An ultralight pseudoscalar boson

Jihn E. Kim, David J.E. Marsh

Research output: Contribution to journalArticlepeer-review

58 Citations (Scopus)


Using a fundamental discrete symmetry, ZN, we construct a two-axion model with the QCD axion solving the strong-CP problem, and an ultralight axion (ULA) with mULA≈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 fULA∼1017 GeV, and fQCD∼1011 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→·B→ and GG couplings. In the simplest models, however, the ULA has identically zero coupling to both GG of QCD and E→·B→ of electromagnetism due to vanishing electromagnetic and color anomalies. The ULA couples to fermions with strength g1/fULA. This coupling causes spin precession of nucleons and electrons with respect to the DM wind with period t∼ 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.

Original languageEnglish
Article number025027
JournalPhysical Review D
Issue number2
Publication statusPublished - 27 Jan 2016


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