Luca Di Luzio, Maurizio Giannotti, Enrico Nardi, Luca Visinelli
We review the landscape of QCD axion models. Theoretical constructions that extend the window for the axion mass and couplings beyond conventional regions are highlighted and classified. Bounds from cosmology, astrophysics and experimental searches are reexamined and updated.
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We propose a landscape of many axions, where the axion potential receives various contributions from shift symmetry breaking effects. We show that the existence of the axion with a super-Planckian decay constant is very common in the axion landscape for a wide range of numbers of axions and shift symmetry breaking terms, because of the accidental alignment of axions. The effective inflation model is either natural or multi-natural inflation in the axion landscape, depending on the number of axions and the shift symmetry breaking terms. The tension between BICEP2 and Planck could be due to smal…
We study the stochastic axion dark matter scenario in the axion landscape, where one of the axions is light and stable and therefore explains dark matter. If the axion mass at the potential is a typical value of the curvature along the direction, the potential can be well approximated by a quadratic mass term. On the other hand, if the axion mass happens to be suppressed in the vicinity of the minimum, the potential may be approximated by a quartic potential plus a suppressed quadratic one, for which the initial angle, and thus the axion abundance, can be significantly suppressed compared to t…
Multiple axions form a landscape in the presence of various shift symmetry breaking terms. Eternal inflation populates the axion landscape, continuously creating new universes by bubble nucleation. Slow-roll inflation takes place after the tunneling event, if a very flat direction with a super-Planckian decay constant arises due to the alignment mechanism. We study the vacuum structure as well as possible inflationary dynamics in the axion landscape scenario, and find that the inflaton dynamics is given by either natural or multi-natural inflation. In the limit of large decay constant, it is a…
We demonstrate that asymmetric reheating arises in a large ensemble of string compactifications with many axions and gauged dark sectors. This phenomenon may help avoid numerous cosmological problems that may arise if the sectors were reheated democratically. Distributions of couplings are presented for two classes of axion reheatons, both of which exhibit very small couplings to most of the gauge sectors. In one class, ratios of reheating couplings and also preferred gauge groups are frequently determined by local regions in the string geometry.
Amnesia in the Axion Misalignment Landscape
该研究通过格点模拟探究暴胀前轴子失准景观的演化，发现meV质量范围存在敏感的“峰顶着陆”解，强自相互作用可使不同早期轴子历史产生相似晚期结构。
If there are a plethora of axions in nature, they may have a complicated potential and create an axion landscape. We study a possibility that one of the axions is so light that it is cosmologically stable, explaining the observed dark matter density. In particular we focus on a case in which two (or more) shift-symmetry breaking terms conspire to make the axion sufficiently light at the potential minimum. In this case the axion has a flat-bottomed potential. In contrast to the case in which a single cosine term dominates the potential, the axion abundance as well as its isocurvature perturbati…
We study in detail MSW-like resonant conversions of QCD axions into hidden axions, including cases where the adiabaticity condition is only marginally satisfied, and where anharmonic effects are non-negligible. When the resonant conversion is efficient, the QCD axion abundance is suppressed by the hidden and QCD axion mass ratio. We find that, when the resonant conversion is incomplete due to a weak violation of the adiabaticity, the CDM isocurvature perturbations can be significantly suppressed, while non-Gaussianity of the isocurvature perturbations generically remain unsuppressed. The isocu…
String theory realisations of the QCD axion are often said to belong to the anthropic window where the decay constant is around the GUT scale and the initial misalignment angle has to be tuned close to zero. In this paper we revisit this statement by studying the statistics of axion physics in the string landscape. We take moduli stabilisation properly into account since the stabilisation of the saxions is crucial to determine the physical properties of the corresponding axionic partners. We focus on the model-independent case of closed string axions in type IIB flux compactifications and find…
We investigate the phenomenology of QCD axions and axionlike particles in a scenario with two eras of inflation. In particular, we describe the possible solutions for the QCD axion field equation after the second inflation and reheating. We calculate the dilution numerically for QCD axions and give an analytic approximation for axionlike particles. While it has been realised before that such a scenario can dilute the axion energy density and open up the parameter space for the axion decay constant $f_A$, we find that even a small increase in the relative QCD axion energy density is possible.
