Electrobaryonic axion: hair of neutron stars

We investigate the general structure of the chiral anomaly AVV/AAA and (LLL,RRR) vertices, in the presence of chemical potentials in perturbation theory. The study finds application in anomalous transport, whenever chirally unbalanced matter is present, with propagating external currents that are classically conserved. Examples are topological materials and the chiral magnetic effect in the plasma state of matter of the early universe. We classify the minimal number of form factors of the AVV parametrization, by a complete analysis of the Schouten identities in the presence of a heat bath. We show that the longitudinal (anomaly) sector in the axial-vector channel, for on-shell and off-shell photons, is protected against corrections coming from the insertion of a chemical potential in the fermion loop. When the photons are on-shell, we prove that the transverse sector, in the same channel, is also μ-independent and vanishes. The related effective action is shown to be always described by the exchange of a massless anomaly pole, as in the case of vanishing chemical potentials. The pole is interpreted as an interpolating axionlike quasiparticle generated by the anomaly. In each axial-vector channel, it is predicted to be a correlated fermion/antifermion pseudoscalar (axionlike) quasiparticle appearing in the response function, once the material is subjected to an external chiral perturbation. The cancellation of the μ dependence extends to any chiral current within the Standard Model, including examples such as B (baryon), L (lepton), and B−L. This holds true irrespective of whether these currents exhibit anomalies. Published by the American Physical Society 2024

Axionlike quasiparticles and topological states of matter: Finite density corrections of the chiral anomaly vertex

Corianò,

Cretì,

Lionetti

et al. 2024

Photoproduction of heavy QCD axions in supernovae

Chakraborty,

Gupta,

Vanvlasselaer

2024

Compact stellar objects like supernovae and neutron stars are believed to cool by emitting axions predominantly via bremsstrahlung (NN→NNa), pion conversion (π−p+→Na), and photoproduction (γN→Na). In this paper, we study new contributions to the photoproduction channel pertaining to axions. We first point out the impact of anomaly-induced unavoidable Wess-Zumino-Witten term ∝εμναβFμν∂αaωβ in the photoproduction of such axions. We then perform a complementary data-driven study where the rate of photoproduction can be estimated from low-energy pion photoproduction data. We found that for such heavier axions, i.e., ma∼O(100) MeV, photoproduction processes can be dominant compared to the usual emission processes. In addition, the spectrum of these particles emitted in the process is significantly harder than those originating from bremsstrahlung. Published by the American Physical Society 2024

Resonant conversion of axion dark radiation into terahertz electromagnetic radiation in a neutron star magnetosphere

Long,

Schiappacasse

2024

In the strong magnetic field of a neutron star’s magnetosphere, axions coupled to electromagnetism develop a nonzero probability to convert into photons. Past studies have revealed that the axion-photon conversion can be resonantly enhanced. We recognize that the axion-photon resonance admits two parametrically distinct resonant solutions, which we call the mass-matched resonance and the Euler-Heisenberg assisted resonance. The mass-matched resonance occurs at a point in the magnetosphere where the radially-varying plasma frequency crosses the axion mass ωpl≈ma. The Euler-Heisenberg assisted resonance occurs where the axion energy satisfies ω≈(2ωpl2/7gγγγγB¯2)1/2. This second resonance is made possible though the strong background magnetic field B¯, as well as the nonzero Euler-Heisenberg four-photon self-interaction, which has the coupling gγγγγ=8α2/45me4. We study the resonant conversion of relativistic axion dark radiation into photons via the Euler-Heisenberg assisted resonance, and we calculate the expected electromagnetic radiation assuming different values for the axion-photon coupling gaγγ and different amplitudes for the axion flux onto the neutron star Φa. We briefly discuss several possible sources of axion dark radiation. Achieving a sufficiently strong axion flux to induce a detectable electromagnetic signal seems unlikely. Published by the American Physical Society 2024