Abstract: The freeze-in production of Feebly Interacting Massive Particle (FIMP) dark matter in the early universe is an appealing alternative to the well-known – and constrained – Weakly Interacting Massive Particle (WIMP) paradigm. Although challenging, the phenomenology of FIMP dark matter has been receiving growing attention and is possible in a few scenarios. In this work, we contribute to this endeavor by considering a Z ' portal to fermionic dark matter, with the Z ' having both vector and axial couplings and a mass ranging from MeV up to PeV. We evaluate the bounds on both freeze-in and freeze-out from direct detection, atomic parity violation, leptonic anomalous magnetic moments, neutrino-electron scattering, collider, and beam dump experiments. We show that FIMPs can already be tested by most of these experiments in a complementary way, whereas WIMPs are especially viable in the Z ' low mass regime, in addition to the Z ' resonance region. We also discuss the role of the axial couplings of Z ' in our results. We therefore hope to motivate specific realizations of this model in the context of FIMPs, as well as searches for these elusive dark matter candidates.

Abstract: We present results of the Relic Axion Dark-Matter Exploratory Setup (RADES), a detector which is part of the CERN Axion Solar Telescope (CAST), searching for axion dark matter in the 34.67 μeV mass range. A radio frequency cavity consisting of 5 sub-cavities coupled by inductive irises took physics data inside the CAST dipole magnet for the first time using this filter-like haloscope geometry. An exclusion limit with a 95% credibility level on the axion-photon coupling constant of g(a gamma) greater than or similar to 4 x 10(-13) GeV-1 over a mass range of 34.6738 μeV < m(a)< 34.6771 μeV is set. This constitutes a significant improvement over the current strongest limit set by CAST at this mass and is at the same time one of the most sensitive direct searches for an axion dark matter candidate above the mass of 25 μeV. The results also demonstrate the feasibility of exploring a wider mass range around the value probed by CAST-RADES in this work using similar coherent resonant cavities.

Abstract: In addition to spectacular signatures such as black hole superradiance and the rotation of CMB polarization, the plenitude of axions appearing in the string axiverse may have potentially dangerous implications. An example is the cosmological overproduction of relic axions and moduli by the misalignment mechanism, more pronounced in regions where the signals mentioned above may be observable, that is for large axion decay constant. In this work, we study the minimal requirements to soften this problem and show that the fundamental requirement is a long period of low-scale inflation. However, in this case, if the inflationary Hubble scale is lower than around O(100) eV, no relic DM axion is produced in the early Universe. Cosmological production of some axions may be activated, via the misalignment mechanism, if their potential minimum changes between inflation and today. As a particular example, we study in detail how the maximal-misalignment mechanism dilutes the effect of dangerous axions and allows the production of axion DM in a controlled way. In this case, the potential of the axion that realises the mechanism shifts by a factor increment theta = pi between the inflationary epoch and today, and the axion starts to oscillate from the top of its potential. We also show that axions with masses m(a) similar to O(1 – 100) H-0 realising the maximal-misalignment mechanism generically behave as dark energy with a decay constant that can take values well below the Planck scale, avoiding problems associated to super-Planckian scales. Finally, we briefly study the basic phenomenological implications of the mechanism and comment on the compatibility of this type of maximally-misaligned quintessence with the swampland criteria.