Search for Axionic Dark Matter

Abstract

Our understanding of the energy and matter composition of the universe has undergone a revolution in recent years.[1‐3] The picture has evolved from a universe dominated by protons, neutrons, and electrons to one where dark matter and dark energy dominate the mass-energy budget. The dark matter is convincingly constrained to be non-baryonic and cold. There are three widely discussed candidates for the dark matter: sterile neutrinos, weakly interacting massive particles (WIMPs), and axions. This paper describes the results and plans for a search for axions: the Axion Dark-Matter eXperiment (ADMX). A discovery of the axion, or placing unambiguous limits on its existence, would have profound implications for the dark matter problem. Either result would also impact a second important problem in contemporary physics: the origin of parity P and the product of charge conjugation with parity CP symmetry in the strong interactions.[4, 5] The axion is thus motivated by and has the potential to solve two rather important issues in particle physics and astrophysics. Moreover, the fact that the LHC has not produced evidence for supersymmetric particles[6] makes the case for WIMP dark matter more dicult to muster.[7] In contrast, the case for axions remains as strong as ever.[8, 9] The most plausible mass for the axion is in the 1-1000µeV range. At the low end of this window axions provide the dark matter.[10‐14] ADMX searches for axions that constitute the dark-matter halo of our galaxy. Many observations imply the existence of large halos of non-luminous matter surrounding galaxies.[15, 16] ADMX exploits the fact that axions may be stimulated to convert into microwave photons in a high Q cavity permeated by a large magnetic field. This detection method was proposed thirty years ago[17] and was developed during pilot experiments [18, 19] ADMX was initially located at the Lawrence Livermore National Laboratory (LLNL) but recently relocated to the University of Washington, Seattle. This axion detector,[8, 20, 21] which improved the sensitivity over the pilot detectors by at least a factor of 400, consists of a large superconducting magnet containing one or more microwave cavities. Axions which overlap the high-field region will be stimulated to decay into microwave photons when the resonant frequency of the cavity equals the mass of the axion. An ultra-sensitive microwave receiver, using superconducting electronics in its front end,[21] amplifies the cavity signal to a point where spectral analysis can search for signatures of axion‐photon emission. Over the past few years, the detector has scanned the 1.9‐3.6 µeV axion mass range with a sensitivity capable of a detection if the axion-photon coupling is near the upper end of theoretical predictions.