Narrow Results

We point out that a well-known axion model with an explicit Z(N) symmetry breaking term predicts both dark energy and cold dark matter. We estimate the parameters of this model which fit the observed densities of the dark components of the universe. We find that the parameters do not conflict with any observations.

The axion–photon system in an external magnetic field, when for example considered with the geometry of the experiments exploring axion–photon mixing, displays a continuous axion–photon duality symmetry in the limit the axion mass is neglected. The conservation law that follows from this symmetry is obtained. The magnetic field interaction is seen to be equivalent to first order to the interaction of a complex charged field with an external electric potential, where this fictitious "electric potential" is proportional to the external magnetic field. This allows one to solve for the scattering amplitudes using already known scalar QED results. It is argued that in more generic conditions (not just related to these experiments) axion–photon condensation could be obtained for high magnetic fields. Finally an exact constraint originating from the current conservation on the amplitudes of reflected and transmitted waves is obtained.

The Supersymmetric Standard Model is a benchmark theoretical framework for particle physics, yet it suffers from a number of deficiencies, the main one among which is the strong CP problem. Solving this with an axion in the context of selected new particles, it is shown in three examples that other problems go away automatically as well, resulting in (-)^L and (-)^3B conservation, viable combination of two dark-matter candidates, successful baryogenesis, seesaw neutrino masses, and verifiable experimental consequences at the TeV energy scale.

We suggest that experiments based on Josephson junctions, SQUIDS, and coupled Josephson qubits may be used to construct a resonant environment for dark matter axions. We propose experimental setups in which axionic interaction strengths in a Josephson junction environment can be tested, similar in nature to recent experiments that test for quantum entanglement of two coupled Josephson qubits. We point out that the parameter values relevant for early-universe axion cosmology are accessible with present day's achievements in nanotechnology. We work out how typical dark matter and dark energy signals would look like in a novel detector that exploits this effect.

The Micromegas detectors have been gaining importance as reliable options in their implementation to Time Projection Chambers (TPCs) in experiments searching for Rare Events mainly due to their demonstrated good performance regarding low background levels, energy and time resolution, gain and stability of operation. In the present paper, we will briefly review the latest developments carried out within the T-REX project of detector R&D, and the performance achieved in the context of several experiments: the CAST solar axion search experiment, the NEXT experiment of double beta decay and the MIMAC dark matter directional search.

The nature of cosmological dark matter finds its explanation in physics beyond the Standard Model of elementary particles. The landscape of dark matter candidates contains a wide variety of species, either elusive or hardly detectable in direct experimental searches. Even in case, when such searches are possible the interpretation of their results implies additional sources of information, which provide indirect effects of dark matter. Some nontrivial probes for the nature of the dark matter are presented in the present issue.

We investigate the decay of condensates of scalars in a field theory defined by $V({𝒜})=m^2{f}^2[1-cos({𝒜}/f)]$, where $m$ and $f$ are the mass and decay constant of the scalar field. An example of such a theory is that of the axion, in which case the condensates are called axion stars. The axion field, ${𝒜}$, is self-adjoint. As a result, the axion number is not an absolutely conserved quantity. Therefore, axion stars are not stable and have finite lifetimes. Bound axions, localized on the volume of the star, have a coordinate uncertainty $\delta x\sim R\sim 1/(m_a\mathrm{\Delta })$, where $R$ is the radius of the star and $\mathrm{\Delta }=\sqrt{1-E_0^2/m_a^2}$. Here $m_a$ and $E_0$ are the mass, and the ground state energy of the bound axion. Then the momentum distribution of axions has a width of $\delta p\sim m_a\mathrm{\Delta }$. At strong binding, $\mathrm{\Delta }={𝒪}(1)$, bound axions can easily transfer a sufficient amount of momentum to create and emit a free axion, leading to fast decay of the star with a transition rate $\mathrm{\Gamma }\sim m_a$. However, when $\mathrm{\Delta }\ll 1$, the momentum distribution is more restricted, and as shown in this paper, the transition rate for creating a free axion decreases as $exp(-p\delta x)\sim exp(-{\mathrm{\Delta }}^{-1})$. Then sufficiently large, weakly bound axion stars, produced after the Big Bang, survive until the present time. We plot the region of their stability, limited by decay through axion loss and by gravitational instability, as a function of the mass of the axion and the mass of the star.

