Astrophysics, Cosmology and Particle Phenomenology at the Energy Frontier

This dissertation consists of two parts, treating significantly separated fields. Each part consists on several chapters, each treating a somewhat isolated topic from the rest. In each chapter, I present some of the work developed during my passage through the graduate program, which has mostly been published elsewhere. Part I – Cosmic Rays and Particle Physics Chapter 1: In this chapter we present an introduction to the topic of cosmic ray physics, with an special focus on the so-called ultra high energy cosmic rays: their potential origins, effects during their propagation between their sources and Earth, the different techniques used for their detection, and the use of cosmic rays to explore fundamental physics at energies unattainable at particle accelerators. Chapter 2: In this chapter we expose some contributions to the understanding of the sources that UHECRs come from. In particular, this chapter explores some aspects of the hypothesis that starburst galaxies may be their origins. We explore the effect that interactions with cosmic microwave background photons within the source may have on the energy spectrum produced by such sources. Chapter 3: This chapter presents the influence of photon backgrounds like the solar radiation field, the cosmic microwave background or the extragalactic background light have on the propagation of cosmic rays from their source to the Earth, via nuclear photo- disintegration. In particular, we explore the the application of this effect to test violations of Lorentz invariance, and we point out the impact that inaccuracies in the description of photo-disintegration cross sections have as we try to understand the whole picture of cosmic ray production and propagation. Chapter 4: This chapter introduces another phenomenon that affects cosmic ray propagation, namely galactic magnetic fields. Magnetic fields bend charged cosmic ray trajectories, obscuring the association between individual events and their sources. This chapter explores a method by which cosmic ray acceleration, magnetic deflections and photo- disintegration are put together with the purpose of understanding cosmic ray composition. Besides that, we explore more closely the galactic magnetic field, and to which extent it can be constrained assuming that UHECRs are produced in starburst galaxies. Chapter 5: In this chapter I present my contributions to the understanding of neutrino cosmic rays. Specifically, the work in this chapter studies the spectrum of astrophysical neutrinos and the possibility that it exhibits a change in the slope at about few hundreds TeV, which could point to a transition between different astrophysical neutrino sources. Besides that, the possibility of using IceCube data to probe the neutrino cross sections beyond accelerator energies is also explored. Chapter 6: This chapter treats topics related to collective effects that may occur in the interactions of UHECRs in the atmosphere. More specifically, it is expected that when high enough energy densities are reached in nuclear collisions, a quark-gluon plasma may be formed. This would modify the produced particle spectrum as well as their angular distributions. This chapter deals with the possibility that a modification of the particle spectrum may suffice to explain the defect of muons at ground level that UHECR experiments observe. Chapter 7: This chapter presents explorations on the application of cosmic ray experi- ments to understand the detectability of more exotic systems. In particular, the absence of cosmic rays beyond some energy threshold allows us to set constraints on the properties of hypothetical super-heavy dark matter particles. On a different note, we explore the observability of the so-called macros: macroscopically sized nuggets composed of dark matter. Part II – Cosmology: Tensions & Conjectures Chapter 8: In this chapter, we present a far from exhaustive review of modern cosmology. Instead, we address the major points cosmology is based around nowadays, without offering a strong historical background. I start with the cosmological principle of homogeneity and isotropy, develop the mathematical framework to describe it, and assess what are the main contributions that drive the evolution of the Universe staying always in touch with observational evidence, that is, highlighting the impact of different cosmological effects on experimentally accessible observables such as the cosmic microwave background or the distance-redshift relation for several astrophysical objects. Chapter 9: In this chapter, we explore the cosmological implications of the Salam-Sezgin six-dimensional supergravity model after its compactification to a four-dimensional space. Such process gives rise to the presence of two coupled scalar fields. This is on par with the distance conjecture of Swampland program, by which the evolution of a rolling scalar field would produce a family of fields with masses related to such field. Here, we consider those fields as potential accounts of the dark energy and dark matter in our Universe, and explore the ability of such model to alleviate the Hubble constant tension. Chapter 10: In this chapter, we continue exploring cosmological models under the Swampland program. In particular, we consider general inflationary models mediated by scalar fields respecting S-duality invariance (that is, under φ → −φ field transformations). We study how these models are constrained by the distance and de Sitter swampland conjectures, which establish limits on the evolution of the fields. We study the feasibility of the conjecture-constrained models against measurements of the spectral index and the tensor-to-scalar ratio, parameters which measure the imprint of inflationary phenomenology on the cosmic microwave background. Chapter 11: In this chapter, we dive deeper into the Hubble constant tension. We consider a model in which several dark matter species evolve throughout the cosmological history. These particles can decay, without mixing with the standard model, into a dark radiation field. We implement a Markov chain Monte Carlo bayesian analysis to confront the large number of model parameters with a dataset comprised of multiple measurements of the Hubble parameter for low redshift, obtained as distance vs. redshift data for supernova and galaxies, as well as large scale structure information from baryon acoustic oscillations. Armed with these tools, we explore how the data constrains the number of fields, as well as their abundances and decay rates