ALPHA – A search for the post-inflation axion

While the open mass range for the axion spans over ten orders of magnitude, in the scenario where Peccei-Quinn symmetry breaking occurs after inflation, the axion mass corresponding to the dark matter density of the universe Ω_DM ~ 0.27 is restricted to the high end of this range. In principle, the post-inflation axion mass is exactly calculable with the best current estimates exceeding 40 μeV [1]. This implies scanning frequencies greater than 10 GHz, a range in which conventional microwave cavities become impractically small. A recent proposal to supplant the microwave cavity with a wire-array metamaterial whose plasma frequency could be engineered and tuned [2] offered the prospect that a resonator could be made both arbitrarily large and arbitrarily high in frequency [3]. A new experiment to search for the post-inflation axion, ALPHA (Axion Longitudinal Plasma HAloscope), is now under construction. Sited at Yale, it will utilize an existing 16 T magnet, quantum-limited amplifiers, and metamaterial-inspired resonators and photonic band gap structures to suppress TE-TM mode hybridization at the very high frequencies required [4,5]. This talk will present the overall experimental design, focusing on the initial resonator design, and the anticipated sensitivity.

[1] M. Buschmann et al., Nature Communications 13 (2022) 1049.

[2] M. Lawson et al., 123 (2019) 141802.

[3] A.J. Millar et al., Physical Review D 107 (2023) 055013.

[4] N. Kowitt et al., contribution to the April 2024 APS meeting (Sacramento, CA).

[5] A. Sindhwad et al., contribution to the April 2024 APS meeting (Sacramento, CA).