Hidden axions may be coupled to the standard model particles through a kinetic or mass mixing with QCD axion. We study a scenario in which a hidden axion constitutes a part of or the whole of dark matter and decays into photons through the mixing, explaining the 3.5 keV X-ray line signal. Interestingly, the required long lifetime of the hidden axion dark matter can be realized for the QCD axion decay constant at an intermediate scale, if the mixing is sufficiently small. In such a two component dark matter scenario, the primordial density perturbations of the hidden axion can be highly non-Gau…
QCD axion bubbles in the presence of ALP resonant conversion
研究ALP共振转换对QCD轴子气泡形成的影响，发现ALP能量转移可抑制背景温度，进而影响原初黑洞形成。
The QCD axion's coupling to photons is often assumed to lie in a narrow band as a function of the axion mass. We demonstrate that several simple mechanisms, in addition to the photophilic clockwork axion already in the literature, can significantly extend the allowed range of couplings. Some mechanisms we present generalize the KNP alignment scenario, widely studied as a model of inflation, to the phenomenology of a QCD axion. In particular we present KSVZ-like realizations of two-axion KNP alignment and of the clockwork mechanism. Such a "confinement tower" realization of clockwork may prove …
We propose a new mechanism to suppress the axion isocurvature perturbation, while producing the right amount of axion dark matter, within the framework of supersymmetric axion models with the axion scale induced by supersymmetry breaking. The mechanism involves an intermediate phase transition to generate the Higgs μ-parameter, before which the weak scale is comparable to the axion scale and the resulting stronger QCD yields an axion mass heavier than the Hubble scale over a certain period. Combined with that the Hubble-induced axion scale during the primordial inflation is well above the inte…
We discuss the idea that the model-independent (MI) axion of string theory is the source of quintessential dark energy. The scenario is completed with a composite QCD axion from hidden sector squark condensation that could serve as dark matter candidate. The mechanism relies on the fact that the hidden sector anomaly contribution to the composite axion is much smaller than the QCD anomaly term. This intuitively surprising scenario is based on the fact that below the hidden sector scale $Λ_h$ there are many light hidden sector quarks. Simply, by counting engineering dimensions the hidden sector…
Searching for the QCD Dark Matter Axion
本文回顾QCD轴子（冷暗物质候选者）的天体物理搜索、理论进展及历史，介绍其搜索现状、未来前景及正在开发的探测技术。
We study a level crossing between the QCD axion and an axion-like particle, focusing on the recently found phenomenon, the axion roulette, where the axion-like particle runs along the potential, passing through many crests and troughs, until it gets trapped in one of the potential minima. We perform detailed numerical calculations to determine the parameter space where the axion roulette takes place, and as a result domain walls are likely formed. The domain wall network without cosmic strings is practically stable, and it is nothing but a cosmological disaster. In a certain case, one can make…
We study the cosmological evolution of the QCD axion coupled to hidden photons. For a moderately strong coupling, the motion of the axion field leads to an explosive production of hidden photons by tachyonic instability. We use lattice simulations to evaluate the cosmological abundance of the QCD axion. In doing so, we incorporate the backreaction of the produced hidden photons on the axion dynamics, which becomes significant in the non-linear regime. We find that the axion abundance is suppressed by at most ${\cal O}(10^{2})$ for the decay constant $f_a = 10^{16}$ GeV, compared to the case wi…
Cosmic strings and domain walls of the QCD quark condensate with and without a hidden axion
本文研究QCD夸克凝聚体形成的轴子型弦-壁系统，指出其独立于隐藏轴子存在并可能主导早期宇宙演化，同时分析了费米子零模、反常流入及其在QCD相变和重离子物理中的意义。
Discrete symmetries allowed in string compactification are the mother of all global symmetries which are broken at some level. We discuss the resulting pseudo-Goldstone bosons, in particular the QCD axion and a temporary cosmological constant, and inflatons. We also comment on some implications of the recent BICEP2 data.
