A new discrete ordinates method designed for use on modern, large memory, vector and/or parallel processing super computers has been developed. The method is similar to conventional SN techniques in that the medium is divided into spatial mesh cells and that discrete directions are used. However, in place of an approximate differencing scheme, a nearly exact matrix representation of the streaming operator is determined. Although extremely large, this matrix can be stored on today's computers for repeated use in the source iteration. Since the source iteration is cast in matrix form it benefits enormously from vector and/or parallel processing, if available. Several test results are presented demonstrating the reduction in numerical diffusion and elimination of ray effects.

The MicroBooNE experiment is a liquid argon time projection chamber with 85-ton active volume at Fermilab, operated from 2015 to 2020 to collect neutrino data from Fermilab’s Booster Neutrino Beam. One of MicroBooNE’s physics goals is to investigate possible explanations of the low-energy excess observed by the MiniBooNE experiment in 𝜈_𝜇 → 𝜈_𝘦 neutrino oscillation measurements. MicroBooNE has performed searches to test hypothetical interpretations of the MiniBooNE low-energy excess, including the underestimation of the photon background or instrinic 𝜈_𝘦 background. This thesis presents MicroBooNE’s searches for two neutral current (NC) single-photon production processes that contribute to the photon background of the MiniBooNE measurement: NC Δ resonance production followed by Δ radiative decay: Δ → 𝛮_𝛾, and NC coherent single-photon production. Both searches take advantage of boosted decision trees to yield efficient background rejection, and a high-statistic NC ?0 measurement to constrain dominant background, and make use of MicroBooNE’s first three years of data. The NC Δ → 𝛮_𝛾 measurement yielded a bound on the Δ radiative decay process at 2.3 times the predicted nominal rate at 90% confidence level (C.L.), disfavoring a candidate photon interpretation of the MiniBooNE low-energy excess as a factor of 3.18 times the nominal NC radiative decay rate at the 94.8% C.L. The NC coherent single-photon measurement leads to the world’s first experimental limit on the cross-section of this process below 1 GeV, of 1.49 × 10−41cm² at 90% C.L., corresponding to 24.0 times the nominal prediction.

The Cosmic Microwave Background (CMB) serves as a powerful backlight, illuminating thestructures throughout the universe. As the CMB photons travel to our telescopes from the surface of last scattering, their interactions with matter imprint detectable signatures in the CMB spectrum, known as the CMB secondary anisotropies. Among these late-time phenomena, the Sunyaev–Zel’dovich (SZ) effect—caused by scattering of the CMB photons off free electrons—is one of the most powerful, providing a unique window into the pressure and density of the electron gas. As the ionized gas and its feedback on the underlying dark matter distribution via high-energy processes present a significant obstacle to obtain precise cosmological constraints from the matter power spectrum, the SZ effect serves as an invaluable tool to address these challenges. This thesis uses the measurements of the CMB secondary anisotropies, particularly the SZ effect, from the state-of-the-art experiments, the Planck satellite and the ground-based Atacama Cosmology Telescope (ACT), in combination with Large-Scale Structure data to probe the ionized gas in distant galaxies. Chapter 2 presents the second measurement of the kinetic SZ effect in the unWISE galaxies with Planck using the projected-fields estimator. This work concludes that the ionized gas abundance in these galaxies matches the primordial-CMB predictions. Chapter 3 describes the work done to model the galaxy-halo connection of the unWISE catalog with Planck CMB lensing data using the halo model framework. It constrained the halo masses of these samples to ≈ 2 ×10¹³ _⊙/ℎ, and found that they are dominated by central galaxies, rather than satellites. These constraints can be directly used in other cross-correlations of unWISE with, e.g., the tSZ or the kSZ effect in the halo model with the upcoming CMB experiments. Chapter 5 discusses the ongoing work of cross-correlating the Dark Energy Survey Maglim galaxies with the thermal SZ maps from ACT. It measures very extended pressure profiles around Maglim, which suggest strong feedback activity in low mass objects, pushing the ionized gas far outside of the halo. This thesis also presents novel techniques to tackle the key systematics in cosmological cross-correlations. The analysis of Maglim galaxies employs the new Cosmic Infrared Background (CIB) cleaning technique, the moment-deprojection method, which ensures that the measurement is robust to this foreground. Chapter 4 discusses three new methods to remove the CIB and tSZ contamination, using the external Large-Scale Structure data which show a large correlation with both fields (e.g., the unWISE catalog). With the new methods presented, it is possible to remove those contaminants to enhance the measurements of the blackbody component of a CMB map. The results presented in this thesis offer a unique window into the baryons residing in distant galaxies through the SZ effect, confirming there is no missing baryons, and indicating that the feedback is stronger than predicted in simulations. These analyses lay the groundwork for cross-correlations of the upcoming high-resolution, low-noise CMB experiments such as the Simons Observatory, and high density galaxy surveys, including DESI, Euclid, or LSST. The upcoming measurements will yield precise constraints on gas physics, transforming our understanding of galaxy formation, and enabling cosmological constraints from the matter power spectrum, where baryons currently represent the primary uncertainty.

In this thesis, two studies of the diffractive phenomena in the electron proton collisions with the H1 detector at HERA are presented.<p>The first is the study of the inclusive elastic diffractive events $ep \ / Doctorat en Sciences / info:eu-repo/semantics/nonPublished