The nature of dark matter is one of the longest enduring cosmological mysteries. The axion is one of the few candidates for dark matter that would not only explain most of the matter in the universe, but another outstanding problem in physics: why is the strong force time-symmetric? The ALPHA (Axion Longitudinal Plasma HAloscope) Consortium is working to detect the axion and help explain the underlying structure of the universe. By using a wire metamaterial to create an artificial plasma with designer properties, ALPHA will create a kind of “radio” capable of picking up axion dark matter.

The ALPHA Consortium (logo on left) was formed in May 2021, bringing together physicists from around the world, involving researchers at Stockholm University, UC Berkeley, MIT, ITMO, the University of Arizona, Cambridge, the University of Maryland and UC Davis with the goal of developing a novel axion-detector – a plasma haloscope.

We recently (October 2022) published a white paper describing our efforts: https://arxiv.org/abs/2210.00017

Finding the axion is a bit like tuning a radio: you have to tune your antenna until you pick up the right frequency. Rather than music, experimentalists in ALPHA would be rewarded with ‘hearing’ the dark matter that the Earth is travelling through. This signal is extremely weak and so requires a powerful antenna and very low noise environment to be detected.

Figure 1 (right): Schematic for the ALPHA Pathfinder. A tuneable wire metamaterial is placed inside a high Tesla solenoidal magnet and tunes into different potential axion frequencies, using ultra low noise electronics to read out any resulting signals.

Inside a magnetic field axions would generate a small electric field that could be used to drive oscillations in the plasma. A plasma is a material where charged particles, such as electrons, can flow freely as a fluid. These oscillations change the wavelength of light in the material, leading to a better “axion radio” by matching the wavelength of light to that of the axion. Unlike traditional experiments based on resonant cavities, the size of the plasma is not limited by the Compton wavelength of the axion, thus giving a larger signal at high frequencies. The difference is somewhat like the difference between a walkie talkie and a radio broadcast tower.

To have the right properties, the plasma must be cold, low loss and have a tuneable plasma frequency. Wire metamaterials have exactly these properties, as they can be tuned by modifying the geometry of the wires. This artificial plasma is then placed inside a strong (10,000 times stronger than a fridge magnet) large bore solenoid magnet. Ultra low-noise electronics would then be used to amplify any resultant signal operating in the range of 5-50 GHz.

Figure 2: (left) Axion parameter space showing the dimensionless axion-photon coupling constant as a function of the axion mass, ma, with various bounds. This dimensionless coupling is defined so that it is O(1) for models which solve the strong CP problem regardless of the axion mass. Haloscope experiments looking for dark matter are shown in grey, the limit from the CAST helioscope looking for solar axions is shown in blue and astrophysical limits are shown in green. The extended QCD axion model band is shown in yellow, with traditional KSVZ and DFSZ models shown as black lines. The orange region depicts the latest prediction for axions produced in a post-inflationary scenario assuming a scaling solution. The region to be explored by ALPHA is shown in purple, covering a significant portion of well-motivated parameter space.

Key publications

Probing dark photons with plasma haloscopes
G. B. Gelmini, A. J. Millar, V. Takhistov and E. Vitagliano
Phys. Rev. D 102 (2020) 043003 [arXiv:2006.06836]

Revisiting longitudinal plasmon-axion conversion in external magnetic fields
A. Caputo, A. J. Millar and E. Vitagliano
Phys. Rev. D 101 (2020) 123004 [arXiv:2005.00078]

Tunable axion plasma haloscopes
M. Lawson, A. J. Millar, M. Pancaldi, E. Vitagliano and F. Wilczek
Phys. Rev. Lett. 123 (2019) 141802 (Editor’s Suggestion) [arXiv:1904.11872]