Axion Decay Research Articles

SU(5)×U(1)X axion model with observable proton decay

We propose a $SU(5) \times U(1)_X \times U(1)_{PQ}$ model, where $U(1)_X$ is the generalization of the $B-L$ (baryon minus lepton number) gauge symmetry and $U(1)_{PQ}$ is the global Peccei-Quinn (PQ) symmetry. There are four fermions families in $\bf{{\overline 5}} + \bf{10}$ representations of $SU(5)$, a mirror family in $\bf{5}+\bf{{\overline {10}}}$ representations, and three $SU(5)$ singlet Majorana fermions. The $U(1)_X$ related anomalies all cancel in the presence of the Majorana neutrinos. The $SU(5)$ symmetry is broken at $M_{GUT} \simeq (6-9)\times 10^{15}$ GeV and the proton lifetime $\tau_p$ is estimated to be well within the expected sensitivity of the future Hyper-Kamiokande experiment, $\tau_p \lesssim 1.3 \times 10^{35}$ years. The $SU(5)$ breaking also triggers the breaking of the PQ symmetry, resulting in axion dark matter (DM), with the axion decay constant $f_a$ of order $M_{GUT}$ or somewhat larger. The CASPEr experiment can search for such an axion DM candidate. The Hubble parameter during inflation must be low, $H_{inf} \lesssim 10^9 $ GeV, in order to successfully resolve the axion domain wall, axion DM isocurvature and $SU(5)$ monopole problems. With the identification of the $U(1)_X$ breaking Higgs field with the inflaton field, we implement inflection-point inflation, which is capable of realizing the desired value for $H_{inf}$. The vectorlike fermions in the model are essential for achieving successful unification of the SM gauge couplings as well as the phenomenological viability of both axion DM and inflation scenario.

Axion–Sterile Neutrino Dark Matter

Extending the standard model with three right-handed neutrinos and a simple QCD axion sector can account for neutrino oscillations, dark matter and baryon asymmetry; at the same time, it solves the strong CP problem, stabilizes the electroweak vacuum and can implement critical Higgs inflation (satisfying all current observational bounds). We perform here a general analysis of dark matter (DM) in such a model, which we call the aνMSM. Although critical Higgs inflation features a (quasi) inflection point of the inflaton potential, we show that DM cannot receive a contribution from primordial black holes in the aνMSM. This leads to a multicomponent axion–sterile neutrino DM and allows us to relate the axion parameters, such as the axion decay constant, to the neutrino parameters. We include several DM production mechanisms: the axion production via misalignment and decay of topological defects as well as the sterile neutrino production through the resonant and non-resonant mechanisms and in the recently proposed CPT-symmetric universe.

Challenges for an axion explanation of the muon g \u2212 2 measurement

The discrepancy between the muon g − 2 measurement and the Standard Model prediction points to new physics around or below the weak scale. It is tantalizing to consider the loop effects of a heavy axion (in the general sense, also known as an axion-like particle) coupling to leptons and photons as an explanation for this discrepancy. We provide an updated analysis of the necessary couplings, including two-loop contributions, and find that the new physics operators point to an axion decay constant on the order of 10s of GeV. This poses major problems for such an explanation, as the axion couplings to leptons and photons must be generated at low scales. We outline some possibilities for how such couplings can arise, and find that these scenarios predict new charged matter at or below the weak scale and new scalars can mix with the Higgs boson, raising numerous phenomenological challenges. These scenarios also all predict additional contributions to the muon g−2 itself, calling the initial application of the axion effective theory into question. We conclude that there is little reason to favor an axion explanation of the muon g – 2 measurement relative to other models postulating new weak-scale matter.

