Search

We propose that the dynamical axion field can be realized in a magnetic topological insulator superlattice or a topological paramagnetic insulator. The magnetic fluctuations of these systems produce a pseudoscalar field which has an axionic coupling to the electromagnetic field, and thus it gives a condensed-matter realization of the axion electrodynamics. Compared to the previously proposed dynamical axion materials where a longrange antiferromagnetic order is required, the systems proposed here have the advantage that only a uniform magnetization or a paramagnetic state is needed for the dynamic axion. We further propose several experiments to detect such a dynamical axion field. [ABSTRACT FROM AUTHOR]

Recently, a new mechanism to generate a naturally small electroweak scale has been proposed. It exploits the coupling of the Higgs boson to an axionlike field and a long era in the early Universe where the axion unchains a dynamical screening of the Higgs mass. We present a new realization of this idea with the new feature that it leaves no sign of new physics at the electroweak scale, and up to a rather large scale, 109 GeV, except for two very light and weakly coupled axionlike states. One of the scalars can be a viable dark matter candidate. Such a cosmological Higgs-axion interplay could be tested with a number of experimental strategies. [ABSTRACT FROM AUTHOR]

We point out that kinetic and Stückelberg mixings that are generically present in the low energy effective action of axions can significantly widen the window of axion decay constants. We show that an effective super-Planckian decay constant can be obtained even when the axion kinetic matrix has only sub-Planckian entries. Our minimal model involves only two axions, a Stückelberg U(1) and a modest rank instanton generating non-Abelian group. Below the mass of the Stückelberg U(1), there is only a single axion with a nonperturbatively generated potential. In contrast to previous approaches, the enhancement of the axion decay constant is not tied to the number of degrees of freedom introduced. We also discuss how kinetic mixings can lower the decay constant to the desired axion dark matter window. String theory embeddings of this scenario and their phenomenological features are briefly discussed. [ABSTRACT FROM AUTHOR]

Several emergent phenomena and phases in solids arise from configurations of the electronic Berry phase in momentum space that are similar to gauge field configurations in real space such as magnetic monopoles. We show that the momentum-space analogue of the "axion electrodynamics" term E . B plays a fundamental role in a unified theory of Berry-phase contributions to optical gyrotropy in time-reversal invariant materials and the chiral magnetic effect. The Berry-phase mechanism predicts that the rotatory power along the optic axes of a crystal must sum to zero, a constraint beyond that stipulated by point-group symmetry, but observed to high accuracy in classic experimental observations on alpha quartz. Furthermore, the Berry mechanism provides a microscopic basis for the surface conductance at the interface between gyrotropic and nongyrotropic media. [ABSTRACT FROM AUTHOR]

Searches for dark matter imprints are one of the most active areas of current research. We focus here on light fields with mass mB, such as axions and axionlike candidates. Using perturbative techniques and full-blown nonlinear numerical relativity methods, we show the following. (i) Dark matter can pile up in the center of stars, leading to configurations and geometries oscillating with a frequency that is a multiple of f=2.5×1014(mBc2/eV) Hz. These configurations are stable throughout most of the parameter space, and arise out of credible mechanisms for dark-matter capture. Stars with bosonic cores may also develop in other theories with effective mass couplings, such as (massless) scalar-tensor theories. We also show that (ii) collapse of the host star to a black hole is avoided by efficient gravitational cooling mechanisms. [ABSTRACT FROM AUTHOR]

Once a light axionlike scalar field couples to the electroweak gauge bosons, its classical motion during reheating induces an effective chemical potential for the fermion number. In the presence of rapid lepton number (L)-violating processes in the plasma, such a chemical potential provides a favorable opportunity for baryogenesis via leptogenesis. We are able to demonstrate that L violation due to the exchange of heavy Majorana neutrinos is sufficient for a successful realization of this idea. Our mechanism represents a novel and minimal alternative to thermal leptogenesis, which turns out to be insensitive to the masses and CP-violating phases in the heavy neutrino sector. It is consistent with heavy neutrino masses close to the scale of grand unification and, quite complementary to thermal leptogenesis, requires the reheating temperature to be at least of order 1012 GeV. [ABSTRACT FROM AUTHOR]

