UC Berkeley

Hints of new physics Beyond the Standard Model (BSM) range from dark matter and the strong CP problem to grand unification and the origin of the matter-antimatter asymmetry. Historically, colliders have been the principal engines of discovery, but with no new physics discovered at the Large Hadron Collider (LHC) except the expected Higgs, and decades until the next collider may be built, a few questions naturally arise: What if there is no new physics until very high scales? How can we discover high energy physics which may hide at energies far above the reach of next-generation colliders? This dissertation focuses on answering these questions in three parts.Part (I) discusses early-Universe cosmology and model building guided by hints from Standard Model parameters as measured by the LHC, particularly Higgs Parity phenomenology. Higgs Parity is a two Higgs doublet mirror extension of the Standard Model that provides an explanation for the peculiar vanishing of the Higgs quartic coupling at very high energies due to quantum corrections from Standard Model particles. Higgs Parity comes in many rich variations, but all share the key mechanism of making the Standard Model Higgs a pseudo- Goldstone boson at the Higgs quartic scale, thereby giving the Standard Model Higgs a vanishing mass and hence vanishing quartic coupling at this scale. The phenomenology of these variations of Higgs Parity are discussed in Chapters 1-3. We find that Higgs Parity admits a natural dark matter candidate in the mirror electron, which can be detected from its scattering with protons due to unavoidable kinetic mixing between the mirror photon and our photon (Ch. 1); generation of dark radiation from the decay of mirror glueballs that can be detected by CMB Stage IV (Ch. 2); and generation of our observed matter-antimatter asymmetry via leptogenesis associated with warm and hot sterile neutrino dark matter (Ch. 3). In all Higgs Parity models, future precision measurements of the top quark mass, strong coupling constant, and Higgs mass will hone in on the precise scale at which the Higgs quartic vanishes and hence predict the aforementioned signals. The reader will thus find signal plots in this part of the dissertation that indicate how the various Higgs Parity signals change as a function of these Standard Model parameters. Finally, Part (I) concludes with discussion on physics inspired by, or in similar spirit to, Higgs Parity: general cosmological constraints on sterile neutrino dark matter in left-right symmetric theories (Ch. 4) and Higgsino dark matter in Intermediate Scale Supersymmetry models (Ch. 5). Part (II) focuses on astrophysical probes of BSM physics at energies and couplings unreach- able at current colliders. We first turn to Nature’s own accelerator, supernova shocks, to search for undiscovered CHarged Massive Particles (CHAMPs) that may make up a compo- nent of dark matter (Ch 6). Such undiscovered particles with minuscule electric charges are well motivated in particle physics (kinetic mixing between the photon and a dark photon), and in cosmology. For example, a particle with electric charge about one trillionth that of an electron can be thermally produced via freeze-in in the early Universe with a relic abundance matching that of the dark matter we see today. Typically, such small electrically charged particles are too weakly interacting or too massive to be discovered at colliders. However, the plasma of the interstellar medium provides a unique laboratory to search for such particles. We trace the dynamics of CHAMPs in the Milky Way and their acceleration by supernova shocks and find this Fermi-accelerated component of dark matter can provide unique experimental signatures typically absent from dark matter moving at virial speeds, such as from their Cherenkov light produced in water or ice. From this analysis, we disfavor CHAMP dark matter with mass less than 10^5 GeV and charge greater than 10^-9 e. In the following chapter, we examine how Magnetic White Dwarfs (MWDs) can generate leading constraints on the coupling of low mass axions to photons (Ch. 7). Axions — well- motivated particles that arise in many theories beyond the Standard Model, such as from the breaking of a global U (1) or from string compactifications — are extremely weakly coupled to Standard Model particles and are thus difficult to probe. However MWDs possess enormous static (B ? 100 MG) and large scale (coherence ? 1R?) magnetic fields that can provide another unique laboratory to test the axion-modified Maxwell equations. In particular, we calculate the axion-induced polarization of MWD starlight arising from the conversion of photons leaving the MWD atmosphere and converting to axions in the MWD magnetosphere. Taking into account astrophysical polarizations and uncertainties, we exclude, at 2σ, axion- photon couplings greater than 5.4 × 10^−12 GeV^−1 for axion masses below 3 × 10^−7 eV. Part (III), which concludes this dissertation, considers other novel signals of high energy physics from the sky, namely gravitational waves. Gravitational waves provide a particularly promising way of studying ultra-high energy physics since gravitational waves produced in the early Universe can travel unimpeded through the primordial plasma and be detected today, carrying information about the BSM physics that sourced them. Moreover, it is often the case that the higher the scale of the BSM physics, the stronger the gravitational wave signal. In contrast, with state-of-the-art technology, a collider far larger than the size of the solar system is needed to reach energies approaching grand unification scales. We first study the gravitational wave signals from a stochastic cosmic string background experiencing an exotic equation of state in the early Universe known as kination, which can arise from the rotation of an axion field (Ch. 8). We find that the change in the expansion rate of the Universe due to the rotation of the axion field imprints a unique triangular peaked gravitational wave spectrum that encodes enformation about the duration and energy scale of the kination era. We determine the parameter space where current and future gravitational wave detectors can distinguish the kination cosmology from the standard ΛCDM cosmology. In the final chapter (Ch. 9), we investigate more generally the gravitational wave signals from hybrid topological defects such as cosmic strings bounded by magnetic monopoles or domain walls bounded by cosmic strings. We show that many grand unification paths generate hybrid topological defects in the early Universe that decay via gravitational waves from the ‘eating’ of one defect by the other via the conversion of its rest mass into the other defect’s kinetic energy. We calculate these gravitational wave ‘gastronomy’ signals and show how observation of these relic gravitational wave signatures can be used to distinguish many unification paths, providing extraordinary insight into ultra-high energy physics.