Investigation of Hadronic Higgs Decays at CLIC at 350 GeV & Scintillator Studies for a Highly Granular Calorimeter

The energy frontier of accelerator-based physics has been dominated, for the best part of the last ten years, by the Large Hadron Collider (LHC). This remarkable accelerator has provided scientists with proton-proton collisions up to 13 TeV in energy, that led to exciting progress in the understanding of particle physics, culminating in the discovery of the Higgs boson in 2012. Despite its successes, the LHC carries an intrinsic limitation: since it collides composite particles, the initial conditions of each interaction cannot be completely determined. This limits the precision with which some observables can be measured. A new generation of colliders, designed for the acceleration of elementary electrons and positrons, is being developed to reach higher precision and to provide complementary discovery potential for new phenomena. The two most mature projects in this category are the Compact LInear Collider (CLIC) and the International Linear Collider (ILC). One key component of the physics program at CLIC is the full exploration of the Higgs sector to understand the mass generation mechanism in detail. This experimental program includes the measurement of branching fractions of decays into various fermions and bosons, the coupling to the top quark and the self coupling. This thesis evaluates the achievable accuracies of the measurement of the Higgs boson branching fractions to b and c quarks, as well as to gluons, for the CLIC_ILD detector using Monte Carlo generated samples with a Higgs mass of 126 GeV at a center-of-mass energy of 350 GeV. The Higgs production mechanisms under investigation are: ZH production ("Higgsstrahlung"), with the Z further decaying into neutrinos or quarks, and Vector Boson Fusion ("VBF"), in which two W bosons, radiated off a colliding electron-positron pair, combine to form a Higgs boson. The jet reconstruction for the hadronic decays benefits from the high granularity of the calorimetric subdetectors and from particle flow algorithms that allow for unprecedented accuracies in the channels involving jets in the final state. Multivariate techniques are applied to determine the jet flavor, separating b, c and light jets, and to separate signal events from Standard Model physics background. A multidimensional likelihood fit is then applied, in a toy Monte Carlo procedure, to simultaneously extract the uncertainties of the Higgs hadronic branching fractions, as well as their correlations. Possible sources of systematic uncertainties are investigated as well. This leads to precisions at the 1-2%, 14-26% and 6-10% levels, for decays into b-quarks, c-quarks and gluons respectively, the exact value for each depending on the particular Higgs production mechanism. Complementary to the core analysis, the thesis also investigates a novel technique, that could potentially lower the costs and production time for the segmentation of scintillators for a highly granular calorimeter. This is based on an industrial process known as sub-surface laser engraving. After engraving, the response of the scintillator prototypes is analyzed by means of a position-sensitive beta ray scan and a silicon photomultiplayer (SiPM) readout, to study their light yield and the amount of intra-channel cross-talk.