QCD Axion Hybrid Inflation
本研究提出QCD轴子通过与重轴子混合充当暴胀子的混合暴胀场景，发现QCD轴子初始失配角由暴胀动力学决定且接近π，实现了π位移暴胀，表明QCD轴子暗物质与暴胀密切相关，重轴子或可由未来加速器实验探测。
Among solutions of the strong CP problem, the "invisible" axion in the narrow axion window is argued to be the remaining possibility among natural solutions on the smallness of $\barθ$. Related to the gravity spoil of global symmetries, some prospective invisible axions from theory point of view are discussed. In all these discussions, including the observational possibility, cosmological constraints must be included.
The QCD axion acquires the potential through the non-perturbative effect of the QCD matters around the QCD phase transition. During this period, the direct interaction between the axion and the QCD matters sets in. Focusing on the impact of this direct interaction, we propose two scenarios where the fluctuation of the axion can rapidly grow, potentially leading to the formation of axion miniclusters even if the Peccei-Quinn (PQ) symmetry was already broken during inflation. The first scenario assumes that the primordial curvature perturbation at the horizon scale during the QCD epoch was signi…
The axion solution to the strong CP problem is delicately sensitive to Peccei-Quinn breaking contributions that are misaligned with respect to QCD instantons. Heavy QCD axion models are appealing because they avoid this so-called "quality problem". We show that generic realizations of this framework can be probed by the LIGO-Virgo-KAGRA interferometers, through the stochastic gravitational wave (GW) signal sourced by the long-lived axionic string-domain wall network, and by upcoming measurements of the neutron and proton Electric Dipole Moments. Additionally, we provide predictions for searche…
We construct models in which the SM Higgs mass scans in a landscape. This is achieved by coupling the SM to a monodromy axion field through Minkowski 3-forms. The Higgs mass scans with steps given by delta(m_H^2)= eta mu f, where mu and f are the axion mass and periodicity respectively, and eta measures the coupling of the Higgs to the associated 3-form. The observed Higgs mass scale could then be selected on anthropic grounds. The monodromy axion may have a mass mu in a very wide range depending on the value of eta, and the axion periodity f. For eta=1 and f = 10^10 GeV, one has 10^{-3}eV < m…
A previously discussed clockwork mechanism within heterotic M-theory is applied to its axion landscape. We identify a unique candidate for a QCD-axion with a decay constant in the preferred "axion window" around $10^{11}\,$GeV. It is accompanied by at least one ultra-light axion that couples predominantly to hidden sector gauge groups.
The serious cosmological problems created by the axion-string/axion-domain-wall system in standard axion models are alleviated by positing the existence of a new confining force. The instantons of this force can generate an axion potential that erases the axion strings long before QCD effects become important, thus preventing QCD-generated axion walls from ever appearing. Axion walls generated by the new confining force would decay so early as not to contribute significantly to the energy in axion dark matter.
Induced Domain Walls of QCD Axion, and Gravitational Waves
提出重轴子与QCD轴子混合产生感应畴壁的新机制，解释暗物质并可能对应NANOGrav观测的引力波背景。
The recent BICEP2 result shows that the Universe once has gone through the vacuum with the GUT scale energy density. The implied high scale inflation nullifies the dilution idea of topological defects, strings and domain walls, of the axionic system. In particular, domain walls are disastrous if the domain wall problem with domain-wall number greater than 1 is present. We argue that the model-independent axion in string compactification with the anomalous U(1) gauge symmetry resolves the domain wall problem naturally with a symmetry principle.
