Light axions can potentially leave a cosmic background, just like neutrinos. We complete the study of thermal axion production across the electroweak scale by providing a smooth and continuous treatment through the two phases. Focusing on both flavor conserving and violating couplings to third generation quarks, we compute the amount of axions produced via scatterings and decays of thermal bath particles. We perform a model independent analysis in terms of axion effective couplings, and we also make predictions for specific microscopic QCD axion scenarios. This observable effect, parameterized as it is conventional by an effective number of additional neutrinos, is above the $1σ$ sensitivity of future CMB-S4 surveys. Moreover, if one assumes no large hierarchies among dimensionless axion couplings to standard model particles, future axion helioscopes will provide a complementary probe for the parameter region we study.
We revisit the constraint from the recently reported cosmic birefringence on axion-like particles with a general decay constant. A particular attention is paid to the naturalness of the model parameter space, which has been overlooked in the literature. We show that the observed cosmic birefringence is naturally explained by the electroweak axion with a string-theory inspired decay constant $F_A\simeq 10^{16}$ GeV.
The electroweak axion is identified with the fuzzy dark matter of a mass $m\simeq 10^{-20}$--$10^{-19}\,{\rm eV}$. The model predicts two components of dark matter, one is ultralight and the other is WIMP-like. The Chern-Simons-type interaction between the fuzzy dark matter and photon and the $B+L$ breaking proton decays are predicted.
We discuss the electroweak quintessence axion as a candidate for dark energy, taking into account observational as well as quantum-gravity constraints.
We study the axion strings with the electroweak gauge flux in the DFSZ axion model and show that these strings, called the electroweak axion strings, can exhibit superconductivity without fermionic zero modes. We construct three types of electroweak axion string solutions. Among them, the string with $W$-flux can be lightest in some parameter space, which leads to a stable superconducting cosmic string. We also show that a large electric current can flow along the string due to the Peccei-Quinn scale much higher than the electroweak scale. This large current induces a net attractive force betw…
Spontaneous Baryogenesis from Axions on Induced Electroweak Walls
Departament de Física Quàntica i Astrofísica and Institut de Ciències del Cosmos (ICC), Universitat de Barcelona（巴塞罗那大学量子物理与天体物理系及宇宙科学研究所）；Department of Physics, Tokyo Metropolitan University（东京都立大学物理系）
An axion can make the electroweak phase transition strongly first-order as required for electroweak baryogenesis even if it is weakly coupled to the Higgs sector. This is essentially because the axion periodicity naturally allows the structure of phase transition to be insensitive to the axion decay constant that determines the strength of axion interactions. Furthermore, the axion can serve as a CP phase relevant to electroweak baryogenesis if one introduces an effective axion coupling to the top quark Yukawa operator. Then, for $f$ between about TeV and order $10$~TeV, the observed baryon as…
Recently, a new mechanism to generate a naturally small electroweak scale has been proposed. It exploits the coupling of the Higgs to an axion-like field and a long era in the early universe where the axion unchains a dynamical screening of the Higgs mass. We present a new realization of this idea with the new feature that it leaves no signs of new physics up to a rather large scale, 10^9 GeV, except for two very light and weakly coupled axion-like states. One of the scalars can be a viable Dark Matter candidate. Such a cosmological Higgs-axion interplay could be tested with a number of experi…
Taking the recently reported non-zero rotation angle of the cosmic microwave background (CMB) linear polarization $β=0.35\pm0.14{\rm\, deg}$ as the hint for a pseudo Nambu-Goldstone boson quintessence dark energy (DE), we study the electroweak (EW) axion quintessence DE model where the axion mass is generated by the EW instantons. We find that the observed value of $β$ implies a non-trivial $U(1)$ electromagnetic anomaly coefficient ($c_γ$), once the current constraint on the DE equation of state is also taken into account. With the aid of the hypothetical high energy structure of the model in…
If there is explicit violation of baryon plus lepton number at some energy scale, then the electroweak theory depends upon a θ-angle. Due to a singular integration over small scale size instantons, this θ-dependence is sensitive to very high momentum scales. Assuming that there is no new physics between the electroweak and Planck scales, for an electroweak axion the energy difference between the vacuum at θ /= 0, and that at θ = 0, is of the correct order of magnitude to be the dark energy observed in the present epoch.
