Axion Particles Research Articles

Axions: Bose Einstein condensate or classical field?

The axion is a motivated dark matter candidate, so it would be interesting to find features in Large Scale Structures specific to axion dark matter. Such features were proposed for a Bose Einstein condensate of axions, leading to confusion in the literature (to which I contributed) about whether axions condense due to their gravitational interactions. This note argues that the Bose Einstein condensation of axions is a red herring: the axion dark matter produced by the misalignment mechanism is already a classical field, which has the distinctive features attributed to the axion condensate (BE condensates are described as classical fields). This note also estimates that the rate at which axion particles condense to the field, or the field evaporates to particles, is negligible.

Sensitivity of the Cherenkov Telescope Array to the detection of axion-like particles at high gamma-ray opacities

Extensions of the Standard Model of particles commonly predict the existence of axion(-like) particles (ALPs) that could be detected through their coupling to photons in external magnetic fields. This coupling could lead to modifications of γ-ray spectra from extragalactic sources. Above a certain energy, the γ-ray flux should be exponentially damped due to the interaction with photons of background radiations fields. ALPs, on the other hand,propagate unimpeded over cosmological distances and a reconversion into γ-rays could lead to an additional component in the spectra.Here, we present the sensitivity of the proposed Cherenkov Telescope Array (CTA) to detect this spectral hardening. Using the full instrumental response functions of CTA, a combined likelihood analysis of four γ-ray sources shows that a significant detection of the ALP signal is possible for couplings gaγ ≳ 2 × 10−11 GeV−1 and ALP masses ma ≲ 100 neV.We discuss the dependency of these values on different model assumptions and magnetic-field scenarios and identify the best observation strategy to search for an ALP induced boost of the γ-ray flux.

Constraints on primordial magnetic fields from CMB distortions in the axiverse

Measuring spectral distortions of the cosmic microwave background (CMB) is attracting considerable attention as a probe of high energy particle physics in the cosmological context, since PIXIE and PRISM have recently been proposed. In this paper, CMB distortions due to resonant conversions between CMB photons and light axion like particles (ALPs) are investigated, motivated by the string axiverse scenario which suggests the presence of a plenitude of light axion particles. Since these resonant conversions depend on the strength of primordial magnetic fields, constraints on CMB distortions can provide an upper limit on the product of the photon-ALP coupling constant g and the comoving strength of primordial magnetic fields B. Potentially feasible constraints from PIXIE/PRISM can set a limit g B < 10^{-16} GeV^{-1} nG for ALP mass, m_\phi < 10^{-14} eV. Although this result is not a direct constraint on g and B, it is significantly tighter than the product of the current upper limits on g and B.

Design and performance of the ADMX SQUID-based microwave receiver

The Axion Dark Matter eXperiment (ADMX) was designed to detect ultra-weakly interacting relic axion particles by searching for their conversion to microwave photons in a resonant cavity positioned in a strong magnetic field. Given the extremely low expected axion–photon conversion power we have designed, built and operated a microwave receiver based on a Superconducting QUantum Interference Device (SQUID). We describe the ADMX receiver in detail as well as the analysis of narrow band microwave signals. We demonstrate the sustained use of a SQUID amplifier operating between 812 and 860 MHz with a noise temperature of 1 K. The receiver has a noise equivalent power of 1.1×10−24W/Hz in the band of operation for an integration time of 1.8×103s.

Self-gravitating system made of axions

We show that the inclusion of an axion-like effective potential in the construction of a self-gravitating system made of scalar fields leads to a decrease on its compactness when the value of the self-interaction coupling constant is increased. By including the current values for the axion mass m and decay constant f_a, we have computed the mass and the radius for self-gravitating systems made of axion particles. It is found that such objects will have asteroid-size masses and radius of few meters, then, the self-gravitating system made of axions could play the role of scalar mini-machos that are mimicking a cold dark matter model for the galactic halo.

Axion searches with helioscopes and astrophysical signatures for axion(-like) particles

Axions should be produced copiously in stars such as the Sun. The first part of this paper reviews the capabilities and performance of axion helioscopes. The mechanism they rely on is described and the experimental results for the interaction of solar axions and axion-like particles with matter are given. The second part is actually observationally driven. New results obtained with Monte Carlo simulation reconstruct solar observations, previously dismissed, supporting an axion(-like) involvement with ma≈1–2×10−2 eV c−2. To further quantify the suggested solar observations as being originated by axions, additional theoretical work is needed. However, the recently suggested axion interaction with magnetic field gradients is a generic theoretical example that seems to reconcile for the first time current limits, derived from axion helioscopes, and potential axion-related solar x-ray activity, thus avoiding contradictions with the best experimental limits. Magnetic quadrupoles can be used to experimentally test this idea, thus becoming a new catalyst in axion experiments. Finally, a short outlook for the future is given, in view of the experimental expansion of axion research with the state-of-the-art orbiting x-ray observatories.

