Cosmological Domain Wall Problem Research Articles

Revisiting the minimal Nelson-Barr model

Abstract We revisit the minimal Nelson-Barr model for solving the strong CP problem through the idea of spontaneous CP breaking. The minimal model suffers from the quality problem, which means that the strong CP angle is generated by higher-dimensional operators and one-loop effects. Consequently, it has been considered that there is a cosmological domain wall problem and that leptogenesis does not work. We point out that just imposing an additional approximate global symmetry solves the quality problem. We also propose a simple solution to the domain wall problem and show that the thermal leptogenesis scenario works.

How viable is a QCD axion near 10 MeV?

There has been an attempt to revive the visible QCD axion at the 10 MeV scale assuming that it exclusively couples to the first-generation quarks and the electron. This variant of the QCD axion is claimed to remain phenomenologically viable, partly due to a clever model construction that induces tree-level pion-phobia and exploits uncertainties inherent in the chiral perturbation theory. We confront this model with the cosmological domain wall problem, the quality issue and constraints arising from the electron electric dipole moment. It is also pointed out that the gluon loop-generated axion-top coupling can provide a very large contribution to rare B-meson decays, such that the present LHCb data for B0 → K*0e+e− rule out the model for the axion mass larger than 30 MeV. There is a strong motivation for pushing the experimental analysis of B → K(*)e+e− to a lower e+e− invariant mass window, which will conclusively determine the fate of the model, as its contribution to this branching ratio significantly exceeds the Standard Model prediction.

The quality/cosmology tension for a post-inflation QCD axion

It is difficult to construct a post-inflation QCD axion model that solves the axion quality problem (and hence the Strong CP problem) without introducing a cosmological disaster. In a post-inflation axion model, the axion field value is randomized during the Peccei-Quinn phase transition, and axion domain walls form at the QCD phase transition. We emphasize that the gauge equivalence of all minima of the axion potential (i.e., domain wall number equals one) is insufficient to solve the cosmological domain wall problem. The axion string on which a domain wall ends must exist as an individual object (as opposed to a multi-string state), and it must be produced in the early universe. These conditions are often not satisfied in concrete models. Post-inflation axion models also face a potential problem from fractionally charged relics; solving this problem often leads to low-energy Landau poles for Standard Model gauge couplings, reintroducing the quality problem. We study several examples, finding that models that solve the quality problem face cosmological problems, and vice versa. This is not a no-go theorem; nonetheless, we argue that it is much more difficult than generally appreciated to find a viable post-inflation QCD axion model. Successful examples may have a nonstandard cosmological history (e.g., multiple types of cosmic axion strings of different tensions), undermining the widespread expectation that the post-inflation QCD axion scenario predicts a unique mass for axion dark matter.

Classification of three-family flavoured DFSZ axion models that have no domain wall problem

We provide an exhaustive classification of three-family DFSZ axion models that have no cosmological domain wall problem. This result is obtained by making the Peccei-Quinn symmetry flavour dependent in certain specific ways, thus reinforcing a possible connection between the strong CP problem and the flavour puzzle. Known DFSZ flavour variants such as the top-specific model emerge as special cases. Key features of the phenomenology of these models are briefly discussed.

S.M.A.S.H.E.D.: Standard Model Axion Seesaw Higgs inflation Extended for Dirac neutrinos

Inspired by the S.M.A.S.H. framework we construct a model that addresses the strong CP problem, axion dark matter, inflation and Dirac neutrino masses as well as leptogenesis. The model possesses only two dynamical scales, namely the SM breaking scale vH and the Peccei Quinn (PQ) breaking scale v .We introduce heavy vector-like quarks in the usual KSVZ fashion to implement the PQ mechanism for the strong CP problem. To generate neutrino masses via a dimension six operator scaling as mν ∼ v 3 H / v 2 σ we add heavy triplet and doublet leptons, which are vector-like under the SM but chiral under PQ symmetry. The model is free from the cosmological domain wall problem and predicts an axion to photon coupling which is about an order of magnitude larger than in conventional DFSZ and KSVZ models. Thus our scenario can be probed and potentially excluded by current and next generation axion experiments such as ORGAN or MADMAX.In addition we numerically demonstrate that our construction can generate the observed baryon asymmetry by realizing a version of the Dirac-Leptogenesis scenario. As a consequence of our neutrino mass mechanism we find that the asymmetry in triplet fermion decays can also be significantly enhanced by up to six orders of magnitude when compared to typical Seesaw scenarios without needing to invoke a resonant enhancement.In passing we note that a decaying Dirac fermion with multiple decay modes contains all the necessary ingredients required for the “quasi optimal efficiency”-scenario previously encountered in the context decaying scalar triplets. The impact of the right handed neutrinos and the axion on ΔN eff is estimated and lies within current bounds.