Implications of Protecting the QCD Axion in the Dual Description
本文在对偶2-形式场描述下研究QCD轴子质量保护，提出用多轴子耦合任意多3-形式，并讨论其对亚eV ALP搜寻、大衰变常数及宇宙双折射的唯象意义。
We study the mirror world with dark matter arising from the thermal freeze-out of the lightest, stable mirror particle -- the mirror electron. The dark matter abundance is achieved for mirror electrons of mass 225 GeV, fixing the mirror electroweak scale near $10^8$ GeV. This highly predictive scenario is realized by an axion that acts as a portal between the two sectors through its coupling to the QCD and mirror QCD sectors. The axion is more massive than the standard QCD axion due to additional contributions from mirror strong dynamics. Still, the strong CP problem is solved by this "heavy" …
Taken individually, magnetic monopoles and axions are both well-motivated aspects of physics beyond the Standard Model. We demonstrate that by virtue of the Witten effect, their interplay is furthermore nontrivial; monopoles break the corresponding axial symmetries explicitly and result in an axion potential lacking the usual instanton-derived exponential suppression factor. As axions are distinguished by their extremely weak interactions, any regime where their effects are enhanced may be of interest. As an application of these findings, we demonstrate that a phenomenologically acceptable pop…
Assuming QCD exhibits an interacting fixed-point behaviour in the ultraviolet regime, I argue that the axion can be substantially heavier than in the conventional case of asymptotically free QCD due to the enhanced contribution of small size instantons to its mass.
In this paper, we consider a possibility that the temperature anisotropy of cosmic microwave background (CMB) is dominantly generated by the primordial fluctuations of QCD-axion like particles under a circumstance that inflaton's perturbation is too small to explain the CMB anisotropy. Since the axion potential is generated by acquiring its energy from radiation, the primordial fluctuations of the axion field generated in the inflation era are correlated with the CMB anisotropies. Consequently, the observations stringently constrain a model of the axion and the early universe scenario. The fol…
Inching toward the QCD Axions with Axion Magnetic Resonance in Helioscopes
该研究将轴子-磁共振（AMR）机制从激光实验拓展至轴子太阳望远镜，应用于CAST实验遗留数据并对IAXO实验做预测，显著提升了对QCD轴子等的探测灵敏度。
The QCD axion is one of the most appealing candidates for the dark matter in the Universe. In this article, we discuss the possibility to predict the axion mass in the context of a simple renormalizable grand unified theory where the Peccei-Quinn scale is determined by the unification scale. In this framework, the axion mass is predicted to be in the range $m_a \simeq (3 - 13) \times 10^{-9} \ \rm{eV}$. We study the axion phenomenology and find that the ABRACADABRA and CASPEr-Electric experiments will be able to fully probe this mass window.
String theoretic axion is a prime candidate for the QCD axion solving the strong CP problem. For a successful realization of the QCD axion in string theory, one needs to stabilize moduli including the scalar partner (saxion) of the QCD axion, while keeping the QCD axion unfixed until the low energy QCD instanton effects are turned on. We note that a simple generalization of KKLT moduli stabilization provides such set-up realizing the axion solution to the strong CP problem. Although some details of moduli stabilization are different from the original KKLT scenario, this set-up leads to the mir…
We discuss inflation models within supersymmetry and supergravity frameworks with a landscape of chiral superfields and one $U(1)$ shift symmetry which is broken by non-perturbative symmetry breaking terms in the superpotential. We label the pseudo scalar component of the chiral fields axions and their real parts saxions. Thus in the models only one combination of axions will be a pseudo-Nambu-Goldstone-boson which will act as the inflaton. The proposed models constitute consistent inflation for the following reasons: The inflation potential arises dynamically with stabilized saxions, the axio…
Mass Mixing between QCD Axions
本文研究了规范QCD轴子与$Z_{\mathcal N}$轴子质量混合中的能级交叉现象，揭示了其发生条件取决于衰变常数之比约1.69，并估算了QCD轴子暗物质遗迹密度及其宇宙学意义。
String allowed discrete symmetry is the mother of acceptable effective global symmetries at low energy. With this philosophy, we discuss dark energy, QCD axion, and inflation, and speculate some implications of the recent BICEP2 data.
