Extending preferred axion models via heavy-quark induced early matter domination

We examine the cosmological consequences of the heavy quarks in KSVZ-type axion models. We find that their presence often causes an early matter domination phase, altering the evolution of the Universe. This extends the axion mass into the region where standard cosmology leads to overproduction, and allows for a greater number of axion models with non-renormalizable terms to be viable. Quantitatively, we find that decays proceeding through effective terms of up to dimension 9 ($d=9$) remain consistent with cosmological constraints, in contrast with the result $d\leq5$ previously found in the literature. As a consequence, the heavy quarks can be much heavier and the axion mass window with the correct relic density for dark matter is extended by orders of magnitude, down to $m_a\approx 6\times 10^{-9} \,{\rm eV}$. This is achieved without resorting to fine-tuning of the initial misalignment angle, bolstering the motivation for many future axion haloscope experiments. Additionally, we explore how these models can be probed through measurements of the number of relativistic degrees of freedom at recombination.Comment: 24 pages, 6 figure

Dark matter production through a non-thermal flavon portal

The Froggatt-Nielsen (FN) mechanism provides an attractive way of generating the determined fermion mass hierarchy and quark mixing matrix elements in the Standard Model (SM). Here we extend it by coupling the FN field, the flavon, to a dark sector containing one or more dark matter particles which are produced non-thermally sequentially through flavon production. Non-thermal flavon production occurs efficiently via freeze-in and through field oscillations. We explore this in the regime of high-scale breaking $\Lambda$ of the global $U(1)_{\textrm{FN}}$ group and at the reheating temperature $T_R\ll \Lambda$ where the flavon remains out of equilibrium at all times. We identify phenomenologically acceptable regions of $T_R$ and the flavon mass where the relic abundance of dark matter and other cosmological constraints are satisfied. In the case of one-component dark matter we find an effective upper limit on the FN charges at high $\Lambda$, i.e. $Q_{\rm FN}^{\rm DM}\leq13$. In the multi-component dark sector scenario the dark particle can be the heaviest dark particle that can be effectively stable at cosmological timescales, alternatively it can be produced sequentially by decays of the heavier ones. For scenarios where dark decays occur at intermediate timescales, i.e. $t\sim 0.1- 10^{28}\,{\rm s}$, we find that existing searches can effectively probe interesting regions of parameter space. These searches include indirect probes on decays such as $\gamma$-ray and neutrino telescopes as well as analyses of the Cosmic Microwave Background, as well as constraints on small scale structure formation from the Lyman-$\alpha$ forest. We comment on the future prospects of such probes and place projected sensitivities.Comment: 21 pages, 4 figure

Dark Matter Axions in the Early Universe with a Period of Increasing Temperature

We consider the production of axion dark matter through the misalignment mechanism in the context of a nonstandard cosmological history involving early matter domination by a scalar field with a time-dependent decay rate. In cases where the temperature of the Universe experiences a temporary period of increase, Hubble friction can be restored in the evolution of the axion field, resulting in the possibility of up to three "crossings" of the axion mass and the Hubble expansion rate. This has the effect of dynamically resetting the misalignment mechanism to a new initial state for a second distinct phase of oscillation. The resultant axion mass required for the present dark matter relic density is never bigger than the standard-history window and can be smaller by more than three orders of magnitude, which can be probed by upcoming experiments such as ABRACADABRA, KLASH, ADMX, MADMAX, and ORGAN, targeting the axion-photon coupling. This highlights the possibility of exploring the cosmological history prior to Big Bang Nucleosynthesis through searches for axion dark matter beyond the standard window.Comment: 28 pages, 8 figure

Revisiting signatures of thermal axions in nonstandard cosmologies

We revisit the formation of a thermal population of hadronic axions in nonstandard cosmologies, in light of the recent developments in obtaining continuous and smooth interaction rates for both the gluon and photon couplings. For certain cosmological histories, such as low-temperature reheating (LTR) and kination-like scenarios, the thermalization of the axion can be severely delayed to higher masses. In the case that thermal equilibrium is achieved, we improve the constraints on LTR for axion masses around the eV scale with respect to previous works and we constrain for the first time early matter-dominated (EMD) cosmologies. We also point out the possibility of having the co-existence of cold and warm dark matter populations of axions in kination-like scenarios in the eV mass range.Comment: 40 pages, 10 figure

Phenomenology of superheavy decaying dark matter from string theory

We study the phenomenology of superheavy decaying dark matter with mass around $10^{10}$ GeV which can arise in the low-energy limit of string compactifications. Generic features of string theory setups (such as high scale supersymmetry breaking and epochs of early matter domination driven by string moduli) can accommodate superheavy dark matter with the correct relic abundance. In addition, stringy instantons induce tiny $R$-parity violating couplings which make dark matter unstable with a lifetime well above the age of the Universe. Adopting a model-independent approach, we compute the flux and spectrum of high-energy gamma rays and neutrinos from three-body decays of superheavy dark matter and constrain its mass-lifetime plane with current observations and future experiments. We show that these bounds have only a mild dependence on the exact nature of neutralino dark matter and its decay channels. Applying these constraints to an explicit string model sets an upper bound of ${\cal O}(0.1)$ on the string coupling, ensuring that the effective field theory is in the perturbative regime

Phenomenology of superheavy decaying dark matter from string theory

Abstract We study the phenomenology of superheavy decaying dark matter with mass around 1010 GeV which can arise in the low-energy limit of string compactifications. Generic features of string theory setups (such as high scale supersymmetry breaking and epochs of early matter domination driven by string moduli) can accommodate superheavy dark matter with the correct relic abundance. In addition, stringy instantons induce tiny R-parity violating couplings which make dark matter unstable with a lifetime well above the age of the Universe. Adopting a model-independent approach, we compute the flux and spectrum of high-energy gamma rays and neutrinos from three-body decays of superheavy dark matter and constrain its mass-lifetime plane with current observations and future experiments. We show that these bounds have only a mild dependence on the exact nature of neutralino dark matter and its decay channels. Applying these constraints to an explicit string model sets an upper bound of O $$\mathcal{O}$$ (0.1) on the string coupling, ensuring that the effective field theory is in the perturbative regime