Gravitational waves in models with multicritical-point principle

The multicritical-point principle (MPP) provides a natural explanation of the large hierarchy between the Planck and electroweak scales. We consider a scenario in which MPP is applied to the Standard Model extended by two real singlet scalar fields phi and S, and a dimensional transmutation occurs by the vacuum expectation value of phi . In this paper, we focus on the critical points that possess a {mathbb {Z}}_2 symmetry Srightarrow -S and all the other fields are left invariant. Then S becomes a natural dark matter (DM) candidate. Further, we concentrate on the critical points where phi does not possess further {mathbb {Z}}_2 symmetry so that there is no cosmological domain-wall problem. Among such critical points, we focus on maximally critical one called CP-1234 that fix all the superrenormalizable parameters. We show that there remains a parameter region that satisfies the DM relic abundance, DM direct-detection bound and the current LHC constraints. In this region, we find a first-order phase transition in the early universe around the TeV-scale temperature. The resultant gravitational waves are predicted with a peak amplitude of {{mathcal {O}}}(10^{-12}) at a frequency of 10^{-2}{-}10^{-1} Hz, which can be tested with future space-based instruments such as DECIGO and BBO.

Harmonic hybrid inflation

We present a mechanism for realizing hybrid inflation using two axion fields with a purely non-perturbatively generated scalar potential. The structure of the scalar potential is highly constrained by the discrete shift symmetries of the axions. We show that harmonic hybrid inflation generates observationally viable slow-roll inflation for a wide range of initial conditions. This is possible while accommodating certain UV arguments favoring constraints f ≲ MP and ∆ϕ60 ≲ MP on the axion periodicity and slow-roll field range, respectively. We discuss controlled ℤ2-symmetry breaking of the adjacent axion vacua as a means of avoiding cosmological domain wall problems. Including a minimal form of ℤ2-symmetry breaking into the minimally tuned setup leads to a prediction of primordial tensor modes with the tensor-to-scalar ratio in the range 10−4 ≲ r ≲ 0.01, directly accessible to upcoming CMB observations. Finally, we outline several avenues towards realizing harmonic hybrid inflation in type IIB string theory.

Stellar cooling anomalies and variant axion models

A number of observations of stellar systems show a mild preference for anomalously fast cooling compared with what predicted in the standard theory, which leads to a speculation that there exists an additional energy loss mechanism originated from the emission of axions in stars. We revisit the global analysis of the stellar cooling anomalies by adopting conservative assessments on several systematic uncertainties and find that the significance of the cooling hints becomes weaker but still indicates a non-vanishing axion-electron coupling at around 2.4 σ. With the revised analysis results, we explore the possibility that such excessive energy losses are interpreted in the framework of variant axion models, which require two Higgs doublets and flavor-dependent Peccei-Quinn charge assignments. These models resolve two fundamental issues faced in the traditional KSVZ/DFSZ models by predicting a sizable axion coupling to electrons required to explain the cooling anomalies and at the same time providing a solution to the cosmological domain wall problem. We also find that a specific structure of the axion couplings to electrons and nucleons slightly relaxes the constraint from supernova 1987A and enlarges viable parameter regions compared with the DFSZ models. It is shown that good global fits to the observational data are obtained for axion mass ranges of 0.45 meV ≲ ma ≲ 30 meV, and that the predicted parameter regions can be probed in the forthcoming helioscope searches.