The stochastic axiverse

In addition to spectacular signatures such as black hole superradiance and the rotation of CMB polarization, the plenitude of axions appearing in the string axiverse may have potentially dangerous implications. An example is the cosmological overproduction of relic axions and moduli by the misalignment mechanism, more pronounced in regions where the signals mentioned above may be observable, that is for large axion decay constant. In this work, we study the minimal requirements to soften this problem and show that the fundamental requirement is a long period of low-scale inflation. However, in this case, if the inflationary Hubble scale is lower than around O(100) eV, no relic DM axion is produced in the early Universe. Cosmological production of some axions may be activated, via the misalignment mechanism, if their potential minimum changes between inflation and today. As a particular example, we study in detail how the maximal-misalignment mechanism dilutes the effect of dangerous axions and allows the production of axion DM in a controlled way. In this case, the potential of the axion that realises the mechanism shifts by a factor ∆θ = π between the inflationary epoch and today, and the axion starts to oscillate from the top of its potential. We also show that axions with masses ma ∼ O(1 − 100) H0 realising the maximal-misalignment mechanism generically behave as dark energy with a decay constant that can take values well below the Planck scale, avoiding problems associated to super-Planckian scales. Finally, we briefly study the basic phenomenological implications of the mechanism and comment on the compatibility of this type of maximally-misaligned quintessence with the swampland criteria.

Revisiting isocurvature bounds in models unifying the axion with the inflaton

Axion scenarios in which the spontaneous breaking of the Peccei-Quinn symmetry takes place before or during inflation, and in which axion dark matter arises from the misalignment mechanism, can be constrained by Cosmic Microwave Background isocurvature bounds. Dark matter isocurvature is thought to be suppressed in models with axion-inflaton interactions, for which axion perturbations are assumed to freeze at horizon crossing during inflation. However, this assumption can be an oversimplification due to the interactions themselves. In particular, non-perturbative effects during reheating may lead to a dramatic growth of axion perturbations. We perform lattice calculations in two models in which the Peccei-Quinn field participates in inflation. We find that the growth of axion perturbations is such that the Peccei-Quinn symmetry is restored for an axion decay constant f A ≲ 1016–1017 GeV, leading to an over-abundance of dark matter, unless f A ≲ 2 × 1011 GeV. For f A ≳ 1016–1017 GeV we still find a large growth of axion perturbations at low momentum, such that a naive extrapolation to CMB scales suggests a violation of the isocurvature bounds.

Impact of axions on the Cassiopea A neutron star cooling

The observed anomalous steady decrease in surface temperature of the supernova remnant Cassiopeia A (Cas A), which was reported about ten years ago, has generated much debate. Several exotic cooling scenarios have been proposed using non-standard assumptions about the physics and evolution of this neutron star (NS). At present, significant corrections have been made to the observational data, which make it possible to numerically simulate the Cas A NS cooling process in the framework of the scenario of minimal neutrino cooling. If there is an additional source of cooling, such as axion emission, the steepness of the Cas A NS surface temperature drop will increase with the growth of the axion-nucleon interaction strength. This makes it possible to limit the minimum value of the axion decay constant f a using the condition that the NS surface temperature should be within the 99% confidence interval obtained from the observational data. Two types of axion models are considered: the Kim-Shifman-Weinstein-Zakharov — KSVZ model and the Dean-Fischler-Srednitsky-Zhitnitsky –DFSZ model. The above criterion gives a lower limit on the axion decay constant, f a > 3 × 107 GeV and f a > 4.5 × 108 GeV for KSVZ and DFSZ axions, respectively.

Constraints on axionic fuzzy dark matter from light bending and Shapiro time delay

Ultralight axion like particles (ALPs) of mass m_a∈ (10-21 eV–10-22 eV) with axion decay constant fa∼ 1017 GeV can be candidates for fuzzy dark matter (FDM). If celestial bodies like Earth and Sun are immersed in a low mass axionic FDM potential and if the ALPs have coupling with nucleons then the coherent oscillation of the axionic field results a long range axion hair outside of the celestial bodies. The range of the axion mediated Yukawa type fifth force is determined by the distance between the Earth and the Sun which fixes the upper bound of the mass of axion as ma≲10-18 eV. The long range axionic Yukawa potential between the Earth and Sun changes the gravitational potential between them and contribute to the light bending and the Shapiro time delay. From the observational uncertainties of those experiments, we obtain an upper bound on the axion decay constant as fa≲ 9.85× 106 GeV, which is the stronger bound obtained from Shapiro time delay. This implies if ALPs are FDM, then they do not couple to quarks.

