Baryogenesis from Strong $CP$ Violation and the QCD Axion

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 baryogenesis if the electroweak phase transition is delayed below the GeV scale. This can occur naturally if the Higgs couples to a $\mathcal{O}(100)\mathrm{GeV}$ 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 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 antimatter asymmetry and dark matter.

Figure 1

Today’s baryon asymmetry as a function of $T_{\text{EWPT}}$ compared with measured value (dotted line). The case $T_{\mathrm{eff}}/T_{\mathrm{reh}}=1$ and $T_{\mathrm{eff}}\sim T_{\mathrm{E}\mathrm{W}\mathrm{P}\mathrm{T}}$ that would characterize standard EW baryogenesis is unfeasible as $T_{\text{reh}}\sim \mathcal{O}(m_H)\gg {\mathrm{\Lambda }}_{\text{QCD}}$. The cases with $T_{\mathrm{eff}}/T_{\mathrm{reh}}\gtrsim 10$ 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 ${\overline{\mathrm{\Theta }}}_i$ in the range $[{10}^{-2},\pi /2]$. Left: $m_a\gtrsim 3H_{\text{EW}}\sim 3\times {10}^{-14}$ GeV, oscillations start at $T=0.3\mathrm{GeV}$ in the supercooling era before the EWPT. Right: $m_a\lesssim 3H_{\text{EW}}$, the axion is frozen to its initial value until after reheating.