The $\pi$-axion and $\pi$-axiverse of dark QCD

Understand

Axions and axion-like particles (ALPs) are a prominent dark matter candidate, drawing motivation in part from the axiverse of string theory.

• Axion-like particles can also arise as composite degrees of freedom of a dark sector, for example, as dark pions in dark Quantum Chromo-Dynamics.
• In a dark Standard Model (SM) wherein all 6 quark flavors are light while the photon is massive, one finds a rich low-energy spectrum of stable and ultralight particles, in the form of neutral and charged dark scalars, and complex neutral scalars analogous to the SM kaon, with mass splittings determined by the mass and charge of the dark quarks.
• The model finds a natural portal to the visible sector via kinetic coupling of the dark and visible photons, and consequent millicharges for dark matter.