Precision axion physics with running axion couplings

We study the renormalization group running of axion couplings while taking into account that the Standard Model can be extended to its supersymmetric extension at a certain energy scale below the axion decay constant. We then apply our results to three different classes of axion models, i.e. KSVZ-like, DFSZ-like, and string-theoretic axions, and examine if string-theoretic axions can be distinguished from others by having a different pattern of low energy couplings to the photon, nucleons and electron. We find that the low energy couplings of string-theoretic axions have a similar pattern as those of KSVZ-like axions but yet reveal a sizable difference which might be testable in future axion search experiments. We also note that the coupling of KSVZ-like QCD axions to the electron is dominated by a three-loop contribution involving the exotic heavy quark, gluons, top quark and Higgs field.

Strong CP problem and axion dark matter with small instantons

The axion mass receives a large correction from small instantons if the QCD gets strongly coupled at high energies. We discuss the size of the new CP violating phases caused by the fact that the small instantons are sensitive to the UV physics. We also discuss the effects of the mass correction on the axion abundance of the Universe. Taking the small-instanton contributions into account, we propose a natural scenario of axion dark matter where the axion decay constant is as large as 1015-16 GeV. The scenario works in the high-scale inflation models.

Photophilic hadronic axion from heavy magnetic monopoles

We propose a model for the QCD axion which is realized through a coupling of the Peccei-Quinn scalar field to magnetically charged fermions at high energies. We show that the axion of this model solves the strong CP problem and then integrate out heavy magnetic monopoles using the Schwinger proper time method. We find that the model discussed yields axion couplings to the Standard Model which are drastically different from the ones calculated within the KSVZ/DFSZ-type models, so that large part of the corresponding parameter space can be probed by various projected experiments. Moreover, the axion we introduce is consistent with the astrophysical hints suggested both by anomalous TeV-transparency of the Universe and by excessive cooling of horizontal branch stars in globular clusters. We argue that the leading term for the cosmic axion abundance is not changed compared to the conventional pre-inflationary QCD axion case for axion decay constant fa> 1012 GeV.

Properties of ultralight bosons from heavy quasar spins via superradiance

The mass and the spin of accreting and jetted black holes, at the center of Active Galactic Nuclei (AGNs), can be probed by analyzing their electromagnetic spectra. For this purpose, we use the Spin-Modified Fundamental Plane of black hole activity, which non-linearly connects the following four variables (in the source frame): radio luminosity, X-ray or optical luminosity (via the [OIII] emission line), black hole mass and spin. Taking into account the uncertainties in luminosity measurements, conversion factors, relativistic beaming and physical properties of the AGN system, we derive lower bounds on the spins of a group of heavy, jetted AGNs. Using these results, we study the direct implications on the mass spectrum of the ultra-light particles of scalar (axion-like), vector (dark photon) and tensor types (additional spin-2 particles). We close unexplored gap in the parameter space 10-20-10-19eV. We obtain upper bounds on the axion decay constant (equivalently lower bounds on the self-interaction strength) considering self-interactions could prevent the axion particles entering the instability, and be the reason for non-observation of superradiance. Assuming axion is described by mass and decay constant, we obtain upper limits on what fraction of dark matter can be formed by ultra-light particles and find that single spieces axion-like light particle can constitute at most 10% of the dark matter in the mass range: 10-21 < μ (eV) < 10-17.

