On the evolution of global strings in the early universe

We find that a bent global string straightens itself out by dissipation into Nambu-Goldstone (NG) bosons in the order of one oscillation time and that the spectrum of radiated NG bosons is 1/k. In the early universe, the assumption that there is one global string per causal horizon is justified after an initial period during which the effect of the primordial plasma on the motion of the strings is non-negligible. We find that the upper bound that axion radiation by strings places on the axion decay constant is comparable to, but not more stringent than the one derived from coherent oscillations of the axion field, i.e. ∫_a ≲ 10¹²GeV. We also discuss the model-dependence of this bound.     

Axion oscillations and the quark-hadron phase transition

We consider the possibility that the quark-hadron phase transition occurs when the axion field passes through the minimum of its potential during its oscillation cycle. If this were to occur, the axion field would gain no energy from the associated increase in mass thus permitting the cosmological bound on the axion decay constant to be raised. However, we find that the probability of this happening is small.     

Wave propagation in axion electrodynamics

In this paper, the axion contribution to the electromagnetic wave propagation is studied. First we show how the axion electrodynamics model can be embedded into a premetric formalism of Maxwell electrodynamics. In this formalism, the axion field is not an arbitrary added Chern–Simon term of the Lagrangian, but emerges in a natural way as an irreducible part of a general constitutive tensor. We show that in order to represent the axion contribution to the wave propagation it is necessary to go beyond the geometric approximation, which is usually used in the premetric formalism. We derive a covariant dispersion relation for the axion modified electrodynamics. The wave propagation in this model is studied for an axion field with timelike, spacelike and null derivative covectors. The birefringence effect emerges in all these classes as a signal of Lorentz violation. This effect is however completely different from the ordinary birefringence appearing in classical optics and in premetric electrodynamics. The axion field does not simple double the ordinary light cone structure. In fact, it modifies the global topological structure of light cones surfaces. In CFJ-electrodynamics, such a modification results in violation of causality. In addition, the optical metrics in axion electrodynamics are not pseudo-Riemannian. In fact, for all types of the axion field, they are even non-Finslerian.     

String theory and the strong CP problem

The axion solution to the strong CP problem is reexamined. It is noted that in order for the axion to solve the problem, it is necessary that high-energy contributions to the axion potential be sufficiently small. Examples are constructed where this is not the case. It is noted that this problem arises in many interesting compactifications of string theory and in popular supergravity models, and that this provides a significant phenomenological constraint on model building. In string theory, these constraints are investigated both for compactifications with unification in E₆ and in O(10) and SU(5). We observe that the latter case is especially exciting, since one can have extra light Higgs doublets while satisfying renormalization group constraints.     

Exotic axion cosmology in theories with phase transitions below the QCD scale

We show that axion phenomenology may be significantly different than conventionally assumed in theories which exhibit late phase transitions (below the QCD scale). In such theories, one can find multiple pseudoscalars with axionlike couplings to matter, including a string scale axion, whose decay constant far exceeds the conventional cosmological bound. Such theories have several dark matter candidates.     

Pion mass effects on axion emission from neutron stars through NN bremsstrahlung processes

The rates of axion emission by nucleon–nucleon bremsstrahlung are calculated with the inclusion of the full momentum contribution from a nuclear one pion exchange (OPE) potential. The contributions of the neutron–neutron (nn), proton–proton ( pp) and neutron–proton (np) processes in both the non-degenerate and degenerate limits are explicitly given. We find that the finite-momentum corrections to the emissivities are quantitatively significant for the non-degenerate regime and temperature-dependent, and should affect the existing axion mass bounds. The trend of these nuclear effects is to diminish the emissivities.     

Axions and “light shining through a wall”: A detailed theoretical analysis

We give a detailed study of axion-photon and photon-axion conversion amplitudes, which enter the analysis of “light shining through a wall” experiments. Several different calculational methods are employed and compared, and in all cases we retain a nonzero axion mass. To leading order, we find that when the photon frequency ω is very close to the axion mass m, there is a threshold cusp which significantly enhances the photon to axion conversion amplitude, by a factor relative to the corresponding axion to photon conversion process. When m=0, the enhancement factor reduces to unity and the results of previous calculations are recovered. Our calculations include an exact wave matching analysis, which shows how unitarity is maintained near threshold at ω=m, and a discussion of the case when the magnetic field extends into the “wall” region.     

