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Register a new userVariations on the Vev Flip-Flop: Instantaneous Freeze-out and Decaying Dark Matter
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In this work we consider a simple model for dark matter and identify regions of parameter space where the relic abundance is set via kinematic thresholds, which open and close due to thermal effects. We discuss instantaneous freeze-out, where dark matter suddenly freezes-out when the channel connecting dark matter to the thermal bath closes, and decaying dark matter, where dark matter freezes-out while relativistic and later decays when a kinematic threshold temporarily opens. These mechanisms can occur in the vicinity of a one-step or a two-step phase transition. In all cases thermal effects provide this dynamic behaviour, while ensuring that dark matter remains stable until the present day.
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We investigate whether right-handed neutrinos can play the role of the dark matter of the Universe and be generated by the freeze-out production mechanism. In the standard picture, the requirement of a long lifetime of the right-handed neutrinos implies a small neutrino Yukawa coupling. As a consequence, they never reach thermal equilibrium, thus prohibiting production by freeze-out. We note that this limitation is alleviated if the neutrino Yukawa coupling is large enough in the early Universe to thermalize the sterile neutrinos, and then becomes tiny at a certain moment, which makes them drop out of equilibrium. As a concrete example realization of this framework, we consider a Froggatt-Nielsen model supplemented by an additional scalar field which obeys a global symmetry (not the flavour symmetry). Initially, the vacuum expectation value of the flavon is such, that the effective neutrino Yukawa coupling is large and unsuppressed, keeping them in thermal equilibrium. At some point the new scalar also gets a vacuum expectation value that breaks the symmetry. This may occur in such a way that the vev of the flavon is shifted to a new (smaller) value. In that case, the Yukawa coupling is reduced such that the sterile neutrinos are rendered stable on cosmological time scales. We show that this mechanism works for a wide range of sterile neutrino masses.
We study the stochastic background of gravitational waves which accompany the sudden freeze-out of dark matter triggered by a cosmological first order phase transition that endows dark matter with mass. We consider models that produce the measured dark matter relic abundance via (1) bubble filtering, and (2) inflation and reheating, and show that gravitational waves from these mechanisms are detectable at future interferometers.
It is quite conceivable that dark matter freeze-out occurred during an early period of matter domination, in which case the evolution and relic abundance differ from standard freeze-out calculations which assume a radiation dominated universe. Here we re-examine the classic models in which dark matter interactions with the Standard Model are mediated via either the Higgs or $Z$ boson in the context of matter dominated freeze-out. We highlight that while these classic models are largely excluded by searches in the radiation dominated case, matter dominated freeze-out can relax these limits and thus revive the Higgs and $Z$ portals. Additionally, we discuss the distinctions between matter dominated freeze-out and decoupling during the transition from matter domination to radiation domination, and we comment on the parameter regimes which lead to non-negligible dark matter production during this transition.
We consider dark matter (DM) with very weak couplings to the standard model (SM), such that its self-annihilation cross section is much smaller than the canonical one, $langlesigma vrangle_{chichi} ll 10^{-26}mathrm{cm}^3/mathrm{s}$. In this case DM self-annihilation is negligible for the dynamics of freeze-out and DM dilution is solely driven by efficient annihilation of heavier accompanying dark sector particles provided that DM maintains chemical equilibrium with the dark sector. This chemical equilibrium is established by conversion processes which require much smaller couplings to be efficient than annihilation. The chemical decoupling of DM from the SM can either be initiated by the freeze-out of annihilation, resembling a co-annihilation scenario, or of conversion processes, leading to the scenario of conversion-driven freeze-out. We focus on the latter and discuss its distinct phenomenology.
The nature of dark matter (DM) and how it might interact with the particles of the Standard Model (SM) is one of greatest mysteries currently facing particle physics, and addressing these issues should provide some understanding of how the observed relic abundance was produced. One widely considered production mechanism, a weakly interacting massive particle (WIMP) produced as a thermal relic, provides a target cross section for DM annihilation into SM particles by solving the Boltzmann equation. In this thermal freeze-out mechanism, dark matter is produced in thermal equilibrium with the SM in the early universe, and drops out of equilibrium to its observed abundance as the universe cools and expands. In this paper, we study the impact of a generalized thermodynamics, known as Tsallis statistics and governed by a parameter $q$, on the target DM annihilation cross section. We derive the phase space distributions of particles in this generalized statistical framework, and check their thermodynamic consistency, as well as analyzing the impact of this generalization on the collisional term of the Boltzmann equation. We consider the case of an initial value of $q_0>1$, with $q$ relaxing to 1 as the universe expands and cools, and solve the generalized Boltzmann numerically for several benchmark DM masses, finding the corresponding target annihilation cross sections as a function of $q_0$. We find that as $q$ departs from the standard thermodynamic case of $q=1$, the collisional term falls less slowly as a function of $x = m_chi/T$ than expected in the standard case. We also find that the target cross section falls sharply from $sigma v simeq 2.2-2.6times10^{-26} textrm{cm}^3/textrm{s}$ for $q_0=1$ to, for example, $sigma v simeq 3times 10^{-34} textrm{cm}^3/textrm{s}$ for $q_0=1.05$ for a 100 GeV WIMP.
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