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A bstract We show that the cosmological abundance of string axions is much smaller than naive estimates if the Hubble scale of inflation, H inf , is sufficiently low (but can still be much higher than the axion masses) and if the inflation lasts sufficiently long. The reason is that the initial misalignment angles of the string axions follow the Bunch-Davies distribution peaked at the potential minima. As a result, the cosmological moduli problem induced by the string axions can be significantly relaxed by low-scale inflation, and astrophysical and cosmological bounds are satisfied over a wide range of the mass without any fine-tuning of the initial misalignment angles. Specifically, the axion with its decay constant f ϕ = 10 16 GeV satisfies the bounds over 10 −18 eV ≲ m ϕ ≲ 10 TeV for H inf ≲ 10 keV-10 6 GeV. We also discuss cases with multiple axions and the QCD axion.

A bstract In this paper, we construct the first asymmetric strongly interacting massive particles (SIMP) dark matter (DM) model, where a new vector-like fermion and a new complex scalar both having nonzero chemical potentials can be asymmetric DM particles. After the spontaneous breaking of a U(1) D dark gauge symmetry, these two particles can have accidental ℤ 4 charges making them stable. By adding one more complex scalar as a mediator between the SIMP DM, the relic density of DM is determined by 3 → 2 and two-loop induced 2 → 2 annihilations in this model. On the other hand, the SIMP DM can maintain kinetic equilibrium with the thermal bath until the DM freeze-out temperature via the new gauge interaction. Interestingly, this model can have a bouncing effect on DM, whereby the DM number density rises after the chemical freeze-out of DM. With this effect, the prediction of the DM self-interacting cross section in this model can be consistent with astrophysical observations, and the ratio of the DM energy density to the baryonic matter energy density can be explained by primordial asymmetries. We also predict the DM-electron elastic scattering cross section that can be used to test this model in future projected experiments.

A bstract In this paper, we construct for the first time a two-component strongly interacting massive particles (SIMP) dark matter (DM) model, where a complex scalar and a vector-like fermion play the role of the SIMP DM candidates. These two particles are stable due to an accidental ℤ 4 symmetry after the breaking of a U(1) D gauge symmetry. By introducing one extra complex scalar as a mediator between the SIMP particles, this model can have 3 → 2 processes that determine the DM relic density. On the other hand, the SIMP DM particles can maintain kinetic equilibrium with the thermal bath until the DM freeze-out temperature via the U(1) D gauge couplings. Most importantly, we find an unavoidable two-loop induced 2 → 2 process tightly connecting to the 3 → 2 process that would redistribute the SIMP DM number densities after the chemical freeze-out of DM. Moreover, this redistribution would significantly modify the predictions of the self-interacting cross section of DM compared with other SIMP models. It is crucial to include the two-loop induced 2 → 2 annihilations to obtain the correct DM phenomenology.

A bstract We propose the first viable radiative seesaw model, in which the neutrino masses are induced radiatively via the two-loop Feynman diagram involving Strongly Interacting Massive Particles (SIMP). The stability of SIMP dark matter (DM) is ensured by a ℤ 5 discrete symmetry, through which the DM annihilation rate is dominated by the 3 → 2 self-annihilating processes. The right amount of thermal relic abundance can be obtained with perturbative couplings in the resonant SIMP scenario, while the astrophysical bounds inferred from the Bullet cluster and spherical halo shapes can be satisfied. We show that SIMP DM is able to maintain kinetic equilibrium with thermal plasma until the freeze-out temperature via the Yukawa interactions associated with neutrino mass generation.

A bstract Hidden monopole is a plausible dark matter candidate due to its stability, but its direct experimental search is extremely difficult due to feeble interactions with the standard model particles in the minimal form. Then, we introduce an axion, a , connecting the hidden monopole and the standard model particles and examine the current limits and future prospects of direct dark matter searches and beam-dump experiments. We find two parameter regions around m a = O (10) MeV, f a = O (10 5 ) GeV and m a = O (100) MeV, f a = O (10 4 ) GeV where monopole dark matter and the axion are respectively within the reach of the future experiments such as PICO-500 and SHiP. We also note that the hidden photons mainly produced by the axion decay contribute to dark radiation with ∆ N eff ≃ 0 . 6 which can relax the H 0 tension.