We explore whether the axion which solves the strong CP problem can naturally be much lighter than the canonical QCD axion. The $Z_\mathcal{N}$ symmetry proposed by Hook, with $\mathcal{N}$ mirror and degenerate worlds coexisting in Nature and linked by the axion field, is considered in terms of generic effective axion couplings. We show that the total potential is safely approximated by a single cosine in the large $\mathcal{N}$ limit, and we determine the analytical formula for the exponentially suppressed axion mass. The resulting universal enhancement of all axion interactions relative to …
Heavy QCD axions are well-motivated extensions of the QCD axion that address the quality problem while still solving the strong CP problem. Owing to the gluon coupling, critical for solving the strong CP problem, these axions can be produced in significant numbers in beam dump and collider environments for axion decay constants as large as PeV, relevant for addressing the axion quality problem. In addition, if these axions have leptonic couplings, they can give rise to long-lived decay into lepton pairs, in particular, dominantly into muons above the dimuon threshold and below the GeV scale in…
A non-negligible fraction of the QCD axion dark matter may form gravitationally bound Bose Einstein condensates, which are commonly known as axion stars or axion clumps. Such astrophysical objects have been recently proposed as the cause for the single candidate event reported by Subaru Hyper Suprime-Cam (HSC) microlensing search in the Andromeda galaxy. Depending on the breaking scale of the Peccei-Quinn symmetry and the details of the dark matter scenario, QCD axion clumps may form via gravitational condensation during radiation domination, in the dense core of axion miniclusters, or within …
Heavy QCD axion dark matter from avoided level crossing
本文提出QCD轴子与惰性轴子混合时，其质量本征态的避免能级交叉可将能量密度几乎完全转移给QCD轴子，从而在更高达meV至eV量级的质量范围内自然产生全部暗物质，为未来直接探测实验提供理论依据。
Toward the Effective Light and Heavy QCD Axion Scenarios
本文研究了轻重QCD轴子情景中轴子质量与衰变常数的参数空间，发现质量比相似而衰变常数比相反，并推广到多轴子情况。
While axions seem ubiquitous in critical string theories, whether they might survive in any string theoretic description of nature is a difficult question. With some mild assumptions, one can frame the issues in the case that there is an approximate supersymmetry below the underlying string scale. The problem of axions is then closely tied to the question of how moduli are fixed. We consider, from this viewpoint, the possibility that supersymmetry is broken at an intermediate scale, as in "gravity mediation," at a low scale, as in gauge mediation, and at a very high scale, to model the possibi…
Resolving the QCD Axion Domain Wall Problem with a Light Axion
通过引入轻轴子，解决QCD轴子的域壁问题，同时保持Peccei-Quinn机制有效，避免强CP问题的重现。
The QCD axion is one of the most compelling solutions of the strong CP problem. There are major current efforts into searching for an ultralight, invisible axion, which is believed to be the only phenomenologically viable realization of the QCD axion. Visible axions with decay constants at or below the electroweak scale are believed to have been long excluded by laboratory searches. Considering the significance of the axion solution of the strong CP problem, we revisit experimental constraints on QCD axions in the O(10 MeV) mass window. In particular, we find a variant axion model that remains…
Can QCD Axions Survive the Cosmological Constant Problem?