Coset cosmology

We show that the potential of Nambu-Goldstone bosons can have two or more local minima e.g. at antipodal positions in the vacuum manifold. This happens in many models of composite Higgs and of composite Dark Matter. Trigonometric potentials lead to unusual features, such as symmetry non-restoration at high temperature. In some models, such as the minimal SO(5)/SO(4) composite Higgs with fermions in the fundamental representation, the two minima are degenerate giving cosmological domain-wall problems. Otherwise, an unusual cosmology arises, that can lead to supermassive primordial black holes; to vacuum or thermal decays; to a high-temperature phase of broken SU(2)L, possibly interesting for baryogenesis.

Spontaneous Breaking of Lepton Number and the Cosmological Domain Wall Problem.

We show that if global lepton number symmetry is spontaneously broken in a postinflation epoch, then it can lead to the formation of cosmological domain walls. This happens in the well-known "Majoron paradigm" for neutrino mass generation. We propose some realistic examples that allow spontaneous lepton number breaking to be safe from such domain walls.

Flavon stabilization in models with discrete flavor symmetry

We propose a simple mechanism for stabilizing flavon fields with aligned vacuum structure in models with discrete flavor symmetry. The basic idea is that flavons are stabilized by the balance between the negative soft mass and non-renormalizable terms in the potential. We explicitly discuss how our mechanism works in A4 flavor model, and show that the field content is significantly simplified. It also works as a natural solution to the cosmological domain wall problem.

Matter parity violating dark matter decay in minimal SO(10), unification, vacuum stability and verifiable proton decay

In direct breaking of non-supersymmetric SO(10) to the standard model, we investigate the possibility that dark matter (DM) decaying through its mixing with right-handed neutrino (RHν) produces high energy IceCube neutrinos having type-I seesaw masses. Instead of one universal mixing and one common heavy RHν mass proposed in a recent standard model extension, we find that underlying quark–lepton symmetry resulting in naturally hierarchical RHν masses predict a separate mixing with each of them. We determine these mixings from the seesaw prediction of the DM decay rates into the light neutrino flavors. We further show that these mixings originate from Planck-scale assisted spontaneously broken matter parity needed to resolve the associated cosmological domain wall problem. This leads to the prediction of a new LHC accessible matter-parity odd Higgs scalar which also completes vacuum stability in the Higgs potential for its mass MχS≃177 GeV. We have also discussed realization of relic density of decaying dark matter in relation to flux of IceCube neutrinos. Two separate minimal SO(10) models are further noted to predict such dark matter dynamics where a single scalar submultiplet from 126H† or 210H of intermediate mass achieves precision gauge coupling unification. Despite the presence of two large Higgs representations and the fermionic dark matter host, 45F, experimentally accessible proton lifetimes are also predicted with reduced uncertainties.

Cosmologically Safe QCD Axion without Fine-Tuning.

Although QCD axion models are widely studied as solutions to the strong CP problem, they generically confront severe fine-tuning problems to guarantee the anomalous Peccei-Quinn (PQ) symmetry. In this Letter, we propose a simple QCD axion model without any fine-tunings. We introduce an extra dimension and a pair of extra quarks living on two branes separately, which is also charged under a bulk Abelian gauge symmetry. We assume a monopole condensation on our brane at an intermediate scale, which implies that the extra quarks develop chiral symmetry breaking and the PQ symmetry is broken. In contrast to Kim's original model, our model explains the origin of the PQ symmetry thanks to the extra dimension and avoids the cosmological domain wall problem because of chiral symmetry breaking in Abelian gauge theory.

Suppressing the QCD axion abundance by hidden monopoles

We study the Witten effect of hidden monopoles on the QCD axion dynamics, and show that its abundance as well as isocurvature perturbations can be significantly suppressed if there is a sufficient amount of hidden monopoles. When the hidden monopoles make up a significant fraction of dark matter, the Witten effect suppresses the abundance of axion with the decay constant smaller than 1012GeV. The cosmological domain wall problem of the QCD axion can also be avoided, relaxing the upper bound on the decay constant when the Peccei–Quinn symmetry is spontaneously broken after inflation.

