Technically Natural Baryogenesis with the QCD Axion

Abstract

The strong CP problem, dark matter, and the observed baryon abundance may all be connected dynamically through the QCD axion, though whether this connection is realized in Nature depends on additional degrees of freedom. We show that the required dynamics can arise from technically natural ingredients if multiple axions mix through the QCD anomaly. We review the connection between axion rotation and the baryon asymmetry, revisit the axiogenesis and kinetic-misalignment proposals, and derive new limits on their dynamics. Motivated by these considerations, naturalness and axion quality, we construct a realization in which mixing with additional axions both enhances the effective QCD axion field range through monodromy and dissipates excess rotational energy through mixing with a dark confining gauge sector, thereby avoiding overclosure. We study the resulting effective dynamics in several illustrative limits, uncovering phenomena that may arise more generally in multi-axion theories. We numerically evolve the full coupled system and show that it can simultaneously reproduce the observed baryon asymmetry and dark matter abundance within a predictive region of parameter space testable by near-future direct and indirect searches. Depending on the model parameters, either the QCD axion or the new heavy axion constitutes the dominant dark matter component. Successful baryogenesis favors a QCD axion decay constant near its current astrophysical lower bound, while the heavy axion lies within reach of forthcoming astrophysical and cosmological observations.