Abstract
The post-inflationary breaking of Peccei-Quinn (PQ) symmetry can lead to the cosmic domain wall catastrophe. In this Letter we show how to avoid domain walls implementing the Instanton Interference Effect (IIE) with a new interaction which itself breaks PQ symmetry and confines at an energy scale smaller than $\Lambda_{QCD}$. We give a general description of the mechanism and consider its cosmological implications and constraints within a minimal model. Contrary to other mechanisms we do not require an inverse phase transition neither fine-tuned bias terms. Incidentally, the mechanism leads to the introduction of new self-interacting dark matter candidates and the possibility of producing gravitational waves in the frequency range of SKA. Unless a fine-tuned hidden sector is introduced, the mechanism predicts a QCD axion in the mass range $1\text{ meV}-15\text{ meV}$.
Highlights
AND MOTIVATIONThe axion solution to the strong CP problem [1,2,3,4,5,6] is a well-known paradigm where domain walls (DWs) bounded by strings1 emerge [9,10]
When NQCD and NHC are coprime numbers, the PQ symmetry is completely broken by the combination of explicit symmetry breaking by instantons and the domain wall problem is solved
We can impose that the domain walls decay before matter domination and obtain the lower bound to the confinement scale, ΛHC ≫ 3.36 × 10−101⁄28Λ2QCDfa1=4 GeV1=4: ð11Þ
Summary
The axion solution to the strong CP problem [1,2,3,4,5,6] is a well-known paradigm where domain walls (DWs) bounded by strings emerge [9,10]. When the cosmic temperature reaches the QCD scale, QCD instantons generate an effective potential for the axion field and domain walls form. At this point, cosmic strings become attached to domain walls. For NQCD > 1 each string gets attached to NQCD walls and the network cannot decay: the walls are topologically protected and stable Their evolution with cosmic expansion is slower than that of matter or radiation and will eventually dominate the energy density of the Universe. In this case, pushed beyond the horizon and will not harm our cosmic evolution This is the case of a PQ symmetry spontaneously broken before inflation and never restored after reheating. We conclude and comment on future directions to follow
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