On spontaneous CP-violation in the Higgs sector

We consider the behaviour of the axion mass as a function of Higgs coupling constants. The analysis is significantly simplified when we identify the axion field with the phase difference of the Higgs neutral components. Spontaneous CP-violation is induced by VEV of the axion field. The estimation of the axion mass for any values of and shows inconsistency of MSSM with the present experimental results. Received: 15 July 1997 / Published online: 26 February 1998     

Fine-tuning favors mixed axion/axino cold dark matter over neutralinos in the minimal supergravity model

Over almost all of minimal supergravity (mSUGRA or CMSSM) model parameter space, there is a large overabundance of neutralino cold dark matter (CDM). We find that the allowed regions of mSUGRA parameter space which match the measured abundance of CDM in the universe are highly fine-tuned. If instead we invoke the Peccei–Quinn–Weinberg–Wilczek solution to the strong CP problem, then the SUSY CDM may consist of an axion/axino admixture with an axino mass of order the MeV scale, and where mixed axion/axino or mainly axion CDM seems preferred. In this case, fine-tuning of the relic density is typically much lower, showing that axion/axino CDM ( $a\tilde{a}$ CDM) is to be preferred in the paradigm model for SUSY phenomenology. For mSUGRA with $a\tilde{a}$ CDM, quite different regions of parameter space are now DM-favored as compared to the case of neutralino DM. Thus, rather different SUSY signatures are expected at the LHC in the case of mSUGRA with $a\tilde{a}$ CDM, as compared to mSUGRA with neutralino CDM.     

Inflation at the TeV scale with a PNGB curvaton

We investigate a particular type of curvaton mechanism, under which inflation can occur at Hubble scale of order 1 TeV. The curvaton is a pseudo Nambu–Goldstone boson, whose order parameter increases after a phase transition during inflation, triggered by the gradual decrease of the Hubble scale. The mechanism is studied in the context of modular inflation, where the inflaton is a string axion. We show that the mechanism is successful for natural values of the model parameters, provided the phase transition occurs much earlier than the time when the cosmological scales exit the horizon. Also, it turns our that the radial mode for our curvaton must be a flaton field.     

Localized axion photon states in a strong magnetic field

We consider the axion field and electromagnetic waves with rapid time dependence, coupled to a strong time independent, asymptotically approaching a constant at infinity “mean” magnetic field, which takes into account the back reaction from the axion field and electromagnetic waves with rapid time dependence in a time averaged way. The direction of the self consistent mean field is orthogonal to the common direction of propagation of the axion and electromagnetic waves with rapid time dependence and parallel to the polarization of these electromagnetic waves. Then, there is an effective U(1) symmetry mixing axions and photons. Using the natural complex variables that this U(1) symmetry suggests we find localized planar soliton solutions. These solutions appear to be stable since they produce a different magnetic flux than the state with only a constant magnetic field, which we take as our “ground state”. The solitons also have non-trivial U(1) charge defined before, different from the uncharged vacuum. These solitons represent a new, non-gravitational mechanism, of trapping light. They could also affect the vacuum structure in models of the QCD vacuum that incorporate a magnetic condensate, introducing may be gluon axion solitons.     

Is it possible to explore Peccei-Quinn axions from frequency-dependence radiation dimming?

We explore how the Peccei-Quinn (PQ) axion parameter space can be constrained by the frequency-dependence dimming of radiation from astrophysical objects. To do so we perform accurate calculations of photon-axion conversion in the presence of a variable magnetic field. We propose several tests where the PQ axion parameter space can be explored with current and future astronomical surveys: the observed spectra of isolated neutron stars, occultations of background objects by white dwarfs and neutron stars, the light-curves of eclipsing binaries containing a white dwarf. We find that the lack of dimming of the light-curve of a detached eclipsing white dwarf binary recently observed, leads to relevant constraints on the photon-axion conversion. Current surveys designed for Earth-like planet searches are well matched to strengthen and improve the constraints on the PQ axion using astrophysical objects radiation dimming.     

