Particle identification by d𝘌/d𝘹 with the ILD TPC. Prototype development and application at the ILC

In this thesis the development of a time projection chamber (TPC) for the International Large Detector (ILD) concept at the International Linear Collider (ILC) is discussed and an analysis is carried out to evaluate the performance of the detector concept. The ILC is a planned linear electron-positron collider. Its first construction stage with a centre-of-mass energy of 250 GeV is primarily intended to perform precision measurements of the Higgs boson that was discovered at the LHC in 2012. In addition, the unprecedented precision that is aimed for also allows to search for indirect signs of new physics that manifest as deviations from the predictions of the standard model. The ILD is one of the detector concepts proposed for the ILC. Its distinctive feature is the large TPC foreseen as its main tracking detector. A TPC provides a large number of measured space points on each track, resulting in an excellent momentum resolution even for low momentum particles. Additionally it allows to measure the specific energy loss (d𝐸/d𝑥) of the particles and thus perform particle identification. In the first part of this thesis the development of a modular readout for the ILD TPC is presented. Each module provides avalanche gas amplification of the electron signal via a stack of three gas electron multiplier (GEM) foils, supported by thin ceramic grids. The signal is read out on the anode plane, which is segmented into pads. For this work the production procedure for the modules was reviewed and improved to achieve a higher accuracy and repeatability. Three newly assembled modules were tested in a prototype TPC at the DESY II Test Beam Facility. The acquired data is used to validate the results regarding the spatial resolution acquired in a previous study. Additionally the d𝐸/d𝑥 resolution of the setup is determined for the first time. An extrapolation of the result to the conditions of the ILD TPC shows that the envisioned relative resolution of 5 % can be achieved. In the second part of this thesis a measurement of the branching fractions of the Z boson into the light quarks d, u and s is presented. In the standard model the coupling of the Z to the quarks is independent of the flavour. Any deviation from this pattern is a hint of new physics. Due to the difficulty in identifying the flavour of the primary quark in jets originating from light quarks, the respective individual branching fractions are only known with uncertainties of 5 % to 10 %. The analysis presented here uses data from a detailed simulation of the ILD, scaled to the full data set of 𝐿${}_{int}$ = 2 ab${}^{-1}$ of the ILC at $\sqrt{s}$ = 250 GeV. To tag the flavour of light quark jets, the high momentum leading hadrons in each jet are used since their flavour content is correlated to the flavour of the primary quark. One challenge of this approach is to identify the species of the tagging hadrons in the detector. The d𝐸/d𝑥 measurement in the TPC is an essential tool for this purpose. The analysis shows that a relative statistical precision of the branching fraction into light down-type quarks of about 1 % can be achieved.