Lepto-axiogenesis

We propose a baryogenenesis mechanism that uses a rotating condensate of a Peccei-Quinn (PQ) symmetry breaking field and the dimension-five operator that gives Majorana neutrino masses. The rotation induces charge asymmetries for the Higgs boson and for lepton chirality through sphaleron processes and Yukawa interactions. The dimension-five interaction transfers these asymmetries to the lepton asymmetry, which in turn is transferred into the baryon asymmetry through the electroweak sphaleron process. QCD axion dark matter can be simultaneously produced by dynamics of the same PQ field via kinetic misalignment or parametric resonance, favoring an axion decay constant fa ≲ 1010 GeV, or by conventional misalignment and contributions from strings and domain walls with fa ∼ 1011 GeV. The size of the baryon asymmetry is tied to the mass of the PQ field. In simple supersymmetric theories, it is independent of UV parameters and predicts the supersymmtry breaking mass scale to be mathcal{O} (10 − 104) TeV, depending on the masses of the neutrinos and whether the condensate is thermalized during a radiation or matter dominated era. The high supersymmetry breaking mass scale may be free from cosmological and flavor/CP problems. We also construct a theory where TeV scale supersymmetry is possible. Parametric resonance may give warm axions, and the radial component of the PQ field may give signals in rare kaon decays from mixing with the Higgs and in dark radiation.

Impact of helical electromagnetic fields on the axion window

Primordial electromagnetic fields can strongly affect the cosmic evolution of axions, and vice versa. We show that if helical electromagnetic fields are coherently produced in the early universe, their remnants source a field velocity to the coupled axions and enhance the relic abundance of axion dark matter. We discuss the implications for the QCD axion and axion-like particles that are coupled to the SM or hidden gauge groups. For a QCD axion coupled to hidden photons, we find that the conventional window for the axion decay constant 108 GeV≲ f ≲ 1012 GeV can be completely closed due to overproduction of axion dark matter by helical electromagnetic fields as little as α Δ Neff≳ 10-12, where α is the gauge coupling and Δ N_eff is the effective extra relativistic degrees of freedom of the hidden photons.

Power spectrum of density perturbations in chain inflation

Chain Inflation is an alternative to slow roll inflation in which the universe undergoes a series of transitions between different vacua. The density perturbations (studied in this paper) are seeded by the probabilistic nature of tunneling rather than quantum fluctuations of the inflaton. We find the scalar power spectrum of chain inflation and show that it is fully consistent with a $\Lambda$CDM cosmology. In agreement with some of the previous literature (and disagreement with others), we show that $10^4$ phase transitions per e-fold are required in order to agree with the amplitude of Cosmic Microwave Background anisotropies within the observed range of scales. Interestingly, the amplitude of perturbations constrains chain inflation to a regime of highly unstable de Sitter spaces, which may be favorable from a quantum gravity perspective since the Swampland Conjecture on Trans-Planckian Censorship is automatically satisfied. We provide new analytic estimates for the bounce action and the tunneling rate in periodic potentials which replace the thin-wall approximation in the regime of fast tunneling. Finally, we study model implications and derive an upper limit of $\sim 10^{10}$ GeV on the axion decay constant in viable chain inflation with axions.

Remarks on axion-electrodynamics

Abstract We propose a simple generalization of axion-electrodynamics (A-ED) for the general case in which both scalar and pseudoscalar axion-like fields are present in the (scalar) Lagrangian of the system. We make some remarks on the problem of finding solutions to the differential equations of motion characterizing the propagation of coupled axion fields and electromagnetic (EM) waves. Our primary goal (which is not explored here) is to understand and predict novel phenomena that have no counterpart in pseudoscalar A-ED. With this end in view, we discuss on very general grounds possible processes related to scalar (and pseudoscalar) axions, e.g., the Primakoff effect; the Compton scattering; and, notably, the EM two-photon axion decay.