Bosons interacting repulsively on a lattice with a flat lowest band energy dispersion may, at sufficiently small filling factors, enter into a Wigner-crystal-like phase. This phase is a consequence of the dispersionless nature of the system, which in turn implies the occurrence of single-particle localized eigenstates. We investigate one of these systems-the sawtooth lattice-filled with strongly repulsive bosons at filling factors infinitesimally above the critical point where the crystal phase is no longer the ground state. We find, in the hard-core limit, that the crystal retains its structure in all but one of its cells, where it is broken. The broken cell corresponds to an exotic kind of repulsively bound state, which becomes delocalized. We investigate the excitation spectrum of the system analytically and find that the bound state behaves as a single particle hopping on an effective lattice with reduced periodicity, and is therefore gapless. Thus, the addition of a single particle to a flat-band system at critical filling is found to be enough to make kinetic behavior manifest. [ABSTRACT FROM AUTHOR]

We connect a family of gauge theories (Maxwell theories with a magnetoelectric coupling θ = 2πk,k ∊ Z) to the family of 3D topological lattice models introduced by Walker and Wang. In particular, we show that the lattice Hamiltonians capture a certain strong-coupling limit of these gauge theories, in which the system enters a gapped (confined) phase. We discuss the relationship between the topological order exhibited by certain of these lattice Hamiltonians and the characteristic electromagnetic response of the symmetry-protected bosonic topological insulator. [ABSTRACT FROM AUTHOR]

We study a Dirac dark matter particle interacting with ordinary matter via the exchange of a light pseudoscalar, and analyze its impact on both direct and indirect detection experiments. We show that this candidate can accommodate the long-standing DAMA modulated signal and yet be compatible with all exclusion limits at 99s% C.L. This result holds for natural choices of the pseudoscalar-quark couplings (e.g., flavor universal), which give rise to a significant enhancement of the dark matter-proton coupling with respect to the coupling to neutrons. We alsofind that this candidate can accommodate the observed 1-3 GeV gamma-ray excess at the Galactic center and at the same time have the correct relic density today. The model could be tested with measurements of rare meson decays, flavor changing processes, and searches for axionlike particles with mass in the MeV range. [ABSTRACT FROM AUTHOR]

We propose a new field theory mechanism for generating an effective trans-Planckian decay constant from sub-Planckian ones. Using the minimal two axions and a hierarchy between two axion decay constants is sufficient for realizing inflation through nonperturbative effects only and with minimal tuning The inflationary motion is kept entirely within a sub-Planckian domain. We outline possible strategies of embedding the model in a string theory setup. [ABSTRACT FROM AUTHOR]

The serious cosmological problems created by the axion-string-axion-domain-wall system in standard axion models are alleviated by positing the existence of a new confining force. The instantons of this force can generate an axion potential that erases the axion strings long before QCD effects become important, thus preventing QCD-generated axion walls from ever appearing. Axion walls generated by the new confining force would decay so early as not to contribute significantly to the energy in axion dark matter. [ABSTRACT FROM AUTHOR]

Dark matter axions may cause transitions between atomic states that differ in energy by an amount equal to the axion mass. Such energy differences are conveniently tuned using the Zeeman effect. It is proposed to search for dark matter axions by cooling a kilogram-sized sample to millikelvin temperatures and count axion induced transitions using laser techniques. This appears to be an appropriate approach to axion dark matter detection in the 10-4 eV mass range. [ABSTRACT FROM AUTHOR]

We derive a strong bound on the axion-photon coupling gαγ from the analysis of a sample of 39 Galactic Globular Clusters. As recognized long ago, the R parameter, i.e., the number ratio of stars in horizontal over red giant branch of old stellar clusters, would be reduced by the axion production from photon conversions occurring in stellar cores. In this regard, we have compared the measured R with state-of-the-art stellar models obtained under different assumptions for gαγ. We show that the estimated value of gay substantially depends on the adopted He mass fraction Υ, an effect often neglected in previous investigations. Taking as a benchmark for our study the most recent determinations of the He abundance in H n regions with O/H in the same range of the Galactic Globular Clusters, we obtain an upper bound gαγ < 0.66 × 10-10 GeV-1 at 95% confidence level. This result significantly improves the constraints from previous analyses and is currently the strongest limit on the axion-photon coupling in a wide mass range. [ABSTRACT FROM AUTHOR]