If electroweak dynamical symmetry breaking is due to a chiral condensate of color sextet quarks, dynamics analagous to ``walking technicolor'' will enhance the condensate by orders of magnitude compared to the electroweak chiral scale. This enhancement compensates for the exponential suppression of electroweak scale color instanton interactions. As a result the $η_6$ axion can naturally aquire an electroweak scale mass.
Recent cosmological observations suggest the presence of small but nonzero cosmological constant $Λ_{cos}^4 \simeq (2 \times 10^{-3} eV)^4$. It is an intriguing possibility that such a small cosmological constant is supplied by the potential energy density of an ultralight axion-like field (called as quintessence axion). If this axion couples to the electroweak gauge fields, its potential may be generated by the electroweak instanton effects. We calculate the axion potential assuming the supersymmetric standard model and obtain a surprising result that the induced energy density of the quintes…
One of the simplest ways to account for the observed W-boson mass shift is to introduce the $SU(2)_L$ triplet Higgs boson with zero hypercharge, whose vacuum expectation value is about 3 GeV. If the triplet is heavy enough at $\mathcal{O}(1)$ TeV, it essentially contributes only to $T$ parameter without any conflict to the observation. The presence of a complex triplet Higgs boson raises the $SU(2)_L$ gauge coupling constant to $α_2(M_{\rm PL} )\simeq 1/44$ at the Planck scale. Thanks to this larger gauge coupling constant, we show that the electroweak axion vacuum energy explains the observed…
In a previous paper, the standard model was generalized to include an electroweak axion which carries baryon plus lepton number, $B+L$. It was shown that such a model naturally gives the observed value of the dark energy, if the scale of explicit baryon number violation was chosen to be of the order of the Planck mass. In this paper, we consider the effect of the modulus of the axion field. Such a field must condense in order to generate the standard Goldstone boson associated with the phase of the axion field. This condensation breaks baryon number. We argue that this modulus might be associa…
An axion-like particle (ALP) offers a new direction in electroweak baryogenesis because the periodic nature enables it to trigger a strong first-order phase transition insensitively to the decay constant $f$. For $f$ much above TeV, the ALP-induced electroweak phase transition is approximately described by adiabatic processes, distinguishing our scenario for electroweak baryogenesis from the conventional ones. We show that, coupled to the electroweak anomaly, the ALP can naturally realize spontaneous electroweak baryogenesis to solve the matter-antimatter asymmetry problem for $f$ in the range…
We determine the model-independent component of the couplings of axions to electroweak gauge bosons, induced by the minimal coupling to QCD inherent to solving the strong CP problem. The case of the invisible QCD axion is developed first, and the impact on $W$ and $Z$ axion couplings is discussed. The analysis is extended next to the generic framework of heavy true axions and low axion scales, corresponding to scenarios with enlarged confining sector. The mass dependence of the coupling of heavy axions to photons, $W$ and $Z$ bosons is determined. Furthermore, we perform a two-coupling-at-a-ti…
The complex field $Φ$ containing the QCD axion has a natural portal coupling to the Higgs doublet of the form $λ_{h ϕ}|Φ|^2 |H|^2$. Here we consider the possibility that $λ_{h ϕ}$ has a natural magnitude for a dimensionless coupling, $λ_{h ϕ} \sim 0.1-1$. This is possible if the total mass squared parameter of the Higgs in the vacuum, including quadratic divergent and the $Φ$ vacuum expectation value contributions, is renormalized to reproduce the observed Higgs boson mass. It is then possible for the axion sector to stabilize the electroweak vacuum. We show the requirement of electroweak vacu…
The dark axion portal is a coupling of an axion-like particle to a dark photon kinetically mixed with the visible photon. We show how this portal, when applied to the relaxion, can lead to cosmological relaxation of the weak scale using dark photon production. The key backreaction mechanism involves the Schwinger effect: As long as electroweak symmetry is unbroken, Schwinger production of massless Standard Model fermions, which carry dark millicharges, suppresses the dark photon production. Once the electroweak symmetry is broken, the fermions acquire mass and the suppression is lifted. An enh…
The dynamics of saxion in a supersymmetric axion model and its effect on the axion production is studied in detail. We find that the axion production is very efficient when the saxion oscillation amplitude is much larger than the Peccei-Quinn scale, due to a spike-like behavior of the effective axion mass. We also consider the axino production and several cosmological consequences. The possibility of detection of gravitational waves from the non-linear dynamics of the saxion and axion is discussed.
