance is expected for low-voltage operation of higher resistivity samples. A simplified model of the physical processes contributing to the detector resolution has been constructed, shown to reasonably reproduce the observed resolution as a function of angle and bias voltage. This model is extrapolated to potential future directions in the design of pixel sensors, highlighting the differences between various technology choices.

The integration of the ATLAS FE-I4 ASIC into the telescope has been carried out, and the performance of an unirradiated planar silicon sensor was shown in order to verify this. Efficiency measurements show that the device is fully efficient in the angular range measured. The tracking performance of two irradiated sensors mounted on FE-I4 ASICs has been investigated, in addition to the mapping of collected charge over the pixel unit cell under various biasing conditions and at varying incident angles with respect to the incoming particles. For the sample irradiated to 2e15 1 MeV neq /cm2 the single hit resolution was 12.5 μm at perpendicular incidence, dropping to 8 μm at 22 degrees. The sample irradiated to 4e15 1 MeV neq /cm2 was found to have a resolution of around 13.5 μm, which remained relatively insensitive to the incident track angle. The conclusions drawn suggest that the upgraded VELO detector will be able to overcome the difficult radiation environment if it is able to reach the high voltage operation required.

The implementation of these observations in the LHCb simulation environment has allowed some initial studies on the likely degradation of the detector performance to take place, showing that the high tracking efficiency (99.4 % for Long tracks) is likely to be maintained throughout the full lifetime of the upgrade. The impact parameter resolution was not observed to vary significantly. These studies have been carried out alongside simulations to gauge the expected compression that can be achieved in the data transmission of the VELOPix ASIC. Different designs of the front-end have been implemented, leading to the adoption of binary readout for the upgraded VELO. The uniformity of the pixel pitch across the detector has additionally been used to show the sensitivity of the system to multiple scattering, shown to be a credible tool with which to control the event reconstruction in the online LHCb trigger. A reduction of the number of VELO tracks passed to the forward reconstruction of almost 50 % has been shown, for the loss of only 5 % of tracks with momentum above 20 GeV/c. This could potentially replace the lifetime biasing cuts currently envisaged in the trigger.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: LHCb, VELO, vertex detector, silicon detector, resolution.
Subjects: Q Science > QC Physics
Colleges/Schools: College of Science and Engineering > School of Physics and Astronomy
Supervisor's Name: Eklund, Dr. Lars and Soler, Professor Paul and Parkes, Professor Christopher
Date of Award: 2014
Depositing User: Mr Daniel Hynds
Unique ID: glathesis:2014-6169
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 24 Mar 2015 11:26
Last Modified: 02 May 2018 14:36

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