The QCD axion fails to solve the strong CP problem unless all explicit PQ violating, Planck-suppressed, dimension n<10 operators are forbidden or have exponentially small coefficients. We show that all theories with a QCD axion contain an irreducible source of explicit PQ violation which is proportional to the determinant of the Yukawa interaction matrix of colored fermions. Generically, this contribution is of low operator dimension and will drastically destabilize the axion potential, so its suppression is a necessary condition for solving the strong CP problem. We propose a mechanism whereby the PQ symmetry is kept exact up to n=12 with the help of the very same flavor symmetries which generate the hierarchical quark masses and mixings of the SM. This "axion flavor protection" is straightforwardly realized in theories which employ radiative fermion mass generation and grand unification. A universal feature of this construction is that the heavy quark Yukawa couplings are generated at the PQ breaking scale.
We construct a string-inspired model, motivated by the flavored Peccei-Quinn (PQ) axions, as a useful bridge between flavor physics and string theory. The key feature is two anomalous gauged $U(1)$ symmetries, responsible for both the fermion mass hierarchy problem of the standard model and the strong CP problem, that combine string theory with flavor physics and severely constrain the form of the F- and D-term contributions to the potential. In the context of supersymmetric moduli stabilization we stabilize the size moduli with positive masses while leaving two axions massless and one axion m…
We consider extensions of the standard model inspired by intersecting D-brane constructions, in order to address flavor mass textures. We include additional anomalous gauge symmetries, and scalar fields to break them and to generate Froggatt-Nielsen mass terms. Green-Schwarz axions are included to cancel mixed anomalies rendering the models consistent. At low energies, a residual anomalous global symmetry remains, and its associated pseudo-Goldstone mode becomes the physical axion, which can be interpreted as an axion arising from open string modes. General considerations show that such axions…
We consider a string inspired non-supersymmetric extension of the standard model with gauged anomalous U(1) flavor symmetries. Consistency requires the Green-Schwarz mechanism to cancel mixed anomalies. The additional required scalars provide Stuckelberg masses for the $Z'$ particles associated to the gauged flavor symmetry, so they decouple at low energies. Our models also include a complex scalar field $ϕ$ to generate Froggatt-Nielsen mass terms for light particles giving a partial solution to the fermion mass problem. A residual approximate (anomalous) global symmetry survives at low energi…
Precision flavor experiments can look for the QCD axion complementarily to usual searches with axion helio- and haloscopes, allowing to test PQ breaking scales as high as $10^{12} \, {\rm GeV}$. Such searches are sensitive to flavor-violating axion couplings, which are generic and potentially sizable whenever SM fermions carry flavor non-universal PQ charges. A particularly predictive scenario is obtained when PQ is identified with the simplest FN flavor symmetry, so that all flavor-violating axion couplings are related to Yukawa hierarchies, up to ${\cal O}(1)$ coefficients.