In this paper, we construct the first asymmetric strongly interacting massive particles (SIMP) dark matter (DM) model, where a new vector-like fermion and a new complex scalar both having nonzero chemical potentials can be asymmetric DM particles. After the spontaneous breaking of a U(1)\\(^{}_\\textsf{D}\\) dark gauge symmetry, these two particles can have accidental \\(\\mathbb{Z}^{}_4\\) charges making them stable. By adding one more complex scalar as a mediator between the SIMP DM, the relic density of DM is determined by \\(3 \\to 2\\) and two-loop induced \\(2 \\to 2\\) annihilations in this model. On the other hand, the SIMP DM can maintain kinetic equilibrium with the thermal bath until the DM freeze-out temperature via the new gauge interaction. Interestingly, this model can have a bouncing effect on DM, whereby the DM number density rises after the chemical freeze-out of DM. With this effect, the prediction of the DM self-interacting cross section in this model can be consistent with astrophysical observations, and the ratio of the DM energy density to the baryonic matter energy density can be explained by primordial asymmetries. We also predict the DM-electron elastic scattering cross section that can be used to test this model in future projected experiments.

In this paper, we construct a viable model for a GeV scale self-interacting dark matter (DM), where the DM was thermally produced in the early universe. Here, a new vector-like fermion with a dark charge under the \\(U(1)_{D}\\) gauge symmetry serves as a secluded WIMP DM and it can dominantly annihilate into the light dark gauge boson and singlet scalar through the dark gauge interaction. Also, the self-interaction of DM is induced by the light dark gauge boson via the same gauge interaction. In addition to these particles, we further introduce two Weyl fermions and a doublet scalar, by which the dark gauge boson produced from s-wave DM annihilations can mostly decay into active neutrinos after the dark symmetry breaking such that the CMB bound on the DM with low masses can be eluded. In order to have a common parameter region to explain the observed relic abundance and self-interaction of DM, we also study this model in a non-standard cosmological evolution, where the cosmic expansion driven by a new field species is faster than the standard radiation-dominated universe during the frozen time of DM. Reversely, one can also use the self-interacting nature of light thermal DM to examine the non-standard cosmological history of the universe.

In this work, we reanalyze the multi-component strongly interacting massive particle (mSIMP) scenario using an effective operator approach. As in the single-component SIMP case, the total relic abundance of mSIMP dark matter (DM) is determined by the coupling strengths of \\(3 \\to 2\\) processes achieved by a five-point effective operator. Intriguingly, we notice that there is an irreducible \\(2 \\to 2\\) process induced by the corresponding five-point interaction in the dark sector, which would reshuffle the mass densities of SIMP DM after the chemical freeze-out. We dub this DM scenario as reshuffled SIMP (\\(r\\)SIMP). Given this observation, we then numerically solve the coupled Boltzmann equations including the \\(3 \\to 2\\) and \\(2 \\to 2\\) processes to get the correct yields of \\(r\\)SIMP DM. It turns out that the masses of \\(r\\)SIMP DM must be nearly degenerate for them to contribute sizable abundances. On the other hand, we also introduce effective operators to bridge the dark sector and visible sector via a vector portal coupling. Notably, we find that the reshuffled mechanism in the \\(r\\)SIMP scenario is sensitive to the size of the DM self-interacting cross section.

We show that the cosmological abundance of string axions is much smaller than naive estimates if the Hubble scale of inflation, \\(H_{\\rm inf}\\), is sufficiently low (but can still be much higher than the axion masses) and if the inflation lasts sufficiently long. The reason is that the initial misalignment angles of the string axions follow the Bunch-Davies distribution peaked at the potential minima. As a result, the cosmological moduli problem induced by the string axions can be significantly relaxed by low-scale inflation, and astrophysical and cosmological bounds are satisfied over a wide range of the mass without any fine-tuning of the initial misalignment angles. Specifically, the axion with its decay constant \\(f_\\phi = 10^{16}\\)\\,GeV satisfies the bounds over \\(10^{-18}{\\rm \\, eV} \\lesssim m_\\phi \\lesssim 10{\\rm\\,TeV}\\) for \\(H_{\\rm inf} \\lesssim 10{\\rm\\,keV}- 10^{6}\\)\\,{\\rm GeV}. We also discuss cases with multiple axions and the QCD axion.

We revisit the adiabatic conversion between the QCD axion and axion-like particle (ALP) at level crossing, which can occur in the early universe as a result of the existence of a hypothetical mass mixing. This is similar to the Mikheyev-Smirnov-Wolfenstein effect in neutrino oscillations. After refining the conditions for the adiabatic conversion to occur, we focus on a scenario where the ALP produced by the adiabatic conversion of the QCD axion explains the observed dark matter abundance. Interestingly, we find that the ALP decay constant can be much smaller than the ordinary case in which the ALP is produced by the realignment mechanism. As a consequence, the ALP-photon coupling is enhanced by a few orders of magnitude, which is advantageous for the future ALP and axion-search experiments using the ALP-photon coupling.