Domain wall formation from level crossing in the axiverse

We point out that domain wall formation is a more common phenomenon in the Axiverse than previously thought. Level crossing could take place if there is a mixing between axions, and if some of the axions acquire a non-zero mass through non-perturbative effects as the corresponding gauge interactions become strong. The axion potential changes significantly during the level crossing, which affects the axion dynamics in various ways. We find that, if there is a mild hierarchy in the decay constants, the axion starts to run along the valley of the potential, passing through many crests and troughs, until it gets trapped in one of the minima; the {\it axion roulette}. The axion dynamics exhibits a chaotic behavior during the oscillations, and which minimum the axion is finally stabilized is highly sensitive to the initial misalignment angle. Therefore, the axion roulette is considered to be accompanied by domain wall formation. The cosmological domain wall problem can be avoided by introducing a small bias between the vacua. We discuss cosmological implications of the domain wall annihilation for baryogenesis and future gravitational wave experiments.

Observable dark radiation from a cosmologically safe QCD axion

We propose a QCD axion model that avoids the cosmological domain wall problem, introducing a global SU(3)_f family symmetry to which we embed the unwanted PQ discrete symmetry. The spontaneous breaking of SU(3)_f and PQ symmetry predicts eight NG bosons as well as axion, all of which contribute to dark radiation in the Universe. The derivation from the standard model prediction of dark radiation can be observed by future observations of CMB fluctuations. Our model also predicts a sizable exotic kaon decay rate, which is marginally consistent with the present collider data and would be tested by future collider experiments.

NMSSM in gauge-mediated SUSY breaking without domain wall problem

A problem of the gauge-mediated SUSY breaking model is its difficulty to generate a natural value of μ/Bμ, while the NMSSM is a natural framework to solve the μ/Bμ-problem. The NMSSM in gauge-mediated SUSY breaking in its original form does not work well since the singlet field cannot develop a desired vacuum expectation value. It also suffers from the cosmological domain wall problem. We study an extension of the model to include additional vector-like matter, which is charged under the hidden QCD. It is shown that this simple extension solves both the problems. We study phenomenological and cosmological implications of this extended models. The lightest Higgs mass can be as large as 130–140 GeV for some model points.

Casimir force for cosmological domain walls

We calculate the vacuum fluctuations that may affect the evolution of cosmological domain walls. Considering domain walls, which are classically stable and have interaction with a scalar field, we show that explicit symmetry violation in the interaction may cause quantum bias that can solve the cosmological domain wall problem.

Superconformal symmetry, NMSSM, and inflation

We identify a particularly simple class of supergravity models describing superconformal coupling of matter to supergravity. In these models, which we call the canonical superconformal supergravity models, the kinetic terms in the Jordan frame are canonical, and the scalar potential is the same as in the global theory. The pure supergravity part of the total action has a local Poincar\'e supersymmetry, whereas the chiral and vector multiplets coupled to supergravity have a larger local superconformal symmetry. The scale-free globally supersymmetric theories, such as the NMSSM with a scale-invariant superpotential, can be naturally embedded into this class of theories. After the supergravity embedding, the Jordan frame scalar potential of such theories remains scale free; it is quartic, it contains no mass terms, no nonrenormalizable terms, no cosmological constant. The local superconformal symmetry can be broken by additional terms, which, in the small field limit, are suppressed by the gravitational coupling. This can be achieved by introducing the nonminimal scalar-curvature coupling, and by taking into account interactions with a hidden sector. In this approach, the smallness of the mass parameters in the NMSSM may be traced back to the original superconformal invariance. This allows one to address the $\ensuremath{\mu}$ problem and the cosmological domain wall problem in this model, and to implement chaotic inflation in the NMSSM. We discuss the gravitino problem in the NMSSM inflation, as well as the possibility to obtain a broad class of new versions of chaotic inflation in supergravity.

Probing variant axion models at LHC

We study collider implications of variant axion models which naturally avoid the cosmological domain wall problem. We find that in such models the branching ratio of $h \to \gamma\gamma$ can be enhanced by a factor of 5 up to 30 as compared with the standard model prediction. The $h \to \gamma\gamma$ process is therefore a promising channel to discover a light Higgs boson at the LHC and to probe the Peccei-Quinn charge assignment of the standard model fields from Yukawa interactions.