Standard Model extensions with light axions are well-motivated by the observed Dark Matter abundance and the Peccei-Quinn solution to the Strong CP Problem. In general such axions can have large flavor-violating couplings to SM fermions, which naturally arise in scenarios where the Peccei-Quinn symmetry also explains the hierarchical pattern of fermion masses and mixings. I will discuss how these couplings allow for efficient axion production from the decays of SM particles, giving the opportunity to probe flavored axion Dark Matter with precision flavor experiments, astrophysics and cosmology…
We extend the work of Carone, Chaurasia and Vasquez on non-supersymmetric models of flavor based on the double tetrahedral group. Three issues are addressed: (1) the sector of flavor-symmetry-breaking fields is simplified and their potential studied explicitly, (2) a flavorful axion is introduced to solve the strong CP problem and (3) the model is extended to include the neutrino sector. We show how the model can accommodate the strong hierarchies manifest in the charged fermion Yukawa matrices, while predicting a qualitatively different form for the light neutrino mass matrix that is consiste…
We show how the scales responsible for Peccei-Quinn (PQ), seesaw, and Froggatt and Nielsen (FN) mechanisms can be fixed, by constructing a compact model for resolving rather recent, but fast-growing issues in astro-particle physics, including quark and leptonic mixings and CP violations, high-energy neutrinos, QCD axion, and axion cooling of stars. The model is motivated by the flavored PQ symmetry for unifying the flavor physics and string theory. The QCD axion decay constant congruent to the seesaw scale, through its connection to the astro-particle constraints of both the stellar evolution …
The hierarchy of fermion masses in the standard model may arise via the breaking of discrete gauge symmetries. The renormalizable interactions of the flavor-symmetry-breaking potential can have accidental global symmetries that are spontaneously broken, leading to pseudo-goldstone bosons that may drive inflation. We consider two-field, axion-monodromy inflation models in which the inflaton is identified with a linear combination of pseudo-goldstone bosons of the flavor sector. We show that the resulting models are nontrivially constrained by current cosmological data as well as the requirement…
We study the sensitivity of the existing MEG data to lepton flavor violating axion-like particles produced through $μ^+ \to e^+ a γ$ and estimate the discovery potential for the upcoming MEG II experiment in this channel. The MEG II signal efficiency can be improved significantly if a new trigger can be implemented in a dedicated run with a reduced beam intensity. This search would establish the world leading measurement in this channel with only 1 month of data taking.
We propose a new flavor paradigm for models with warped extra dimensions. The idea is to impose the minimal amount of flavor protection to make warped models compatible with all current flavor and electroweak precision constraints. We discuss a particular realization of this minimal flavor protection in the quark sector, by means of a flavor symmetry acting on the right handed down sector. Hierarchical quark masses and mixing angles are naturally reproduced through wave function localization, and flavor violating processes are predicted, in the absence of large brane kinetic terms for the righ…
Inspired by recent studies of high-scale decay constant or flavorful QCD axions, we review and clarify their existence in effective string models with anomalous $U(1)$ gauge groups. We find that such models, when coupled to charged scalars getting vacuum expectation values, always have one light axion, whose mass can only come from nonperturbative effects. If the main nonperturbative effect is from QCD, then it becomes a Peccei-Quinn axion candidate for solving the strong CP problem. We then study simple models with universal Green-Schwarz mechanism and only one charged scalar field: in the mi…
Flavor violating axion couplings can be in action before recombination, and they can fill the early universe with an additional radiation component. Working within a model-independent framework, we consider an effective field theory for the axion field and quantify axion production. Current cosmological data exclude already a fraction of the available parameter space, and the bounds will improve significantly with future CMB-S4 surveys. Remarkably, we find that future cosmological bounds will be comparable or even stronger than the ones obtained in our terrestrial laboratories.
A Peccei-Quinn~(PQ) symmetry is proposed in order to generate in the Standard Model~(SM) quark sector a realistic mass matrix ansatz with five texture-zeros. Limiting our analysis to Hermitian mass matrices, we show that this requires a minimum of 4 Higgs doublets. This model allows assigning values close to 1 for several Yukawa couplings, giving insight into the origin of the mass scales in the SM. Since the PQ charges are non-universal, the model features Flavor-Changing Neutral Currents~(FCNC) at the tree level. From the analytical expressions for the FCNC we report the allowed region in th…
We revisit the flavor-changing processes involving an axion-like particle (ALP) in the context of generic ALP effective lagrangian with a discussion of possible UV completions providing the origin of the relevant bare ALP couplings. We focus on the minimal scenario that ALP has flavor-conserving couplings at tree level, and the leading flavor-changing couplings arise from the loops involving the Yukawa couplings of the Standard Model fermions. We note that such radiatively generated flavor-changing ALP couplings can be easily suppressed in field theoretic ALP models with sensible UV completion…
We show that the hierarchically small mu term in supersymmetric theories is a consequence of two identical pairs of Higgs doublets taking a democratic form for their mass matrix. We briefly discuss the discrete symmetry S2xS2 toward the democratic mass matrix. Then, we show that there results an approximate Peccei-Quinn symmetry and hence the value mu is related to the axion decay constant.