We explore the connection between tree-level Dirac neutrino masses and axion physics in a scenario where the PQ symmetry enforces lepton number conservation perturbatively. Requiring that the PQ scale $f_a$ is the only heavy scale to play a role in neutrino mass generation, we are led to the construction of a KSVZ-type model where Dirac neutrino masses are inversely proportional to $f_a$, provided a real scalar triplet (zero hypercharge) is added to the SM scalar sector. We analyse this extended scalar sector, focusing on the stabilisation of the electroweak vacuum. The contribution of the tri…
We show that an axionlike particle (ALP) naturally implements spontaneous electroweak baryogenesis through a cosmic evolution strongly tied to the electroweak phase transition (EWPT) if it feebly couples to the Higgs field while giving a small contribution to the Higgs boson mass. The observed baryon asymmetry can be generated successfully if the ALP couples strongly enough to the electroweak anomaly. Also interesting is that the ALP contributes to dark matter, and its coupling to a hidden gauge sector makes the relic abundance insensitive to the cosmic history before the EWPT. The ALP explain…
We reconsider thermal production of axions in the early universe, including axion couplings to all Standard Model (SM) particles. Concerning the axion coupling to gluons, we find that thermal effects enhance the axion production rate by a factor of few with respect to previous computations performed in the limit of small strong gauge coupling. Furthermore, we find that the top Yukawa coupling induces a much larger axion production rate, unless the axion couples to SM particles only via anomalies.
We study thermal axion production around the confinement scale. At higher temperatures, we extend current calculations to account for the masses of heavy quarks, whereas we quantify production via hadron scattering at lower temperatures. Matching our results between the two opposite regimes provides us with a continuous axion production rate across the QCD phase transition. We employ such a rate to quantify the axion contribution to the effective number of neutrino species.
We study derivatively coupled fermions in axion-driven inflation, specifically $m_ϕ^2ϕ^2$ and monodromy inflation, and calculate particle production during the inflationary epoch and the post-inflationary axion oscillations. During inflation, the rolling axion acts as an effective chemical potential for helicity which biases the gravitational production of one fermion helicity over the other. This mechanism allows for efficient gravitational production of heavy fermion states that would otherwise be highly suppressed. Following inflation, the axion oscillates and fermions with both helicities …
We point out that the swampland conjectures, forbidding the presence of global symmetries and (meta-)stable de Sitter vacua within quantum gravity, pick up a dynamical axion for the electroweak SU(2) gauge theory as a natural candidate for the quintessence field. The potential energy of the electroweak axion provides an attractive candidate for the dark energy. We discuss constraints from the weak gravity conjecture, from the conjecture of no global symmetry, and from observations, which can be satisfied elegantly in a supersymmetric extension of the standard model.