Ultralight DM bosons with an axion-like potential: scale-dependent constraints revisited

A scalar field ϕ endowed with a trigonometric potential has been proposed to play the role of Dark Matter. A deep study of the cosmological evolution of linear perturbations, and its comparison to the Cold Dark Matter (CDM) and Fuzzy Dark Matter (FDM) cases (scalar field with quadratic potential), reveals an enhancement in the amplitude of the mass power spectrum for large wave numbers due to the non-linearity of the axion-like potential. For the first time, we study the scale-dependence on physical quantities such as the growth factor Dk, the velocity growth factor fk, and fk σ8. We found that for z<10, all these quantities recover the CDM evolution, whereas for high redshift there is a clear distinction between each model (FDM case, and axion-like potential) depending on the wavenumber k and on the decay parameter of the axion-like potential as well. A semi-analytical Halo Mass Function is also revisited, finding a suppression of the number of low mass halos, as in the FDM case, but with a small increment in the amplitude of the variance and halo mass function due to the non-linearity of the axion-like potential. Finally, we present constraints on the axion mass and the axion decay parameter by using data of the Planck Collaboration 2018 and Lyman-α forest.

Goldstone Boson Decays and Chiral Anomalies

Martinus Veltman was the first to point out the inconsistency of the experimental value for the decay rate of $\pi^0\rightarrow\gamma\gamma$ and its calculation by J. Steinberger with the very successful concept of the pion as the (pseudo)Nambu-Goldstone boson of the spontaneously broken global axial symmetry of strong interactions. That inconsistency has been resolved by J. Bell and R. Jackiw in their famous paper on the chiral anomalies. We review the connection between the decay amplitudes of an axion into two gauge bosons in Abelian vector-like and chiral gauge theories. The axion is the Nambu-Goldstone boson of a spontaneously broken axial global symmetry of the theory. Similarly as for the vector-like gauge theory, also in the chiral one the axion decay amplitude is determined by the anomaly of the current of the axial symmetry in its non-linear realization. Certain subtlety in the calculation of the anomaly in chiral gauge theories is emphasised.

Photon directional profile from stimulated decay of axion clouds with nonspherical axion spatial distributions

We model clusters of axions with spherically symmetric momentum but arbitrary spatial distributions and study the directional profile of photos produced in their evolution through spontaneous and stimulated decay of axions via the process $a \rightarrow \gamma + \gamma$. Several specific examples are presented.

Primordial black holes from QCD axion bubbles

We propose a scenario in which a strong Peccei-Quinn (PQ) symmetry breaking in the early universe results in large inhomogeneities of the initial QCD axion field value, leading to the formation of very dense axion bubbles. Some of the axion bubbles subsequently collapse into primordial black holes (PBHs). The spatially homogeneous part of the QCD axion explains dark matter of the universe, while the PBHs arising from the axion bubbles can explain the LIGO events or the seed of supermassive black holes. Interestingly, the mass of PBH is determined by the axion decay constant; for fa = 1017 (1016) GeV, the PBH mass is heavier than about 10 (104) M⊙. In addition, axion miniclusters are also formed from the axion bubbles more abundantly than PBHs, and their masses are expected to be heavier than in the usual scenario based on the spontaneous breaking of the PQ symmetry after inflation.

Probing the angle of birefringence due to long range axion hair from pulsars

Rotating neutron star or pulsar can be a possible source of pseudo Nambu Goldstone bosons or axions which can mediate long range axionic hair outside of the pulsar. When the electromagnetic radiation is emitted from pulsar and passes through the long range axion hair, the axion rotates the polarization of the electromagnetic radiation and produces birefringence. We obtain the angle of birefringence due to this long range axionic hair as $0.42^\circ$. This result is independent of the rotational frequency, radius of the pulsar, mass of the axion, and axion photon coupling constant. This value is within the accuracy of measuring the linear polarization angle of pulsar light which is $\leq 1.0^\circ$. Our result continues to hold as long as the range of the axion hair (inverse of axion mass) is greater than the radius of the pulsar, i.e; $m_a<10^{-11}$eV and the axion decay constant $f_a\lesssim \mathcal{O}(10^{17}\rm{GeV})$.