The Flux‐Scaling scenario: De sitter uplift and axion inflation

Non‐geometric flux‐scaling vacua provide promising starting points to realize axion monodromy inflation via the F‐term scalar potential. We show that these vacua can be uplifted to Minkowski and de Sitter by adding an ‐brane or a D‐term containing geometric and non‐geometric fluxes. These uplifted non‐supersymmetric models are analyzed with respect to their potential to realize axion monodromy inflation self‐consistently. Admitting rational values of the fluxes, we construct examples with the required hierarchy of mass scales.     

Dark and visible matter with broken R-parity and the axion multiplet

A small breaking of R-parity reconciles thermal leptogenesis, gravitino dark matter and primordial nucleosynthesis. We find that the same breaking relaxes cosmological bounds on the axion multiplet. Naturally expected spectra become allowed and bounds from late particle decays become so weak that they are superseded by bounds from non-thermal axion production. In this sense, the strong CP problem serves as an additional motivation for broken R-parity.     

Heavy axions from strong broken horizontal gauge symmetry

We study the consequences of the existence and breaking of a Peccei-Quinn symmetry within the context of a dynamical model of electroweak symmetry breaking based on broken gauged flavour symmetries. We perform an estimate of the axion mass by including flavour instanton effects and show that, for low cut-offs, the axion is sufficiently massive to prevent it from being phenomenologically unacceptable. We conclude with an examination of the strong CP problem and show that our axion cannot solve the problem, though we indicate ways in which the model can be extended so that the strong CP problem is solved.     

Harmless axions in superstring theories

We explore the possibility of obtaining an axion in superstring theories whose decay constant lies in a phenomenologically acceptable range. We find in agreement with Choi and Kim that this is not possible in SO(32) theories. However, we find that it appears to be possible in the E₈ × E′₈ heterotic string theory.     

Large Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn_2Bi_2Te_5

The dynamical axion field is a new state of quantum matter where the magnetoelectric response couples strongly to its low-energy magnetic fluctuations.It is fundamentally different from an axion insulator with a static quantized magnetoelectric response.The dynamical axion field exhibits many exotic phenomena such as axionic polariton and axion instability.However,these effects have not been experimentally confirmed due to the lack of proper topological magnetic materials.Combining analytic mode...     

The grand unified axion not observable?

It is a characteristic of grand unified theories with an axion of the type suggested by Dine, Fischler and Srednicki that they predict a very small neutron electric dipole moment because the QCD θ vacuum parameter is suppressed. The minimal SU(5) axion model of Wise, Georgi and Glashow yields a cosmological baryon-to-photon ratio in the same order as that in the minimal SU(5) model with one 5 of Higgs, which is much smaller than required by astrophysics. More complicated grand unified axion models can avoid this problem.     

Strong CP problem and mirror world: the Weinberg–Wilczek axion revisited

A new possibility for solving the strong CP problem is suggested. It is based on the concept of a mirror world of particles, with the gauge symmetry and Lagrangian completely identical to that of the observable particles. We assume that the ordinary and mirror sectors share the same Peccei–Quinn symmetry realized à la Weinberg–Wilczek, so that the θ-terms are simultaneously canceled by the axion VEV in both worlds. This property remains valid even if the symmetry between two sectors is spontaneously broken and the weak scale of the mirror world is larger than the ordinary weak scale, in which case also the mirror QCD scale becomes larger than the ordinary one. In this situation our axion essentially represents a Weinberg–Wilczek axion of the mirror world with quite a large mass, while it couples the ordinary particles like an invisible axion. The experimental and astrophysical limits are discussed and an allowed parameter window is found with the Peccei–Quinn scale f_a10⁴–10⁵ GeV and the axion mass m_a1 MeV, which can be accessible for future experiments. We also show that our solution to the strong CP problem is stable against the Planck scale induced effects.