We describe a method based on precision magnetometry that can extend the search for axion-mediated spin-dependent forces by several orders of magnitude. By combining techniques used in nuclear magnetic resonance and short-distance tests of gravity, our approach can substantially improve upon current experimental limits set by astrophysics, and probe deep into the theoretically interesting regime for the Peccei-Quinn (PQ) axion. Our method is sensitive to PQ axion decay constants between 109 and 1012 GeV or axion masses between 10-6 and 10-3, independent of the cosmic axion abundance. [ABSTRACT FROM AUTHOR]

Evidence from the BICEP2 experiment for a significant gravitational-wave background has focused attention on inflaton potentials V(ϕ)∝ϕα with α=2 ("chaotic" or "m²ϕ²" inflation) or with smaller values of α, as may arise in axion-monodromy models. Here we show that reheating considerations may provide additional constraints to these models. The reheating phase preceding the radiation era is modeled by an effective equation-of-state parameter wre. The canonical reheating scenario is then described by wre=0. The simplest α=2 models are consistent with wre=0 for values of ns well within the current 1σ range. Models with α=1 or α=2/3 require a more exotic reheating phase, with −1/31/3, unless ns is close to the lower limit of the 2σ range. For m²ϕ² inflation and canonical reheating as a benchmark, we derive a relation log10(Tre/106 GeV)≃2000(ns-0.96) between the reheat temperature Tre and the scalar spectral index ns. Thus, if ns is close to its central value, then Tre≲106 GeV, just above the electroweak scale. If the reheat temperature is higher, as many theorists may prefer, then the scalar spectral index should be closer to ns≃0.965 (at the pivot scale k=0.05 Mpc-1), near the upper limit of the 1σ error range. Improved precision in the measurement of ns should allow m²ϕ², axion monodromy, and ϕ4 models to be distinguished, even without precise measurement of r, and to test the m²ϕ² expectation of ns≃0.965. [ABSTRACT FROM AUTHOR]

We consider the competition between the conventional s-wave and the triplet Balian-Werthamer or the B-phase pairings in doped three-dimensional narrow-gap semiconductors, such as CuxBi2Se3 and Sn1-xInxTe. When the coupling constants of the two contending channels are comparable, we find a simultaneously time-reversal and parity violating p+is state at low temperatures, which provides an example of a dynamic axionic state of matter. In contradistinction to the time-reversal invariant, topological B phase, the p+is state possesses gapped Majorana fermions as surface Andreev bound states, which give rise to an anomalous surface thermal Hall effect. The anomalous gravitational and electrodynamic responses of the p+is state can be described by the ϑ vacuum structure, where ϑ ≠ 0 or π. [ABSTRACT FROM AUTHOR]

The properties of axions that constitute 100% of cold dark matter (CDM) depend on the tensor-to-scalar ratio r at the end of inflation. If r = 0.20+0.07-0.05 as reported by the BICEP2 Collaboration, then "half" of the CDM axion parameter space is ruled out. Namely, in the context of single-field slow-roll inflation, for axions to be 100% of the CDM, the Peccei-Quinn symmetry must be broken after the end of inflation, so that axion nonadiabatic primordial fluctuations are compatible with observational constraints. The cosmic axion density is then independent of the tensor-to-scalar ratio r, and the axion mass is expected to be in a narrow range that, however, depends on the cosmological model before primordial nucleosynthesis. In the standard Lambda CDM cosmology, the CDM axion mass range is ma = (71 ± 2 μeV)(αdec +1)6/7, where αdec is the fractional contribution to the cosmic axion density from decays of axionic strings and walls. [ABSTRACT FROM AUTHOR]

