Baryogenesis from Strong CP Violation and the QCD Axion

TL;DR

The paper explores whether strong CP violation from the QCD axion can generate the cosmic baryon asymmetry in a cosmology where electroweak symmetry breaking is delayed by a dilaton-driven Higgs quench, enabling cold baryogenesis. The mechanism hinges on a time-varying CP-violating source $\mu=\dot{\zeta}$ linked to axion dynamics, producing a chemical potential for Chern-Simons number that biases SM sphaleron processes during a delayed EWPT. Under plausible axion parameters and a dilaton-induced supercooling scenario, the observed baryon asymmetry can be achieved while preserving axion dark matter, and the model makes testable predictions for axion experiments, LHC dilaton searches, and a stochastic gravitational-wave background detectable by eLISA. This framework connects strong CP physics, baryogenesis, and dark matter, and relies on a naturally light, TeV-scale dilaton and a QCD axion with $f_a$ in the usual window to satisfy cosmological constraints. The scenario is falsifiable by upcoming axion searches, collider tests of dilaton-like scalars, and gravitational-wave observations.

Abstract

The strong CP-violating parameter is small today as indicated by constraints on the neutron electric dipole moment. In the early universe, the QCD axion has not yet relaxed to its QCD-cancelling minimum and it is natural to wonder whether this large CP violation could be responsible for baryogenesis. We show that strong CP violation from the QCD axion can be responsible for the matter antimatter asymmetry of the universe in the context of cold electroweak (EW) baryogenesis if the EW phase transition is delayed below the GeV scale. This can occur naturally if the Higgs couples to a O(100) GeV dilaton, as expected in some models where the Higgs is a pseudo-Nambu Goldstone boson of a new strongly interacting sector at the TeV scale. The only new relevant ingredients beyond the Standard Model in our framework are the QCD axion and an EW scale dilaton-like scalar field. The existence of such a second scalar resonance with a mass and properties similar to the Higgs boson will soon be tested at the LHC. In this context, the QCD axion would not only solve the strong CP problem, but also the matter anti-matter asymmetry and dark matter.

Figures (5)

• Figure 1: Evolution of $\bar{\Theta}/\bar{\Theta}_i$ assuming axion oscillations start respectively at $T_i =0.1, 0.3, 1$ GeV, $\bar{\Theta}_i$ being the initial value. From Eq.( \ref{['eq:axionbau']}), the B asymmetry is proportional to $\bar{\Theta}(T_{EWPT})$, where $T_{EWPT}$ is the temperature of the EW phase transition.
• Figure 2: Prediction for today's B asymmetry as a function of the temperature of the EWPT compared with measured value (dotted line). The case $T_{\hbox{\tiny eff}}/T_{\hbox{\tiny reh}}=1$ (light gray) and $T_{\hbox{\tiny eff}} \sim T_{\hbox{\tiny EWPT}}$ that would characterize standard EW baryogenesis is unfeasible as $T_{reh}\sim {\cal O}(m_H) \gg \Lambda_{QCD}$. The cases with $T_{\hbox{\tiny eff}}/T_{\hbox{\tiny reh}} \gtrsim 10$ can easily account for a large B asymmetry and correspond to a quenched EWPT, as in cold EW baryogenesis. Each band corresponds to varying the initial angle value $\bar{\Theta}_i$ in the range $[10^{-2},\pi/2]$. Left: $m_a\gtrsim 3H_{EW}\sim3\times 10^{14}$ GeV, for oscillations starting at $T=0.3$ GeV in the supercooling era before the EWPT. Right: $m_a\lesssim 3H_{EW}$, the axion is frozen to its initial value until after reheating.
• Figure 3: Value of the dilaton field for which the cold baryogenesis quenching condition is satisfied as a function of the Higgs quartic coupling $\lambda$, for a typical dilaton potential (we used the Goldberger-Wise potential from Randall:2006py for this illustration). As $\sigma$ approaches the minimum $\langle \sigma \rangle =f$, the quenching condition is satisfied even for relatively small values of $\lambda$.
• Figure 4: Upper bound on the dilaton mass for the reheat temperature to be below the sphaleron freese-out temperature, $T=130$ GeV, as a function of the dilaton VEV $\langle \sigma\rangle$, for three choices of dilaton potentials, taken respectively from Ref. Bellazzini:2013fga (solid line) and Ref. Konstandin:2011dr (dotted and dashed lines).
• Figure 5: Comparison between the axion oscillation rate $m_a$ for various PQ scales $f_a$ with the expansion rate of the universe in the radiation era and the supercooling era. Axion oscillations start when $3H\lesssim m_a$.