One of the most powerful probes of new physics is the polarized cosmic microwave background (CMB). The detection of a nonzero polarization angle rotation between the CMB surface of last scattering and today could provide evidence of Lorentz-violating physics. The purpose of this paper is two-fold. First, we review one popular mechanism for polarization rotation of CMB photons: the pseudo-Nambu–Goldstone boson (PNGB). Second, we propose a method to use the Polarbear experiment to constrain Lorentz-violating physics in the context of the Standard Model Extension (SME), a framework to standardize a large class of potential Lorentz-violating terms in particle physics.

We revisit the issue of the vacuum angle $𝜃$ dependence in weakly coupled (Higgsed) Yang–Mills theories. Two most popular mechanisms for eliminating physical $𝜃$ dependence are massless quarks and axions. Anselm and Johansen noted that the vacuum angle ${𝜃}_{\mathrm{\text{EW}}}$, associated with the electroweak SU(2) in the Glashow–Weinberg–Salam model (Standard Model, SM), is unobservable although all fermion fields obtain masses through Higgsing and there is no axion. We generalize this idea to a broad class of Higgsed Yang–Mills theories.

In the second part, we consider the consequences of Grand Unification. We start from a unifying group, e.g. SU(5), at a high ultraviolet scale and evolve the theory down within the Wilson procedure. If on the way to infrared the unifying group is broken down into a few factors, all factor groups inherit one and the same $𝜃$ angle — that of the unifying group. We show that embedding the SM in SU(5) drastically changes the Anselm–Johansen conclusion: the electroweak vacuum angle ${𝜃}_{\mathrm{\text{EW}}}$, equal to ${𝜃}_{\mathrm{\text{QCD}}}$ becomes in principle observable in $\mathrm{\Delta }B=\mathrm{\Delta }L=\pm 1$ processes. We also note in passing that if the axion mechanism is set up above the unification scale, we have one and the same axion in the electroweak theory and QCD, and their impacts are interdependent.

We study a model where photons interact with hidden photons and millicharged particles through a kinetic mixing term. Particularly, we focus on vacuum birefringence effects and we find a bound for the millicharged parameter assuming that hidden photons are a piece of the local dark matter density.

We suggest that the dark matter halo in some of the spiral galaxies can be described as the ground state of the Bose–Einstein condensate of ultra-light self-gravitating axions. We have also developed an effective “dissipative” algorithm for the solution of nonlinear integro-differential Schrödinger equation describing self-gravitating Bose–Einstein condensate. The mass of an ultra-light axion is estimated.

We consider a two-component dark matter halo (DMH) of a galaxy containing ultra-light axions (ULA) of different mass. The DMH is described as a Bose–Einstein condensate (BEC) in its ground state. In the mean-field (MF) limit, we have derived the integro-differential equations for the spherically symmetrical wave functions of the two DMH components. We studied, numerically, the radial distribution of the mass density of ULA and constructed the parameters which could be used to distinguish between the two- and one-component DMH. We also discuss an interesting connection between the BEC ground state of a one-component DMH and Black Hole temperature and entropy, and Unruh temperature.

We consider a dark matter halo (DMH) of a spherical galaxy as a Bose–Einstein condensate (BEC) of the ultra-light axions (ULA) interacting with the baryonic matter. In the mean-field (MF) limit, we have derived the integro-differential equation of the Hartree–Fock type for the spherically symmetrical wave function of the DMH component. This equation includes two independent dimensionless parameters: (i) $\beta$ is the ratio of baryon and axion total mases and (ii) $\xi$ is the ratio of characteristic baryon and axion spatial parameters. We extended our “dissipation algorithm” for studying numerically the ground state of the axion halo in the gravitational field produced by the baryonic component. We calculated the characteristic size, $x_c$ of DMH as a function of $\beta$ and $\xi$ and obtained an analytical approximation for $x_c$.

