Cosmology of the invisible axion

A cosmological bound on the invisible axion

The dark matter that appears to be gravitationally dominant on all scales larger than galactic cores may consist of axions, stable photinos, or other collisionless particles whose velocity dispersion in the early Universe is so small that fluctuations of galactic size or larger are not damped by free streaming. An attractive feature of this cold dark matter hypothesis is its considerable predictive power: the post-recombination fluctuation spectrum is calculable, and it in turn governs the formation of galaxies and clusters. Good agreement with the data is obtained for a Zeldovich (|δk|2 ∝ k) spectrum of primordial fluctuations.

/pdf/formation-of-galaxies-and-large-scale-structure-with-cold-20ypzbmycs.pdf

Formation of galaxies and large-scale structure with cold dark matter

R-Parity-violating supersymmetry

Current upper bounds on the neutron electric dipole moment constrain the physically observable quantum chromodynamic (QCD) vacuum angle $|\overline{\ensuremath{\theta}}|\ensuremath{\lesssim}{10}^{\ensuremath{-}11}$. Since QCD explains a great deal of experimental data from the $100\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ to the TeV scale, it is desirable to explain this smallness of $|\overline{\ensuremath{\theta}}|$ in the QCD framework; this is the strong $CP$ problem. There now exist two plausible solutions to this problem, one of which leads to the existence of a very light axion. The axion decay constant window, ${10}^{9}\ensuremath{\lesssim}{F}_{a}\ensuremath{\lesssim}{10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$ for an $O(1)$ initial misalignment angle ${\ensuremath{\theta}}_{1}$, has been obtained from astrophysical and cosmological data. For ${F}_{a}\ensuremath{\gtrsim}{10}^{12}\phantom{\rule{0.3em}{0ex}}\mathrm{GeV}$ with ${\ensuremath{\theta}}_{1}lO(1)$, axions may constitute a significant fraction of the dark matter of the universe. The supersymmetrized axion solution of the strong $CP$ problem introduces its superpartner the axino, which might have affected the evolution of the Universe significantly. The very light axion (theory, supersymmetrization, and models) using recent particle, astrophysical, and cosmological data, and present prospects for its discovery is reviewed here.

/pdf/axions-and-the-strong-cp-problem-4shyva93ac.pdf

Axions and the Strong CP Problem

A bound on the coupling strengths of a very light pseudoscalar particle is derived from stellar energy loss at various stages of evolution: the sun, red giants, and cooling white dwarfs. The dimensionless Yukawa and $2\ensuremath{\gamma}$ couplings must be less than ${10}^{\ensuremath{-}11}$.

Light Pseudoscalar Particle and Stellar Energy Loss

Constraints on the light axion of Dine, Fischler, and Srednicki are critically reexamined and upper bounds on its mass are derived from stars at various stages of evolution. A conservative upper bound for the axion mass is about 1 eV, while a model-dependent argument gives a better upper bound of mass approx.0.07 eV. The same argument also applies to coupling of any massless pseudoscalar particle to electrons giving an upper bound of Vertical Barg/sub e/Vertical Bar<1 x 10/sup -11/.

M. Yoshimura

Papers

Light Pseudoscalar Particle and Stellar Energy Loss

Astrophysical constraints on a new light axion and other weakly interacting particles

Baryon nonconservation with a harmless axion

Astrophysical Constraints on a New Light Axion and Other Weakly Interacting Particles