We investigate the interplay between early universe cosmology and dark matter direct detection, considering axion models with naturally suppressed couplings to photons. In the context of the cosmological relaxation of the electroweak scale, we focus on a scenario of \emph{Relaxion Dark Matter}, in which the relaxion field constitutes all the observed dark matter relic density and its allowed mass range is fixed to a few $\mathrm{keV}$ by construction. In particular, we show that a relaxion particle with mass $m_ϕ= 3.0 \,\mathrm{keV}$ which couples to electrons with $g_{ϕ, e}= 6.8 \times 10^{-1…
We consider a hidden-valley gauge sector, G, with strong coupling scale Lambda~TeV and CP-violating topological parameter, theta, as well as a new axion degree of freedom which adjusts theta to near zero in the current universe. If the G-sector couples to the Standard Model via weak-scale states charged under both, then in the early universe it is possible for the CP-violation due to theta (which has not yet been adjusted to zero by the hidden axion) to feed in to the SM and drive efficient baryogenesis during the electroweak (EW) phase transition, independent of the effectively small amount o…
Electroweak Baryogenesis from Collapsing Domain Walls
University of Wisconsin-Madison（威斯康星大学麦迪逊分校）；Argonne National Laboratory（阿贡国家实验室）；University of Oklahoma（俄克拉荷马大学）；The Hong Kong University of Science and Technology（香港科技大学）
New Source for QCD Axion Dark Matter Production: Curvature Induced
İstinye University（伊斯坦耶大学）；Tel-Aviv University（特拉维夫大学）；Johannes Gutenberg University（约翰内斯·古腾堡大学）；The University of Osaka（大阪大学）；Deutsches Elektronen-Synchrotron DESY（德国电子同步加速器中心）；Universität Hamburg（汉堡大学）；Universitat de València-CSIC（瓦伦西亚大学-西班牙国家研究委员会）；Khon Kaen University（孔敬大学）
We present a new mechanism to produce the dark photon ($γ'$) in the early universe with a help of the axion ($a$) using a recently proposed dark axion portal. The dark photon, a light gauge boson in the dark sector, can be a relic dark matter if its lifetime is long enough. The main process we consider is a variant of the Primakoff process $f a \to f γ'$ mediated by a photon, which is possible with the axion--photon--dark photon coupling. The axion is thermalized in the early universe because of the strong interaction and it can contribute to the non-thermal dark photon production through the …
We implement inflation in a non-supersymmetric SU(5) model based on a non-minimal coupling of the axion field to gravity. The isocurvature fluctuations are adequately suppressed, axions comprise the dark matter, proton lifetime estimates are of order 8e34-3e35 years, and the observed baryon asymmetry arises via non-thermal leptogenesis. The presence of low scale colored scalars ensures unification of the Standard Model gauge couplings and also helps in stabilizing the electroweak vacuum.
Production of axionlike particles (ALPs) by primordial magnetic fields may have significant impacts on cosmology. We discuss the production of ALPs in the presence of the primordial magnetic fields. We find a region of the ALP mass and photon coupling which realizes the observed properties of the dark matter with appropriate initial conditions for the magnetic fields. This region may be interesting in light of recent indications for the 3.5 keV lines from galaxy clusters. For a small axion mass, a region of previously allowed parameter spaces is excluded by overproduction of ALPs as a hot/warm…
We analyze the spectrum of axions radiated from collapse of domain walls, which have received less attention in the literature. The evolution of topological defects related to the axion models is investigated by performing field-theoretic lattice simulations. We simulate the whole process of evolution of the defects, including the formation of global strings, the formation of domain walls and the annihilation of the defects due to the tension of walls. The spectrum of radiated axions has a peak at the low frequency, which implies that axions produced by the collapse of domain walls are not hig…
We propose that the observed matter-antimatter asymmetry can be naturally produced as a byproduct of axion-driven slow-roll inflation by coupling the axion to standard-model neutrinos. We assume that GUT scale right-handed neutrinos are responsible for the masses of the standard model neutrinos and that the Higgs field is light during inflation and develops a Hubble scale vacuum expectation value (VEV). In this set up, the rolling axion generates a helicity asymmetry in standard-model neutrinos. Following inflation, this helicity asymmetry becomes equal to a net lepton number as the Higgs VEV …
We show that a recently proposed framework that provides a simple connection between Majorana neutrinos and an invisible axion in minimal scalar extensions of the standard electroweak model can be naturally embedded in a classically scale-invariant setup. The explicit breaking of the scale invariance à la Coleman-Weinberg generates the Peccei-Quinn and electroweak scales. The spontaneous breaking of the chiral $U(1)_{PQ}$ triggers the generation of neutrino masses via Type-II seesaw and, at the same time, provides a dynamical solution to the strong CP problem as well as the axion as a dark mat…