We reconsider entropy arguments which have been previously argued to support the idea that the dark matter constituents are primordial black holes with many solar masses. It has recently been shown that QCD axions which solve the strong CP problem may have masses $m_a$ in the extended range $10^{-3}\mathrm{\text{eV}}>m_a>10^{-33}\mathrm{\text{eV}}$. Ultralight axions provide so many degrees of freedom that their entropy can exceed that of primordial black holes. This suggests that ultralight axions are more suited than primordial black holes to be constituents of dark matter.

A deeper understanding of the vacuum structure in QCD invites one to rethink certain aspects of axion physics. The recent advances are mostly due to developments in super-symmetric gauge theories and the brane theory, in which QCD can be embedded. They include, but are not limited to, the studies of metastable vacua in multicolor gluo-dynamics, and the domain walls. We briefly review basics of axion physics and then present a modern perspective on a rich interplay between the QCD vacuum structure and axion physics.

In this work, we analyze the propagation of photons in an environment where a strong magnetic field (perpendicular to the photon momenta) coexists with an oscillating cold axion background with the characteristics expected from dark matter in the galactic halo. Qualitatively, the main effect of the combined background is to produce a three-way mixing among the two photon polarizations and the axion. It is interesting to note that in spite of the extremely weak interaction of photons with the cold axion background, its effects compete with those coming from the magnetic field in some regions of the parameter space. We determine (with one plausible simplification) the proper frequencies and eigenvectors as well as the corresponding photon ellipticity and induced rotation of the polarization plane that depend both on the magnetic field and the local density of axions. We also comment on the possibility that some of the predicted effects could be measured in optical table-top experiments.

We discuss the astronomical methods of searching for light Goldstone bosons (axions and arions). The basic idea is to use processes of coupling between axions and photons: a) the axion decay into two photons; b) the transformation process of photons into axions (arions) in the magnetic fields of stars and also of interstellar and intergalactic media; c) the inverse process of transformations of axions (arions) which are generated into cores of stars into X-ray photons. The decaying axions affect upon the diffuse extragalactic background radiation, the brightness of the night sky and especially on the intergalactic light of clusters of galaxies due to generation of the axion radiative decay emission line. The processes (b) and (c) are strongly dependent on polarization state of photon and may produce a noticeable amount of linear polarization.

Development of low radioactive technique allows to investigate many other rare nuclear and subnuclear processes: double $\beta$ decay, rare $\beta$ and $\alpha$ decays, and to search for hypothetical particles and processes like axions, charge nonconserving decays, the nucleon, di-nucleon and tri-nucleon decays into invisible channels, to test the Pauli principle. Here, we present results of the rare processes searches and development of instrumentation for low counting experiments.

It is discussed how the ideas of entropy and the second law of thermodynamics, conceived long ago during the nineteenth century, underly why cosmological dark matter exists and originated in the first three years of the universe in the form of primordial black holes, a very large number of which have many solar masses including up to the supermassive black holes at the centres of galaxies. Certain upper bounds on dark astrophysical objects with many solar masses based on analysis of the CMB spectrum and published in the literature are criticised. For completeness we discuss WIMPs and axions which are leading particle theory candidates for the constituents of dark matter. The PIMBHs (Primordial Intermediate Mass Black Holes) with many solar masses should be readily detectable in microlensing experiments which search the Magallenic Clouds and measure light curves with durations of from one year up to several years.

The axions and axion-like particles can be detected via a resonant atomic or molecular transition induced by axion absorption. The signal obtained in this process is second order in the axion-electron interaction constant and hence small. In this chapter, it is demonstrated that this signal may become first order in the axion-electron interaction constant if we allow the interference between the axion-induced transition amplitude and the transition amplitude induced by the electromagnetic radiation. Additionally, we show that the conventional scheme of producing axions from photons in a magnetic field may be improved if the field is replaced by an atomic medium in which photons scattering off the atoms in the forward direction are transformed into axions.