The Field Theory Axiverse

Abstract: Axion and axion-like particles (ALPs) are a prominent candidate for physics beyond the Standard Model, and can play an important role in cosmology, serving as dark matter or dark energy, or both, drawing motivation in part from the string theory axiverse. Axion-like particles (ALPs) can also arise as composite degrees of freedom following chiral symmetry breaking in a dark confining gauge theory, analogous to the Standard Model (SM) pion. A dark sector with arbitrary $N_f$ flavors of dark quarks leads to $N_f^2-1$ axion-like states, effectively a field theory axiverse (or '$\pi$-axiverse'). A portal to the visible sector can be achieved through the standard kinetic mixing between the dark photon and SM photon, generating millicharges for the dark quarks and consequently couplings, both parity-even and parity-odd, between the SM and the dark pions. This scenario has been studied for the $N_f=2$ case and more recently for a dark Standard Model with $N_f=6$. In this work, we study the spectrum of this field theory axiverse for an arbitrary number of flavors, and apply this to the example $N_f=10$. We calculate the couplings to the SM photon analogous to the conventional axion-photon coupling, including the $N_f$ and $N_c$ dependence, and compute the present and future constraints on the $N_f=10$, $N_c=3$, $\pi$-axiverse. We elucidate the accompanying 'bary-verse' of superheavy dark baryons, namely an ensemble of charged and neutral dark baryons with a mass set by the dark pion decay constant.