Berkeley Axion Works


ALPHA – the Axion Longitudinal Plasma Haloscope

The original axion haloscope proposed by Sikivie is based on the resonant conversion of axions to photons in a microwave cavity permeated by a strong magnetic field [1].  Due to the inverse relationship of the size of a microwave cavity and its frequency, however, the experiment is most naturally suited for axions in the ~0.5-5 GHz decade of frequency (~2-20 microelectronvolt).  Lower mass (frequency) cavities require microwave cavities of a size too large to be accommodated in superconducting solenoidal magnets, and thus to reach much lower mass axions associated with the GUT scale, a lumped-element variant of this scheme is being pursued (see DM Radio experiment in our web page).  On the other hand, for much higher mass axions, predicted by simulations of post-inflation axions [2], the microwave cavities become too small to produce detectable power.  Lawson et al. have recently proposed a solution to this conundrum [3], i.e. a resonator based on a wire-array metamaterial whose plasma frequency is determined by its unit cell rather than the physical dimensions of the resonator.  Thus a metamaterial-based haloscope could in principle be designed to be arbitrarily large and arbitrarily high in frequency.  A collaboration has formed to pursue R&D and early design of an experiment named ALPHA to probe axion masses above 10 GHz (>40 microelectronvolt).  Berkeley’s role in the collaboration has been fundamental studies of wire-array metamaterials, and engineering concepts and prototyping of tunable resonators based on this concept [4].

1.  P. Sikivie, Physical Review Letters 51 (1983) 1415; Physical Review D 32 (1985) 2988.

2.  M. Buschmann et al., Nature Communications 13 (2022) 1049.

3.  M. Lawson et al., Physical Review Letters 123 (2019) 141802.

4.  M. Wooten et al.  Annalen der Physik (2022) 2200479.