π-axion and π-axiverse of dark QCD

Axions and axionlike particles are a prominent dark matter candidate, drawing motivation in part from the axiverse of string theory. Axionlike particles can also arise as composite degrees of freedom of a dark sector, for example, as dark pions in dark quantum chromodynamics. In a dark Standard Model (SM) wherein all six 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. The dark matter can be a mixture of all these ultralight bosonic degrees of freedom and exhibits both parity-even and parity-odd interactions, making the theory testable at a wide variety of experiments. In context of dark QCD with Nf flavors of light quarks, this scenario predicts N2 f - 1 ultralight axionlike particles-effectively an axiverse from dark QCD. This "pi-axiverse" is consistent with but makes no recourse to string theory and is complementary to the conventional string theory axiverse.