The recent detection of B modes by the BICEP2 experiment has nontrivial implications for axion dark matter implied by combining the tensor interpretation with isocurvature constraints from Planck observations. In this Letter the measurement is taken as fact, and its implications considered, though further experimental verification is required. In the simplest inflation models, r = 0.2 implies H1 = 1.1 × 1014 GeV. If the axion decay constant fa < H1/2π, constraints on the dark matter (DM) abundance alone rule out the QCD axion as DM for ma ≲ 52ϰ6/7 μeV (where ϰ > 1 accounts for theoretical uncertainty). If f a > H1/2π then vacuum fluctuations of the axion field place conflicting demands on axion DM: isocurvature constraints require a DM abundance which is too small to be reached when the backreaction of fluctuations is included. High- fa QCD axions are thus ruled out. Constraints on axionlike particles, as a function of their mass and DM fraction, are also considered. For heavy axions with ma ≳ 10-22 eV we find Ωa/Ωb ≲ 10-3, with stronger constraints on heavier axions. Lighter axions, however, are allowed and (inflationary) model-independent constraints from the CMB temperature power spectrum and large scale structure are stronger than those implied by tensor modes. [ABSTRACT FROM AUTHOR]

We report a first search for weakly interacting massive particles (WIMPs) using the background rejection capabilities of SuperCDMS. An exposure of 577 kg days was analyzed for WIMPs with mass < 30 GeV/c2, with the signal region blinded. Eleven events were observed after unblinding. We set an upper limit on the spin-independent WIMP-nucleon cross section of 1.2 × 10-42 cm2 at 8 GeV/ c2. This result is in tension with WIMP interpretations of recent experiments and probes new parameter space for WIMP-nucleon scattering for WIMP masses < 6 GeV/c2. [ABSTRACT FROM AUTHOR]

The observed value of the Higgs boson mass indicates that the Higgs potential becomes small and flat at the scale around 1017 GeV. Having this fact in mind, we reconsider the Higgs inflation scenario proposed by Bezrukov and Shaposhnikov. It turns out that the nonminimal coupling ξ of the Higgs squared to the Ricci scalar can be smaller than 10. For example, ξ = 7 corresponds to the tensor-to-scalar ratio r ≃ 0.2, which is consistent with the recent observation by BICEP2. [ABSTRACT FROM AUTHOR]

We show that dark matter axions cause an oscillating electric current to flow along magnetic field lines. The oscillating current induced in a strong magnetic field B⃗0 produces a small magnetic field B⃗a. We propose to amplify and detect B⃗a using a cooled LC circuit and a very sensitive magnetometer. This appears to be a suitable approach to searching for axion dark matter in the 10–7 to 10–9 eV mass range. [ABSTRACT FROM AUTHOR]

The CERN Axion Solar Telescope has finished its search for solar axions with 3He buffer gas, covering the search range 0.64 eV ≲ mα ≲ 1.17 eV. This closes the gap to the cosmological hot dark matter limit and actually overlaps with it. From the absence of excess x rays when the magnet was pointing to the Sun we set a typical upper limit on the axion-photon coupling of gαγ ≲ 3.3 x 10-10 GeV-1 at 95% C.L., with the exact value depending on the pressure setting. Future direct solar axion searches will focus on increasing the sensitivity to smaller values of gαγ, for example by the currently discussed next generation helioscope International AXion Observatory. [ABSTRACT FROM AUTHOR]

We introduce a new N = 1 no-scale supergravity model with F- and D-term breaking. It contains a single chiral supermultiplet T and a single U(1) vector multiplet U, gauging a nonanomalous axionic shift symmetry. Both supersymmetry and the gauge symmetry are spontaneously broken, with the spin-3/2, spin-1, and spin-1/2 masses sliding along a classical flat direction, with a single real massless scalar in the spectrum. The other degrees of freedom are absorbed by the massive gravitino and vector. We extend our model, under very mild conditions, to general gauge groups and matter content. [ABSTRACT FROM AUTHOR]

