Dark matter axion detection in the radio/mm waveband

Abstract

We discuss axion dark matter detection via two mechanisms: spontaneous decays and resonant conversion in neutron star magnetospheres. For decays, we show that the brightness temperature signal, rather than flux, is a less ambiguous measure for selecting candidate objects. This is owing principally to the finite beam width of telescopes which prevents one from being sensitive to the total flux from the object. With this in mind, we argue that the large surface-mass-density of the galactic center or the Virgo cluster center offer the best chance of improving current constraints on the axion-photon coupling via spontaneous decays. For the neutron star case, we first carry out a detailed study of mixing in magnetized plasmas. We derive transport equations for the axion-photon system via a controlled gradient expansion, allowing us to address inhomogeneous mass-shell constraints for arbitrary momenta. We then derive a nonperturbative Landau-Zener formula for the conversion probability valid across the range of relativistic and nonrelativistic axions and show that the standard perturbative resonant conversion amplitude is a truncation of this result in the nonadiabatic limit. Our treatment reveals that infalling dark matter axions typically convert nonadiabatically in magnetospheres. We describe the limitations of one-dimensional mixing equations and explain how three-dimensional effects activate new photon polarizations, including longitudinal modes and illustrate these arguments with numerical simulations in higher dimensions. We find that the bandwidth of the radio signal from neutron stars could be dominated by Doppler broadening from the oblique rotation of the neutron star if the axion is nonrelativistic in the conversion region. Therefore, we conclude that the radio signal from the resonant decay is weaker than previously thought, which means one relies on local density peaks to probe weaker axion-photon couplings.

Modeling dark photon oscillations in our inhomogeneous Universe

“…[16]; it is also similar to expressions for the A 0 production rate in stars under the narrow width approximation [32,33]. The stationary phase approximation has also been used to calculate appearance and disappearance probabilities with resonant oscillations in the context of neutrino oscillations [25] and in axion-photon conversions in magnetic fields [30,31,34]. When multiple resonances exist, the probability becomes…”

“…The plasma mass is an in-medium effect similar to the Mikheyev-Smirnov-Wolfenstein effect in the case of neutrino oscillations [28,29], leading in our case to the familiar correction of m 2 γ =2ω relative to the propagation phase of a massless particle [30,31]. H can be conveniently written in terms of Pauli matrices,…”

“…as defined in Eq. (31). The variable lnð1 þ δ b Þ has a Gaussian distribution with mean −Σ 2 =2 and time-dependent variance Σ 2 .…”

Radio Constraints of Dark Matter: A Review and Some Future Perspectives

“…It is also possible that dark matter consists of very light particles such as axions. The recent misalignment production model of axions predicts that the axion mass should be m DM = 19-23 µeV if axions are all cold dark matter [103]. Theoretically, axions could decay into photons spontaneously with a specific energy E = m DM c 2 /2.…”

“…However, the signal of axions is not easy to detect because the interaction between axions and the standard model particles is very small. The coupling constant between axion and photon interaction can be as small as 10 −11 GeV −1 [104], which gives the decay time of an axion to be more than 10 40 s [103]. Fortunately, the spontaneous decay of axions could be greatly enhanced by the stimulated emission mechanism if the background contains a large amount of photons with the same emission frequency [103,105].…”