If one simultaneously invokes the SUSY solution to the gauge hierarchy problem and the PQ solution to the strong CP problem, then one might expect mixed axion/neutralino dark matter (DM), i.e. two dark matter particles. In this case, dark matter production involves oscillation production of axions, thermal and non-thermal neutralino production and thermal and non-thermal production of axinos and saxions. The latter particles may feed additional neutralinos into the cosmic soup, or dilute all relics via additional entropy production, or feed dark radiation into the cosmos. Depending on the SUSY…
We study the process of primordial black hole (PBH) formation at the beginning of radiation era for the cosmological scenario in which the inflaton is a pseudo-Nambu-Goldstone boson (axion) and there is a coupling of the inflaton with some gauge field. In this model inflation is accompanied by the gauge quanta production and a strong rise of the curvature power spectrum amplitude at small scales (along with non-Gaussianity) is predicted. We show that data on PBH searches can be used for a derivation of essential constraints on the model parameters in such an axion inflation scenario. We compar…
Sterile neutrinos can be generated in the early universe through oscillations with active neutrinos and represent a popular and well-studied candidate for our universe's dark matter. Stringent constraints from X-ray and gamma-ray line searches, however, have excluded the simplest of such models. In this letter, we propose a novel alternative to the standard scenario in which the mixing angle between the sterile and active neutrinos is a dynamical quantity, induced through interactions with a light axion-like field. As the energy density of the axion-like particles is diluted by Hubble expansio…
We survey a few minimal scalar extensions of the standard electroweak model that provide a simple setup for massive neutrinos in connection with an invisible axion. The presence of a chiral $U(1)$ à la Peccei-Quinn drives the pattern of Majorana neutrino masses while providing a dynamical solution to the strong CP problem and an axion as a dark matter candidate. We paradigmatically apply such a renormalizable framework to type-II seesaw and to two viable models for neutrino oscillations where the neutrino masses arise at one and two loops, respectively. We comment on the naturalness of the eff…
We study the production and backreaction of massive vector-like fermions in the background of a classical SU(2) gauge field during axion-driven inflation. We demonstrate all ultraviolet divergences due to the interactions with the fermions can be absorbed by renormalization of the axion wavefunction and the gauge coupling. The effects of the fermion-axion interaction vanish in the massless limit as required by symmetry. For very massive fermions, contact interactions are induced between the axion, the gauge field and the gravitational field. In this massive limit, we find the usual axion-gauge…
Models of natural supersymmetry seek to solve the little hierarchy problem by positing a spectrum of light higgsinos \lesssim 200 GeV and light top squarks \lesssim 500 GeV along with very heavy squarks and TeV-scale gluinos. Such models have low electroweak finetuning and are safe from LHC searches. However, in the context of the MSSM, they predict too low a value of m(h) and the relic density of thermally produced higgsino-like WIMPs falls well below dark matter (DM) measurements. Allowing for high scale soft SUSY breaking Higgs mass m_{H_u}> m_0 leads to natural cancellations during RG runn…
We examine mixed axion/neutralino cold dark matter production in the SUSY DFSZ axion model where an axion superfield couples to Higgs superfields. We calculate a wide array of axino and saxion decay modes along with their decay temperatures, and thermal and non-thermal production rates. For a SUSY benchmark model with a standard underabundance (SUA) of Higgsino-like dark matter (DM), we find for the PQ scale f_a< 10^{12} GeV that the DM abundance is mainly comprised of axions as the saxion/axino decay occurs before the standard neutralino freeze-out and thus its abundance remains suppressed. F…
We apply the axion-photon conversion mechanism to the 130 GeV $γ$-ray line observed by the Fermi satellite. Near the Galactic center, some astrophysical sources and/or particle dark matter can produce energetic axions (or axionlike particles), and the axions convert to $γ$ rays in Galactic magnetic fields along their flight to the Earth. Since continuum $γ$-ray and antiproton productions are sufficiently suppressed in axion production, the scenario fits the 130 GeV $γ$-ray line without conflicting with cosmic ray measurements. We derive the axion production cross section and the decay rate of …
We study the pair production of fermions in a time dependent axion background with and without an electric background. We construct the adiabatic mode functions which incorporate the gauge field and the axion velocity dependence of the dispersion relation. The semiclassical approach using this adiabatic basis shows two types of pair production. One is axion-assisted pair production: the presence of the axion velocity gives enhancement and interference effects on the pair production driven by the electric field. The other is axion-driven pair production: the time variation of the axion velocity…