Indirect signatures for axion(-like) particles

Magnetic field dependent transient solar observations are suggestive for axionphoton oscillations with light axion(-like) particle involvement. Novel dark-moon measurements with the SMART X-ray detectors can be conclusive for radiatively decaying massive exotica like the generic solar Kaluza-Klein (KK) axions. Furthermore, the predicted intrinsic strong solar magnetic fields could be the reason of enhanced low energy axion production. Such an axion component could be the as yet unknown origin of the strong quiet Sun X-ray luminosity at energies below ∼ 1 keV. Solar axion telescopes should lower their threshold, aiming to copy processes that might occur near the solar surface, be it due to spontaneous or magnetically induced radiative decay of axion(-like) particles. This is motivated also by the recent claim of an axion-like particle detection by the laser experiment PVLAS.

Observational evidence for gravitationally trapped massive axion(-like) particles

Several unexpected astrophysical observations can be explained by gravitationally captured massive axions or axion-like particles, which are produced inside the Sun or other stars and are accumulated over cosmic times. Their radiative decay in solar outer space would give rise to a ‘self-irradiation’ of the whole star, providing the time-independent component of the corona heating source (we do not address here the flaring Sun). In analogy with the Sun-irradiated Earth atmosphere, the temperature and density gradient in the corona–chromosphere transition region is suggestive for an omnipresent irradiation of the Sun, which is the strongest evidence for the generic axion-like scenario. The same mechanism is compatible with phenomena like the solar wind, the X-rays from the dark-side of the Moon, the X-ray background radiation, the diffuse X-ray excesses (below ∼1 keV), the non-cooling of oldest stars, etc. A temperature of ∼10 6 K is observed in various places, while the radiative decay of a population of such elusive particles mimics a hot gas, which fits unexpected astrophysical X-ray observations. Furthermore, the recently reconstructed quiet solar X-ray spectrum during solar minimum supports this work, since it covers the expected energy range, and it is consistent with the result of a simulation based on Kaluza–Klein axions above ∼1 keV. The derived axion luminosity ( L a≈0.16 L ⊙) fits the cosmic energy density spectrum and is compatible within 2 σ with the recent SNO result, showing the important interplay between any exotic energy loss mechanism and neutrino production. At lower energies, using also a ROSAT observation, only ∼3% of the X-ray intensity is explained. Data from orbiting X-ray telescopes provide upper limits for particle decay rates 1 a.u. from the Sun, and suggest new types of searches on Earth or in space. In particular, X-ray observatories, with an unrivalled equivalent fiducial volume of ∼10 3 m 3 for the 0.1–10 keV range, can search for the radiative decay of new particles even from existing data. This work introduces the elongation angle of the X-ray telescope relative to the Sun as a relevant new parameter.

Observational evidence for axion(-like) particles

Abstract Several unexpected astrophysical observations can be explained by gravitationally captured massive axions or axion-like particles produced inside the Sun or other stars. Their radiative decay in solar outer space would give rise to a ‘self-irradiation’ of the whole star, providing the missing corona heating source. In analogy with the Sun-irradiated Earth atmosphere, the temperature and density gradient in the corona−chromosphere transition region is suggestive for an omnipresent irradiation of the Sun, which is the strongest evidence for the generic axion-like scenario. The radiative decay of a population of such elusive particles mimics a hot gas. The recently reconstructed quiet solar X-ray spectrum supports this work, since it covers the expected energy range, and it is consistent with the result of a simulation based on Kaluza–Klein axions above ∼1 keV. At lower energies, using also a ROSAT observation, only ∼3% of the solar X-ray intensity is explained. Data from orbiting X-ray Telescopes provide upper limits for particle decay rates 1 AU from the Sun, and suggest new types of searches on Earth or in space. In particular, X-ray observatories, with an unrivalled equivalent fiducial volume of ∼10 3 m 3 for the 0.1–10 keV range, can search for the radiative decay of new particles even from existing data.