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Register a new userAxion dark matter in the post-inflationary Peccei-Quinn symmetry breaking scenario
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We consider extensions of the Standard Model in which a spontaneously broken global chiral Peccei-Quinn (PQ) symmetry arises as an accidental symmetry of an exact $Z_N$ symmetry. For $N = 9$ or $10$, this symmetry can protect the accion - the Nambu-Goldstone boson arising from the spontaneous breaking of the accidental PQ symmetry - against semi-classical gravity effects, thus suppressing gravitational corrections to the effective potential, while it can at the same time provide for the small explicit symmetry breaking term needed to make models with domain wall number $N_{rm DW}>1$, such as the popular Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model ($N_{rm DW}=6$), cosmologically viable even in the case where spontaneous PQ symmetry breaking occurred after inflation. We find that $N=10$ DFSZ accions with mass $m_A approx 3.5$-$4.2,mathrm{meV}$ can account for cold dark matter and simultaneously explain the hints for anomalous cooling of white dwarfs. The proposed helioscope International Axion Observatory - being sensitive to solar DFSZ accions with mass above a few meV - will decisively test this scenario.
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We show that, for values of the axion decay constant parametrically close to the GUT scale, the Peccei-Quinn phase transition may naturally occur during warm inflation. This results from interactions between the Peccei-Quinn scalar field and the ambient thermal bath, which is sustained by the inflaton field through dissipative effects. It is therefore possible for the axion field to appear as a dynamical degree of freedom only after observable CMB scales have become super-horizon, thus avoiding the large-scale axion isocurvature perturbations that typically plague such models. This nevertheless yields a nearly scale-invariant spectrum of axion isocurvature perturbations on small scales, with a density contrast of up to a few percent, which may have a significant impact on the formation of gravitationally-bound axion structures such as mini-clusters.
The primordial irreducible gravitational-wave background due to quantum vacuum tensor fluctuations produced during inflation spans a large range of frequencies with an almost scale-invariant spectrum but is too low to be detected by the next generation of gravitational-wave interferometers. We show how this signal is enhanced by a short temporary kination era in the cosmological history (less than 10 e-folds), that can arise at any energy scale between a GeV and the inflationary scale $10^{16}$ GeV. We argue that such kination era is naturally generated by a spinning axion before it gets trapped by its potential. It is usually assumed that the axion starts oscillating around its minimum from its initially frozen position. However, the early dynamics of the Peccei-Quinn field can induce a large kinetic energy in the axion field, triggering a kination era, either before or after the axion acquires its mass, leading to a characteristic peak in the primordial gravitational-wave background. This represents a smoking-gun signature of axion physics as no other scalar field dynamics is expected to trigger such a sequence of equations of state in the early universe. We derive the resulting gravitational-wave spectrum, and present the parameter space that leads to such a signal as well as the detectability prospects, in particular at LISA, Einstein Telescope, Cosmic Explorer and Big Bang Observer. We show both model-independent predictions and present as well results for two specific well-motivated UV completions for the QCD axion dark matter where this dynamics is built-in.
We study the Peccei-Quinn (PQ) symmetry of sterile right-handed neutrino sector and the gauge symmetries of the Standard Model (SM). Due to four-fermion interactions, spontaneous breaking of these symmetries at the electroweak scale generates top-quark Dirac mass and sterile neutrino Majorana mass. The top quark channels yields massive Higgs, $W^pm$ and $Z^0$ bosons. The sterile neutrino channel yields the heaviest sterile neutrino Majorana mass, sterile Nambu-Goldstone axion (or majoron) and massive scalar $chi$boson ($m_chisim 10^2$ GeV). Their tiny couplings to SM particles are effectively induced by four-fermion operators. We show that such sterile axion is the PQ solution to the strong CP problem. The lightest sterile neutrino ($m_N^esim 10^2$ keV), sterile QCD axion ($m_a< 10^{-6}$ eV, $g_{agamma}< 10^{-13} {rm GeV}^{-1}$) and $chi$boson can be dark matter particle candidates, for their tiny couplings and long lifetimes inferred from the Xenon1T experiment. The axion and $chi$boson couplings to SM particles are below the values reached by current laboratory experiments and astrophysical observations for directly or indirectly detecting dark matter particles.
We propose a model where Dirac neutrino mass is obtained from small vacuum expectation value (VEV) of neutrino-specific Higgs doublet without fine-tuning problem. The small VEV results from a seesaw-like formula with the high energy scale identified as the Peccei-Quinn (PQ) symmetry breaking scale. Axion can be introduced {it `a la} KSVZ or DFSZ. The model suggests neutrino mass, solution to the strong CP problem, and dark matter may be mutually interconnected.
We show that the required high quality of the Peccei-Quinn symmetry can be naturally explained in the aligned QCD axion models where the QCD axion arises from multiple axions with decay constants much smaller than the axion window, e.g., around the weak scale. Even in the presence of general Planck-suppressed Peccei-Quinn symmetry breaking operators, the effective strong CP phase remains sufficiently small in contrast to the standard axion models without the alignment. The QCD axion potential has small or large modulations due to the symmetry breaking operators, which can significantly affect the axion cosmology. When the axions are trapped in different minima, domain walls appear and their scaling behavior suppresses the axion isocurvature perturbations at super-horizon scales. Our scenario predicts many axions and saxions coupled to gluons, and they may be searched for at collider experiments. In particular, the recently found diphoton excess at 750 GeV could be due to one of such (s)axions.
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