An ultralight pseudoscalar boson

Abstract

Using a fundamental discrete symmetry, Z(N), we construct a two-axion model with the QCD axion solving the strong-CP problem, and an ultralight axion (ULA) with m(ULA) approximate to 10(-22) eV providing the dominant form of dark matter (DM). The ULA is light enough to be detectable in cosmology from its imprints on structure formation, and may resolve the small-scale problems of cold DM. The necessary relative DM abundances occur without fine-tuning in constructions with decay constants f(ULA) similar to 10(17) GeV, and f(QCD) similar to 10(11) GeV. An example model achieving this has N = 24, and we construct a range of other possibilities. We compute the ULA couplings to the standard model, and discuss prospects for direct detection. The QCD axion may be detectable in standard experiments through the (E) over right arrow . (B) over right arrow and G (G) over tilde couplings. In the simplest models, however, the ULA has identically zero coupling to both G (G) over tilde of QCD and (E) over right arrow . (B) over right arrow of electromagnetism due to vanishing electromagnetic and color anomalies. The ULA couples to fermions with strength g proportional to 1/f(ULA). This coupling causes spin precession of nucleons and electrons with respect to the DM wind with period t similar to months. Current limits do not exclude the predicted coupling strength, and our model is within reach of the CASPEr-Wind experiment, using nuclear magnetic resonance.