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Hadron structure is an important field in particle physics because hadrons make up most of the matter in nature. The theory of the strong nuclear force, via which the partons of hadrons interact, is Quantum Chromodynamics (QCD) and cannot be solved analytically. Lattice QCD (LQCD) is an ideal formulation of QCD and is the only formulation starting from first principles. In this thesis, we use LQCD for two primary topics of study: 1) nucleon structure and 2) pion and kaon structure. In the first study, we calculate the quark momentum fraction, helicity, and transversity for the nucleon. The calculations are performed on three ensembles at the physical point of the pion mass allowing us to study finite volume, discretization, strange and charm quark quenching, and excited-state systematic effects. Our calculations of the helicity and transversity are first predictions at the physical point. In the second study, we investigate pion and kaon structure. We calculate the first three non-trivial Mellin moments of the meson parton distribution functions (PDFs). For the kaon, this is the first direct calculation of the second and third moments. We carefully choose which matrix elements we implement so that there is no mixing with lower derivative operators, avoiding systematic uncertainties which are not well understood. We also perform an extensive study of the excited-state contamination. In a pioneering study, we show that the full x-dependence of the PDFs can be calculated from the first three Mellin moments. Such a calculation was previously thought to be unfeasible using moments calculated from LQCD. Our reconstruction of the PDFs allow us to comment on SU(3) flavor symmetry breaking and the high-x behavior of the pion PDF which are both interesting topics in hadron structure. / Physics

Longstanding evidence from observational astronomy indicates that non-luminous "dark matter" constitutes the majority of all matter in the universe, yet this mysterious form of matter continues to elude experimental detection. This dissertation presents a search for dark matter at the Large Hadron Collider using 139 fb\(^{-1}\) of proton-proton collision data at a centre-of-mass energy of \(\sqrt{s} = 13\,\)TeV, recorded with the ATLAS detector from 2015 to 2018. The search targets a final state topology in which dark matter is produced from the proton-proton collisions in association with a pair of W bosons, one of which decays to a pair of quarks and the other to a lepton-neutrino pair. The dark matter is expected to pass invisibly through the detector, resulting in an imbalance of momentum in the plane transverse to the beam line. The search is optimized to test the Dark Higgs model, which predicts a signature of dark matter production in association with the emission of a hypothesized new particle referred to as the Dark Higgs boson. The Dark Higgs boson is predicted to decay to a W boson pair via a small mixing with the Standard Model Higgs boson discovered in 2012. Collisions that exhibit the targeted final state topology are selected for the search, and an approximate mass of the hypothetical Dark Higgs boson is reconstructed from the particles in each collision. A search is performed by looking for a deviation between distributions of the reconstructed Dark Higgs boson masses and Standard Model predictions for the selected collisions. The data is found to be consistent with the Standard Model prediction, and the results are used to constrain the parameters of the Dark Higgs model. This search complements and extends the reach of existing searches for the Dark Higgs model by the ATLAS and CMS collaborations. / Graduate

Radiotherapy is one of the most used methods for treating cancer and the most common way to execute such treatments is to irradiate tumors with high energy photons. This can damage healthy tissue along the irradiation line. By using hadron therapy and instead irradiate the tumor with charged particles (protons or Carbon 12 ions), the energy can be concentrated to a more specific place in the body. However, the method is not well studied and the tools available for simulating hadron therapy can be hard to use. When simulating hadron therapy and other nuclear interactions a large amount of calculations need to be executed. Monte Carlo methods is a numerical method to solve equations based on repeated number sampling and is used in the simulation program Geant4. Hadron therapy was simulated with Geant4 and the data was analyzed with the data analysis framework ROOT. New macros and analysis scripts were created with the intention to help new Geant4 users. The aim to make Geant4 easier to use was partially met. The implementation of code for the low energy region of Carbon 12 projectiles was unsuccessful.

The Standard Model of particle physics constitutes the most accurate and comprehensive known description of the fundamental building blocks of the universe. However, overwhelming evidence suggests that the theory is incomplete and that new physics may be hiding at the TeV-scale. The Large Hadron Collider (LHC) at CERN probes these high-energy scales, opening a potential gateway to access physics beyond the Standard Model (BSM). Long-lived particles (LLPs) arise in many promising BSM theories, but they remain weakly constrained at the LHC. This thesis presents a novel search for displaced and delayed diphoton and dielectron vertices originating from the decay of a neutral LLP. The analysis uses the full LHC Run 2 dataset of pp collisions at a center-of-mass energy of √s = 13 TeV recorded by the ATLAS detector, corresponding to an integrated luminosity of 139 fb-1. The search harnesses the capabilities of the ATLAS Liquid Argon calorimeter to precisely measure the displacement and delay of the final state electromagnetic objects. The results are interpreted in a gauge-mediated supersymmetry breaking model that features the pair-production of LLPs, with each LLP subsequently decaying into either a Higgs boson or a Z boson. Since no significant excess is observed above the background expectation, the results are used to set upper limits on the cross section of LLP pair-production for signals with an LLP mass between 100 and 725 GeV and lifetime between 0.25 ns and 1 μs. A model-independent limit is also set on the production of pairs of photons or electrons with a significantly delayed arrival at the calorimeter.

<h4>Exotic hadrons are hadrons with constituent quark content different from the normal meson and baryon. The f₀(980) hadron is one of the exotic hadron candidates, which was first discovered in $\pi\pi$ scattering experiments in the 1970's. It could be a ordinary quarnk-antiquark meson, a tetraquark exotic state, a kaon-antikaon molecule, or a quark-antiquark-gluon hybrid state.</h4><p dir="ltr">Anisotropic flow (v_n) is a powerful tool to probe the dynamics of relativistic heavy ion collisions. The observed approximate scaling of v_n by the number of constituent quarks (NCQ) suggests the importance of partonic degree of freedom in these collisions. Large v_n and NCQ-scaling have also been observed in high-multiplicity proton-lead (pPb) collisions at the LHC, suggesting similar dynamics in pPb collisions. The empirically established NCQ-scaling can be exploited to probe the constituent quark content of the f₀(980) hadron.</p><p dir="ltr">We measure the elliptic flow (v₂) of f₀(980) in high-multiplicity pPb collisions at 8 TeV with the Compact Muon Solenoid (CMS) detector at the LHC, through its main pion-pion ($\pi^+\pi^-$) decay channel. The invariant mass spectrum is obtained from opposite sign pion pairs, and the combinatorial background is estimated by same-sign pion pairs and subtracted. The f₀(980) signal and the remaining residual background are modeled by relativistic Breit-Wigner and polynomial functions, respectively. The yield of f₀(980) is extracted as a function of azimuthal angle relative to the event plane, and the f₀(980) v₂ is measured as function of $p_T$, corrected for the event-plane resolution. The non-flow component in f₀(980) v₂ has been estimated by that of K_s⁰. By comparing the measured v₂ of the f₀(980) to those of other hadrons, we infer the number of constituent quarks for the f₀(980) assuming NCQ scaling. The f₀(980) is found to be a 2-quark state in this work. We discuss the systematic uncertainties involved in our study, implications of our result, and possible future works in this endeavor.</p>

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