Dissertation/introduction.tex

\chapter{Introduction}
\label{ch:introduction}

The Standard Model (SM) of particle physics~\autocite{Glashow, Salam, Weinberg} is a well tested and very successful theory that explains the structure of matter and the interactions between the elementary particles.
According to the SM, all visible forms of matter are composed of 12 fundamental particles, which are divided into the groups of quarks and leptons.
Furthermore, it includes the gauge bosons that are the mediators of the interactions between these particles.
One of them is the well-known photon, which is the force carrier of the electromagnetic force.
Together with the strong and weak interaction, they represent the three fundamental forces included in the model.
Lastly, there is the Higgs boson, which was predicted by Peter Higgs in 1964~\autocite{higgs-theory}.
The associated Higgs field plays an important role in the SM, as it explains how the elementary particles gain their masses.

While almost any experimental observation in the field of particle physics is in agreement with the SM predictions, it is not a complete theory.
This can already be seen from the fact that the gravitational force, although negligible at the subatomic scale, is not covered by it.
Other observations that cannot be explained by the SM are the asymmetry between matter and antimatter in the universe~\autocite{baryon-asymmetry}, the existence of dark matter~\autocite{dark-matter}, and the measurement of neutrino oscillations~\autocite{neutrino-oscillations}.
It is therefore essential to continue the thorough testing of the SM, as well as theories beyond it.

In 2012, with the observation of the Higgs boson at the Large Hadron Collider (LHC), the last missing elementary particle of the SM was discovered~\autocite{atlas-higgs, cms-higgs}.
The LHC near Geneva, Switzerland, is the most powerful particle accelerator in the world.
By colliding high-energy protons at enormous intensities, it provides the ideal conditions to probe the SM and search for new physics phenomena.

The \lhcbexperiment{} is one of four major experiments that are located at the LHC.
It is a dedicated experiment specialised in studying hadrons\footnote{Hadrons are composite particles made of two or more quarks that are bound by the strong interaction.
They are further divided into mesons (usually containing two quarks) and baryons (usually three quarks).} containing $b$ and $c$ quarks.
They are produced in the proton-proton collisions and decay again after a short time into other particles, which can be detected in the experiment.
During the first years of operation starting from 2010, the \lhcbexperiment{} has recorded the world's largest samples of events containing $b$ and $c$ hadrons~\autocite{lhcb-performance}.

The collected samples allowed for performing important measurements in the field of flavour physics such as the observation of the rare decay ${B^0_s \rightarrow \mu^+\mu^-}$~\autocite{bmumu}, the determination of the CKM\footnote{The Cabibbo–Kobayashi–Maskawa (CKM) matrix parametrises the relative weak coupling strengths between the different quark flavours.} angle $\gamma$~\autocite{gamma} or the test of lepton universality~\autocite{Rk}.
While the latter already revealed a slight tension compared to the SM prediction, the significance of the measurement is statistically limited.
In fact, many key results at LHCb are limited by the statistical sensitivity of the data samples.
Therefore, to be competitive with the uncertainties of the theoretical predictions, the \lhcbexperiment{} will collect data at five times the previous luminosities\footnote{At particle colliders, the luminosity $\luminosity$ is a measure for the interaction rate $\dot{N}$ relative to the cross-section $\sigma$.} starting from the middle of the year 2022.
In order to prepare the detector for the increasing rates, a three-year upgrade period was required during which in particular the tracking system has been improved.

An essential component of the upgraded tracking system is a detector made of scintillating fibres: the LHCb Scintillating Fibre (SciFi) Tracker.
With a fibre diameter of \qty{250}{\micro\meter}, it provides sufficient granularity for the higher number of particle tracks in each event.
The produced scintillation light is detected by arrays of silicon photomultipliers (SiPMs), whose output signals are processed and digitised by a complex chain of front-end electronics.
The electronics is designed and optimised to sample the signals at a rate of \qty{40}{\mega\hertz}, matching the proton bunch collision rate at the LHC.
While the initial version of the \lhcbexperiment{} was only read out at a fraction of that rate, the so-called trigger-less readout is required to fully exploit the increase in luminosity.

In the context of this thesis, several contributions to the \scifitracker{}, in particular to the front-end electronics, have been made.
While supporting various steps in the research and production phase of the different electronic components, the main responsibility was the initial commissioning of the electronics on a large scale alongside the surrounding infrastructure and the upgraded LHCb DAQ system.
A major challenge that had to be overcame in this context was the establishment of the \qty{40}{\mega\hertz} readout of the complex detector.
In its final configuration, the \scifitracker{} will produce data volumes up to \qty{20}{\tera\bit/\s} corresponding to about \qty{40}{\percent} of the entire \lhcbexperiment{}.

As part of the front-end electronics commissioning, a detailed test procedure has been defined, developed and implemented.
In the process, every component is thoroughly tested and validated in its final environment before being released for installation in the experiment.
This also includes first calibration and tuning steps that are required for the proper operation of the detector.

The following contents will be discussed in the course of this thesis:
In \cref{ch:lhcb}, the \lhcbexperiment{} is further introduced, followed by the \scifitracker{} in \cref{ch:scifi}.
Afterwards, in \cref{ch:fee}, a detailed overview on the SciFi front-end electronics is given, providing a first complete reference of all components involved.
In \cref{ch:commissioning}, the commissioning procedure is presented and the results from the commissioning of a large fraction of the detector are shown.
Finally, performance studies that were conducted alongside the commissioning are discussed in \cref{ch:performance}.


% Old stuff
%The collected samples allowed for performing key measurements of \textit{CP} violation\footnote{\textit{CP} violation is associated to an asymmetry in the behaviour of particles and their corresponding antiparticles} and the observation of (very) rare processes.

%A key measurement enabled by the collected samples was the observation of the decay of a $B^0_s$ meson into two muons: ${B^0_s \rightarrow \mu^+\mu^-}$~\autocite{bmumu}.
%Although it is a very rare process\footnote{About 1 out of 300M $B^0_s$ mesons decays into two muons.} in the SM, it is accurately predicted by the theory~\autocite{bmumu-theory}.
%On the other hand, the clear signature of the decay allows for a clean measurement in the LHCb detector.
%While the current measurements are in agreement with the SM prediction, they are still limited by statistical uncertainties.
%In addition, the related and even rarer decay $B^0 \rightarrow \mu^+\mu^-$ could not be observed yet.

%To allow for even preciser measurements and to uncover even rarer processes like the related $B^0 \rightarrow \mu^+\mu^-$ decay, which could not be observed yet, the \lhcbexperiment{} will collect data at five times the previous luminosities\footnote{At particle colliders, the luminosity $\luminosity$ is a measure for the interaction rate $\dot{N}$ relative to the cross-section $\sigma$.} starting from the middle of this year 2022.