From a direct calculation of the anomalous Hall conductivity and an effective electromagnetic action obtained via Fujikawa's chiral rotation technique, we conclude that an axionic field theory with a nonquantized coefficient describes the electromagnetic response of the (3 + 1 )-dimensional Weyl semimetal. The coefficient is proportional to the momentum space separation of the Weyl nodes. Akin to the Chern-Simons field theory of quantum Hall effect, the axion field theory violates gauge invariance in the presence of the boundary, which is cured by the chiral anomaly of the surface states via the Callan-Harvey mechanism. This provides a unique solution for the radiatively induced CPT-odd term in the electromagnetic polarization tensor of the Lorentz violating spinor electrodynamics, where the source of the Lorentz violation is a constant axial 4-vector term for the Dirac fermion. A direct linear response calculation also establishes anomalous thermal Hall effect and a Wiedemann-Franz law, but thermal Hall conductivity does not directly follow from the well known formula for the gravitational chiral anomaly. [ABSTRACT FROM AUTHOR]

We provide theoretical arguments that dark-matter axions from the galactic halo that pass through Earth may generate a small observable signal in resonant S/N/S Josephson junctions. The corresponding interaction process is based on the uniqueness of the gauge-invariant axion Josephson phase angle modulo 2π and is predicted to produce a small Shapiro steplike feature without externally applied microwave radiation when the Josephson frequency resonates with the axion mass. A resonance signal of so far unknown origin observed by C. Hoffmann et al. [Phys. Rev. B 70, 180503(R) (2004)] is consistent with our theory and can be interpreted in terms of an axion mass mac2 = 0.11 meV and a local galactic axionic dark-matter density of 0.05 GeV/cm3. We discuss future experimental checks to confirm the dark-matter nature of the observed signal. [ABSTRACT FROM AUTHOR]

The red-giant branch (RGB) in globular clusters is extended to larger brightness if the degenerate helium core loses too much energy in "dark channels." Based on a large set of archival observations, we provide high-precision photometry for the Galactic globular cluster M5 (NGC 5904), allowing for a detailed comparison between the observed tip of the RGB with predictions based on contemporary stellar evolution theory. In particular, we derive 95% confidence limits of gae < 4.3 × 10-13 on the axion-electron coupling and μv < 4.5 × 10-12μB (Bohr magneton μB = e/2me) on a neutrino dipole moment, based on a detailed analysis of statistical and systematic uncertainties. The cluster distance is the single largest source of uncertainty and can be improved in the future. [ABSTRACT FROM AUTHOR]

The authors present their study on axion-photon conversion to explain excess emission of soft x-rays from galaxy clusters and thus, contribute to the diffused unresolved cosmic x-rays background. The authors mention that the dark radiation cosmic background is produced by primordial decays of string theory moduli which are detected in axion-photon conversion in astrophysical magnetic fields. The authors mention this leads to quasithermal excess in extreme ultraviolet and soft x-ray bands.

Axions with couplings gaγγ∼few × 10-11 GeV-1 to electromagnetism may resolve a number of astrophysical anomalies, such as unexpected ~TeV transparency, anomalous stellar cooling, and x-ray excesses from nearby neutron stars. We show, however, that such axions are severely constrained by the nonobservation of x rays from the magnetic white dwarf (MWD) RE J0317-853 using ~40 ks of data acquired from a dedicated observation with the Chandra X-ray Observatory. Axions may be produced in the core of the MWD through electron bremsstrahlung and then convert to x rays in the magnetosphere. The nonobservation of x rays constrains the axion-photon coupling to gaγγ ≲ 5.5×10-13√Caγγ/Caee GeV-1 at 95% confidence for axion masses ma ≲ 5×10-6 eV, with Caee and Caγγ the dimensionless coupling constants to electrons and photons. Considering that Caee is generated from the renormalization group, our results robustly disfavor gaγγ ≳ 4.4×10-11 GeV-1 even for models with no ultraviolet contribution to Caee. [ABSTRACT FROM AUTHOR]

We present a novel framework to solve simultaneously the electroweak-hierarchy problem and the strong-CP problem. A small but finite Higgs vacuum expectation value and a small θ angle are selected after the QCD phase transition, without relying on the Peccei-Quinn mechanism or other traditional solutions. We predict a distinctive pattern of correlated signals at hadronic EDM, fuzzy dark matter, and axion experiments. [ABSTRACT FROM AUTHOR]