Nonperturbative dynamics in the early universe: from axion-like particles to dark matter and condensates

Scalar fields play an important role in cosmology. They can be responsible for cosmic inflation in the very early universe, as well as are among well-motivated dark matter (DM) candidates. The aim of this dissertation is to contribute to a better understanding of the dynamics of such fields in the nonperturbative regime. Motivated by cosmological scenarios, we consider coherent oscillations of a scalar field in potentials that are summed from periodic and monomial terms, which have recently attracted much attention in the context of axion-like particles (ALP). We investigate the resonant amplification of quantum fluctuations, as well as the subsequent nonlinear dynamics after the fragmentation of the field. Our studies are extended to the nonthermal production of ALP DM. It is found that the process of fragmentation imprints strong overdensities of DM on small scales, as well as can produce a stochastic gravitational wave background, potentially within reach of future detectors. Finally, we investigate the role of experiments with ultracold atoms for the quantum simulation of nonperturbative dynamics and describe how, by means of a modulation of the interatomic interaction strength, such Bose gases can go through the characteristic stages of the dynamics of relativistic systems in the early universe

The Stochastic Relaxion

We revisit the original proposal of cosmological relaxation of the electroweak scale by Graham, Kaplan and Rajendran in which the Higgs mass is scanned during inflation by an axion field, the relaxion. We investigate the regime where the relaxion is subject to large fluctuations during inflation. The stochastic dynamics of the relaxion is described by means of the Fokker-Planck formalism. We derive a new stopping condition for the relaxion taking into account transitions between the neighboring local minima of its potential. Relaxion fluctuations have important consequences even in the "classical-beats-quantum" regime. We determine that for a large Hubble parameter during inflation, the random walk prevents the relaxion from getting trapped at the first minimum. The relaxion stops much further away, where the potential is less shallow. Interestingly, this essentially jeopardises the "runaway relaxion" threat from finite-density effects, restoring most of the relaxion parameter space. We also explore the "quantum-beats-classical" regime, opening large new regions of parameter space. We investigate the consequences for both the QCD and the non-QCD relaxion. The misalignment of the relaxion due to fluctuations around its local minimum opens new phenomenological opportunities.Comment: 35 pages and 16 figures in main text, and 15 pages and 2 figures in appendice

Real-time dynamics of false vacuum decay

We investigate false vacuum decay of a relativistic scalar field initialized in the metastable minimum of an asymmetric double-well potential. The transition to the true ground state is a well-defined initial-value problem in real time, which can be formulated in nonequilibrium quantum field theory on a closed time path. We employ the non-perturbative framework of the two-particle irreducible (2PI) quantum effective action at next-to-leading order in a large-N expansion. We also compare to classical-statistical field theory simulations on a lattice in the high-temperature regime. By this, we demonstrate that the real-time decay rates are comparable to those obtained from the conventional Euclidean (bounce) approach. In general, we find that the decay rates are time dependent. For a more comprehensive description of the dynamics, we extract a time-dependent effective potential, which becomes convex during the nonequilibrium transition process. By solving the quantum evolution equations for the one- and two-point correlation functions for vacuum initial conditions, we demonstrate that quantum corrections can lead to transitions that are not captured by classical-statistical approximations

Gravitational signatures of ALP dark matter fragmentation

The misalignment mechanism for axion-like particles (ALPs) is a leading explanation for dark matter. In this work we investigate ALPs with non-periodic potentials, which allow for large misalignment of the field from the minimum and make it possible for ALPs to match the relic density of dark matter in a large part of the parameter space. Such potentials give rise to self-interactions which can trigger an exponential growth of fluctuations in the ALP field via parametric resonance, leading to the fragmentation of the field. The fluctuations later collapse to halos that can be dense enough to produce observable gravitational effects. These effects would provide a probe of dark matter even if it does not couple to the Standard Model (or too feebly). We determine the relevant regions of parameter space in the (ALP mass, decay constant)-plane and compare predictions in different axion fragmentation models. These proceedings are a short version of arXiv:2305.03756Comment: 6 pages, 5 figures. Contribution to the proceedings of EPS-HEP202

ALP dark matter with non-periodic potentials: parametric resonance, halo formation and gravitational signatures

Axion-like particles (ALPs) are leading candidates to explain the dark matter in the universe. Their production via the misalignment mechanism has been extensively studied for cosine potentials characteristic of pseudo-Nambu-Goldstone bosons. In this work we investigate ALPs with non-periodic potentials, which allow for large misalignment of the field from the minimum. As a result, the ALP can match the relic density of dark matter in a large part of the parameter space. Such potentials give rise to self-interactions which can trigger an exponential growth of fluctuations in the ALP field via parametric resonance, leading to the fragmentation of the field. We study these effects with both Floquet analysis and lattice simulations. Using the Press-Schechter formalism, we predict the halo mass function and halo spectrum arising from ALP dark matter. These halos can be dense enough to produce observable gravitational effects such as astrometric lensing, diffraction of gravitational wave signals from black hole mergers, photometric microlensing of highly magnified stars, perturbations of stars in the galactic disk or stellar streams. These effects would provide a probe of dark matter even if it does not couple to the Standard Model. They would not be observable for halos predicted for standard cold dark matter and for ALP dark matter in the standard misalignment mechanism. We determine the relevant regions of parameter space in the (ALP mass, decay constant)-plane and compare predictions in different axion fragmentation models.Comment: 50 pages and 22 figures in the main text, and 15 pages and 2 figures in appendices, v2: As published in JCA

Determining Higgs couplings with a model-independent analysis of h ->gamma gamma

Discovering a Higgs boson at the LHC will address a major outstanding issue in particle physics but will also raise many new questions. A concerted effort to determine the couplings of this new state to other Standard Model fields will be of critical importance. Precise knowledge of these couplings can serve as a powerful probe of new physics, and will be needed in attempts to accommodate such a new boson within specific models. In this paper, we present a method for constraining these couplings in a model-independent way, focusing primarily on an exclusive analysis of the gamma gamma final state. We demonstrate the discriminating power of fully exclusive analyses, and discuss ways in which information can be shared between experimentalists and theorists in order to facilitate collaboration in the task of establishing the true origins of any new physics discovered at the LHC.Comment: 24 pages, 4 figure

The stochastic relaxion

We revisit the original proposal of cosmological relaxation of the electroweak scale by Graham, Kaplan and Rajendran in which the Higgs mass is scanned during inflation by an axion-like field, the relaxion. We investigate the regime where the relaxion is subject to large fluctuations during inflation, including the “quantum-beats-classical” regime. The stochastic dynamics is described by means of the Fokker-Planck formalism. We derive a new stopping condition for the relaxion taking into account the transitions between the local minima. For a large Hubble scale during inflation the field can stop much further away from the first minimum, where the potential is less shallow. We investigate the consequences both for the QCD relaxion and the strong CP problem, as well as for non-QCD models. We identify a new region of the parameter space where the stochastic misalignment of the relaxion from its local minimum due to fluctuations can naturally explain the observed dark matter density in the universe

The role of fluctuations in the cosmological relaxation of the weak scale

Cosmological relaxation of the electroweak scale provides an elegant solution to the Higgs mass hierarchy problem. In the simplest model, the Higgs mass is scanned during inflation by another scalar field, the relaxion, whose slow-roll dynamics selects a naturally small Higgs vev. In this work we investigate the mechanism in a less conventional regime where the relaxion is subject to large fluctuations during its dynamics. We identify modified stopping conditions for such dynamics of the relaxion and find the new parameter space. In a certain region of the parameter space, the relaxion can naturally account for the observed dark matter density in the universe