Very weakly interacting slim particles (WISPs), such as axion-like particles (ALPs) or hidden photons (HPs), may be non-thermally produced via the misalignment mechanism in the early universe and survive as a cold dark matter population until today. We find that, both for ALPs and HPs whose dominant interactions with the standard model arise from couplings to photons, a huge region in the parameter spaces spanned by photon coupling and ALP or HP mass can give rise to the observed cold dark matter. Remarkably, a large region of this parameter space coincides with that predicted in well motivated models of fundamental physics. A wide range of experimental searches — exploiting haloscopes (direct dark matter searches exploiting microwave cavities), helioscopes (searches for solar ALPs or HPs), or light-shining-through-a-wall techniques — can probe large parts of this parameter space in the foreseeable future.

We study axion dark matter production from a misalignment mechanism in scenarios featuring a general nonstandard cosmology. Before the onset of Big Bang nucleosynthesis, the energy density of the universe is dominated by a particle field ϕ described by a general equation of state ω. The ensuing enhancement of the Hubble expansion rate decreases the temperature at which axions start to oscillate, opening this way the possibility for axions heavier than in the standard window. This is the case for kination, or in general for scenarios with ω > 1/3. However, if ω < 1/3, as in the case of an early matter domination, the decay of ϕ injects additional entropy relative to the case of the standard model, diluting this way the preexisting axion abundance, and rendering lighter axions viable. For a misalignment angle 0.5 < θ_i < π/√(3), the usual axion window becomes expanded to 4 × 10^-9 eV ≲ m_a ≲ 2 × 10^-5 eV for the case of an early matter domination, or to 2 × 10^-6 eV ≲ m_a ≲ 10^-2 eV for the case of kination. Interestingly, the coupling axion-photon in such a wider range can be probed with next generation experiments such as ABRACADABRA, KLASH, ADMX, MADMAX, and ORGAN. Axion dark matter searches may therefore provide a unique tool to probe the history of the universe before Big Bang nucleosynthesis.

One of the prime tools to search for new light bosons interacting very weakly with photons\char22{}prominent examples are axions, axionlike particles, and extra ``hidden'' U(1) gauge bosons\char22{}are light-shining-through-a-wall experiments. With the current generation of these experiments finishing data taking it is time to plan for the next and search for an optimal setup. The main challenges are clear: on the one hand we want to improve the sensitivity towards smaller couplings, on the other hand we also want to increase the mass range to which the experiments are sensitive. Our main example is axion(like particle)s but we also discuss implications for other WISPs (weakly interacting slim particles) such as hidden U(1) gauge bosons. To improve the sensitivity for axions towards smaller couplings, one can use multiple magnets to increase the length of the interaction region. However, naively the price to pay is that the mass range is limited to smaller masses. We discuss how one can optimize the arrangement of magnets (both in field direction as well as allowing for possible gaps in between) to ameliorate this problem. Moreover, future experiments will include resonant, high quality optical cavities in both the production and the regeneration region. To achieve the necessary high quality of the cavities we need to avoid too high diffraction losses. This leads to minimum requirements on the diameter of the laser beam and therefore on the aperture of the cavity. We investigate what can be achieved with currently available magnets and desirable features for future ones.

We analyze the consequences of violation of Lorentz and $CPT$ invariance in the massless neutrino sector by deforming the canonical anti-commutation relations for the fields. We show that, for particular choices of the deformation, oscillation between massless neutrino species takes place when only Lorentz invariance is violated. On the other hand, if both Lorentz and $CPT$ invariances are violated, we show that there is no oscillation between massless neutrino species. Comparing with the existing experimental data on neutrino oscillations, we obtain bounds on the parameter for Lorentz invariance violation.

We point out that a cold dark matter condensate made of gauge bosons from an extra hidden U(1) sector—dubbed hidden photons—can create a small, oscillating electric density current. Thus, they could also be searched for in the recently proposed LC-circuit setup conceived for axion cold dark matter search by Sikivie, Sullivan and Tanner. We estimate the sensitivity of this setup for hidden-photon cold dark matter and we find it could cover a sizable, so far unexplored parameter space.