Gravitational effects may interfere with the axion solution to the strong-CP problem. We point out that gravity can potentially provide a protection mechanism against itself, in the form of an additional axion-like field associated with torsion.
Axion-like particles (ALPs) with lepton-flavor violating couplings can be probed in exotic muon and tau decays. The sensitivity of different experiments depends strongly on the ALP mass and its couplings to leptons and photons. For ALPs that can be resonantly produced, the sensitivity of three-body decays such as $μ\to 3e$ and $τ\to 3μ$ exceeds by many orders of magnitude that of radiative decays like $μ\to eγ$ and $τ\toμγ$. Searches for these two types of processes are therefore highly complementary. We discuss experimental constraints on ALPs with a single dominant lepton-flavor violating co…
We study a class of DFSZ-like models for the QCD axion that can address observed anomalies in stellar cooling. Stringent constraints from SN1987A and neutron stars are avoided by suppressed couplings to nucleons, while axion couplings to electrons and photons are sizable. All axion couplings depend on few parameters that also control the extended Higgs sector, in particular lepton flavor-violating couplings of the Standard Model-like Higgs boson $h$. This allows us to correlate axion and Higgs phenomenology, and we find that that ${\rm BR}(h \to τe)$ can be as large as the current experimental…
Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model (SM). We investigate the phenomenology of an ALP with flavor-changing couplings, and present a comprehensive analysis of quark and lepton flavor-changing observables within a general ALP effective field theory. Observables studied include rare meson decays, flavor oscillations of neutral mesons, rare lepton decays, and dipole moments. We derive bounds on the general ALP couplings as a function of its mass, consistently taking into account the ALP lifetime and branching ra…
Axion models with generation-dependent Peccei-Quinn charges can lead to flavor-changing neutral currents, thus motivating QCD axion searches at precision flavor experiments. We rigorously derive limits on the most general effective flavor-violating couplings from current measurements and assess their discovery potential. For two-body decays we use available experimental data to derive limits on $q\to q' a$ decay rates for all flavor transitions. Axion contributions to neutral-meson mixing are calculated in a systematic way using chiral perturbation theory and operator product expansion. We als…
We study the reasonable requirements of two anomalous $U(1)$s in a flavored-axion framework for the anomaly cancellations of both $U(1)$-mixed gravity and $U(1)_Y\times[U(1)]^2$ which in turn determine the $U(1)_Y$ charges where $U(1)_Y$ is the hypercharge gauge symmetry of the standard model. We argue that, with a flavor symmetry group, axion-induced topology in symmetry-broken phases plays crucial roles in describing how quarks and leptons are organized at a fundamental level and make deep connections with each other. A unified model, as an example, is then proposed in a simple way to descri…
We assess the impact of searches at flavor factories for new neutral resonances that couple to both photons and gluons. These are well motivated by "heavy axion" solutions of the strong CP problem and by frameworks addressing both Dark Matter and the Higgs hierarchy problem. We use LHCb public diphoton data around the Bs mass to derive the current best limit on these resonances for masses between 4.9 and 6.3 GeV. We estimate that a future LHCb dedicated search would test an axion decay constant of O(TeV) for axion masses in the few-to-tens of GeV, being fully complementary to the low mass ATLA…
Minimal Flavor Protection for TeV-scale New Physics
University of Basel（巴塞尔大学）；Instituto de Física Corpuscular (IFIC), Consejo Superior de Investigaciones Científicas (CSIC) and Universitat de València (UV)（粒子物理研究所（IFIC），西班牙国家科学研究委员会（CSIC）和瓦伦西亚大学（UV））
We present a simple variation of warped flavor models where the hierarchies of fermion masses and mixings are still explained but dangerous flavor violating effects in the Kaon sector are greatly reduced. The key new ingredients are two horizontal U(1) symmetries. These symmetries forbid flavor violation in the down quark sector (with the exception of small IR brane localized kinetic mixing terms for the left-handed quarks) while allowing for flavor violation in the up quark sector. The leading flavor constraints come from D - anti-D mixing, and are safely satisfied for the KK mass scale of or…
