An ultralight pseudoscalar boson

Jihn E. Kim; David J. E. Marsh

doi:10.1103/PhysRevD.93.025027

paper

An ultralight pseudoscalar boson

arXiv:1510.01701 · doi:10.1103/PhysRevD.93.025027

Abstract

Using a fundamental discrete symmetry, $Z_{N}$ , we construct a two-axion model with the QCD axion solving the strong- $C P$ problem, and an ultralight axion (ULA) with $m_{ULA} \approx 1 0^{- 22} eV$ 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 $f_{ULA} \sim 1 0^{17} GeV$ , and $f_{QCD} \sim 1 0^{11} GeV$ . 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 $E \cdot B$ and $G G ~$ couplings. In the simplest models, however, the ULA has identically zero coupling to both $G G ~$ of QCD and $E \cdot B$ of electromagnetism due to vanishing electromagnetic and color anomalies. The ULA couples to fermions with strength $g \propto 1/ f_{ULA}$ . This coupling causes spin precession of nucleons and electrons with respect to the DM wind with period $t \sim$ 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.

14 pages, 3 figures. v2 numerical error on N corrected, conclusions unchanged. Typos and notation corrected. Matches version published in PRD

An ultralight pseudoscalar boson

Abstract

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