Characterizing detector modules for the Upgrade of the Silicon Tracker of the Compact Muon Solenoid experiment

Since its start-up in $2008$ the Large Hadron Collider has been producing proton-proton collisions with a center-of-mass energy approaching $14\,\text{TeV}$ for high-energy physics experiments at the European Center for Nuclear research. The resulting physics events are studied by four large experiments around the $27\,\text{km}$ collider ring one of them being the Compact Muon Solenoid detector. To boost the discovery potential of the Large Hadron Collider it will be upgraded in several phases, such that its particle interaction flux will be increased from $1\cdot10^{34}\,\text{cm}^{-2}\text{s}^{-1}$ to $5\cdot10^{34}\,\text{cm}^{-2}\text{s}^{-1}$. This is referred to as the High Luminosity Upgrade. Consequently, the four large experiments at the laboratory will be improved as well to cope with these operating conditions.

This research is focused on the characterization of prototype modules designed for the upgrade of the Compact Muon Solenoid's tracking sub-detector. It is composed of the Inner Tracker at its center that is surrounded by the Outer Tracker. The prototype modules under study consist of silicon pixel sensors and silicon strip sensors for the Inner and Outer Tracker, respectively. ... mehrPrototype front-end readout chips are connected to the sensors for the initial signal processing. These are mounted on printed circuit boards that are used to supply power to, and to extract data from the readout chips. In the first part of this thesis the calibration of the threshold setting of the prototype binary front-end chips for the Outer Tracker is described. The second part covers a beam test setup that was designed, build, and tested with the aim to perform high-rate studies ($1\,\text{MHz}$) with the tracker prototypes. This kind of setup can be used to characterize any kind of silicon tracker module in particle beams.

Due to the technology that is used to produce the binary readout chips a $10\,\%$ difference in response and threshold calibration value is expected between chips and their individual readout channels. For this reason, a method to calibrate this threshold setting was developed. This method makes use of fluorescent, or radioactive photons of a single energy that irradiate the silicon sensors while a threshold scan is executed with the binary chip. The threshold setting corresponding to the signal in the silicon can be extracted from the scan by estimating at what value the number of particle hits per event reaches zero. A calibration curve is obtained by repeating the threshold scan with photons of different energies, and from this curve the gain of the binary front-end readout chip can be estimated.

The average calibration value found for the thus far tested readout chips, expressed as the signal induced in silicon, was $367$ electrons per threshold unit. Different calibration values for different chips and readout channels were measured. For the chips used in this research the differences were not greater than $10\,\%$. A toy Monte-Carlo simulation was developed in order to study the results of the thresholds scans in further detail. This simulation included a simple model for the charge sharing between the strips of the silicon sensors.

As a preparation for high-rate beam studies with Upgrade Tracker modules that can measure particle fluxes of up to $100\,\text{MHz}/\text{cm}^2$, a new beam telescope was developed. This telescope can be used to reconstruct particle tracks that are then compared to clusters measured by a prototype module that is placed in the center of the setup. Commonly used beam telescopes have a $4600$ times slower readout $113\,\mu\text{m}$ versus $25\,\text{ns}$) than the tracker prototypes concerned in this research. Therefore, this new setup dedicated to stress testing prototype tracker modules was build with pixel detectors with the same readout speed as the tracker prototypes. To validate the setup it was tested in a particle beam with a prototype tracker module installed in its center.

The tracking resolution of the telescope was determined to be $7\,\mu\text{m}$ in the horizontal, and $10\,\mu\text{m}$ in the vertical direction. This is better than what was expected from a straight line fit that takes as the uncertainty of the hit locations measured by the telescope $100\,\mu\text{m}/\sqrt{12}$ and $150\,\mu\text{m}/\sqrt{12}$, where $100\,\mu\text{m}$ is the pixel pitch in the horizontal direction and $150\,\mu\text{m}$ the pixel pitch in the vertical direction. This was explained by the charge-sharing between the pixels of the telescope modules that allows for having sub-pixel precision on the location of the particle hit.

Tests with a Tracker prototype module in the the center of the new beam telescope confirmed that it is fully operational and ready for use in high-rate studies. Both the telescope and the prototype tracker module could be readout simultaneously. This was confirmed by a clear correlation between the tracks reconstructed with the telescope data, and the particle hit positions found with the new tracker module.