本文探讨了真空能松弛机制对QCD轴子的影响，发现其真空势被抑制，导致轴子行为改变，可能违反观测约束。
The QCD axion couplings of various invisible axion models are presented. In particular, the exact global symmetry U(1)$_{\rm PQ}$ in the superpotential is possible for the anomalous U(1) from string compactification, broken only by the gauge anomalies at one loop level, and is shown to have the resultant invisible axion coupling to photon, $c_{aγγ}\ge \frac83-c_{aγγ}^{\rm ch\,br}$ where $c_{aγγ}^{\rm ch\,br}\simeq 2$. However, this bound is not applicable in approximate U(1)$_{\rm PQ}$ models with sufficiently suppressed U(1)$_{\rm PQ}$-breaking superpotential terms. We also present a simple m…
We present a model where the QCD axion is at the TeV scale and visible at a collider via its decays. Conformal dynamics and strong CP considerations account for the axion coupling strongly enough to the standard model to be produced as well as the coincidence between the weak scale and the axion mass. The model predicts additional pseudoscalar color octets whose properties are completely determined by the axion properties rendering the theory testable.
Pre-inflationary QCD axion stars after moduli domination
大爆炸前QCD轴子暗物质在早期物质主导时期经历绝热密度扰动，产生约1个量级的扰动，这些扰动在量子 Jeans 尺寸处坍缩成轴子星，可能包含50%的暗物质。
In the QCD axion scenario, a network of domain walls bounded by cosmic strings fragments into pieces. As these fragments collapse, some of them will form black holes. With standard QCD axion parameters, the black holes will have lunar masses ($M_{\rm bh} \sim 10^{-8}\, {\rm M}_\odot$). Even though their number density is difficult to estimate, arguments suggest that they can constitute a reasonable fraction of the critical cosmological density.
We show that the initial misalignment angle of the QCD axion (or axion-like particles) can be set very close to $π$, if the QCD axion has a mixing with another heavy axion which induces the phase shift $\approx π$ after inflation. In the simplest case, the heavy axion plays the role of the inflaton, and we call such inflation as "$π$nflation." The basic idea was first proposed by Daido and the present authors in Ref. [1702.03284] in 2017 and more recently discussed in Ref. [1903.00462]. We show that the QCD axion with a decay constant $f_a \gtrsim 3 \times 10^9\,$ GeV can explain dark matter b…
The QCD Axion: A Unique Player in the Axiverse with Mixings
本文研究轴子宇宙中QCD轴子与其他轴子混合时的动力学，发现其丰度增强可拓宽暗物质参数区域，并可能形成多组分暗物质。
We revisit the adiabatic conversion between the QCD axion and axion-like particle (ALP) at level crossing, which can occur in the early universe as a result of the existence of a hypothetical mass mixing. This is similar to the Mikheyev-Smirnov-Wolfenstein effect in neutrino oscillations. After refining the conditions for the adiabatic conversion to occur, we focus on a scenario where the ALP produced by the adiabatic conversion of the QCD axion explains the observed dark matter abundance. Interestingly, we find that the ALP decay constant can be much smaller than the ordinary case in which th…
Quantum Gravity Cutoff from Axions: A Type IIB Landscape Study
研究通过IIB型弦论Calabi-Yau紧化中C2和C4轴子的定量分析，验证了量子引力截断与轴子衰变常数及瞬子作用量之间的界限。
We find that the adiabatic fluctuations produced in the primordial plasma by cosmological inflation resonantly excite the axion field during the QCD phase transition by pumping axions from low momentum modes to modes with momentum up to of order $\sqrt{3} m$ where $m$ is the axion mass. We derive the momentum distribution of the excited axions. The fraction of cold axions that get excited is of order one if the axion mass is larger than a few $μ$eV. The effect occurs whether inflation happens before or after the Peccei-Quinn phase transition.
We propose a QCD axion model where the origin of PQ symmetry and suppression of axion isocurvature perturbations are explained by introducing an extra dimension. Each extra quark-antiquark pair lives on branes separately to suppress PQ breaking operators. The size of the extra dimension changes after inflation due to an interaction between inflaton and a bulk scalar field, which implies that the PQ symmetry can be drastically broken during inflation to suppress undesirable axion isocurvature fluctuations.
A non-compact QCD axion
非紧致QCD轴子通过产生域墙并引发引力波背景，解释暗物质并规避传统宇宙学约束。
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 $f_a = 10^{17}…
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