We investigate the most interesting decay processes involving axions, photons and the lightest pseudoscalar mesons, making use of a chiral effective Lagrangian model with $L=3$ light quark flavors, which also includes the flavor-singlet pseudoscalar meson and implements the $U(1)$ axial anomaly of the fundamental theory. In particular, we compute the electromagnetic coupling of the axion to photons and we compare our result with the prediction of the Chiral Effective Lagrangian with $L=2$ light quark flavors. Moreover, we study the decay channels $η/η'\rightarrow ππa$ and we estimate the corre…
We propose new searches for axion-like particles (ALPs) produced in flavor-changing neutral current (FCNC) processes. This proposal exploits the often-overlooked coupling of ALPs to W bosons, leading to FCNC production of ALPs even in the absence of a direct coupling to fermions. Our proposed searches for resonant ALP production in decays such as $B\rightarrow K^{(*)}a,\,a\rightarrow γγ$ and $K\rightarrow πa, \,a\rightarrow γγ$ could greatly improve upon the current sensitivity to ALP couplings to Standard Model particles. We also determine analogous constraints and discovery prospects for inv…
In this work, we study the propagation of high energy neutrinos produced in extragalactic sources including the effect of a possible interaction with ultralight axion-like particles (UALP) with a mass $m_a\sim 10^{-22} {\rm eV}$ as the constituents of dark matter (DM) under the assumption that their coupling to neutrinos is dominant. We compute the cross section and describe the propagation of a diffuse neutrino flux using transport equations for each mass eigenstate. This allows us to obtain the neutrino fluxes of the different flavors to be observed at the Earth with neutrino telescopes unde…
We show that solving the flavor problem of the Standard Model with a simple $U(1)_H$ flavor symmetry naturally leads to an axion that solves the strong CP problem and constitutes a viable Dark Matter candidate. In this framework, the ratio of the axion mass and its coupling to photons is related to the SM fermion masses and predicted within a small range, as a direct result of the observed hierarchies in quark and charged lepton masses. The same hierarchies determine the axion couplings to fermions, making the framework very predictive and experimentally testable by future axion and precision …
We examine various possibilities for realistic 5D higgsless models and construct a full quark sector featuring next-to-minimal flavor violation (with an exact bulk SU(2) protecting the first two generations) satisfying electroweak and flavor constraints. The "new custodially protected representation" is used for the third generation to protect the light quarks from flavor violations induced due to the heavy top. A combination of flavor symmetries, and RS-GIM for the right-handed quarks suppresses flavor-changing neutral currents below experimental bounds, assuming CKM-type mixing on the UV bra…
We study charged lepton flavor violation associated with a light leptophilic axion-like particle (ALP), $X$, at the $B$-factory experiment Belle II. We focus on production of the ALP in the tau decays $τ\to X l$ with $l=e,μ$, followed by its decay via $X\to l^- l^+$. The ALP can be either promptly decaying or long-lived. We perform Monte-Carlo simulations, recasting a prompt search at Belle for lepton-flavor-violating $τ$ decays, and propose a displaced-vertex (DV) search. For both types of searches, we derive the Belle~II sensitivity reaches in both the product of branching fractions and the …
We propose new perspective in searching for axion-like particles (ALPs) from quark and lepton flavor physics: measurements of the time-dependent CP asymmetry in $B^0 \to K_S^0 π^0 γ$ and the branching ratio of $B_s \to e^\pm μ^\mp$ decay possess, along with the anomalous magnetic moment of muon. In the mass range of sub-GeV, accessible by the flavorful ALPs search, the experimental sensitivity for these flavor observables reaches the maximum at around the pion mass scale (called the {\it sweetest} spots), where a couple of loopholes (unexplored regions) for the ALP parameter space have heretof…
The invisible variant axion model is very attractive as it is free from the domain wall problem. This model requires two Higgs doublets at the electroweak scale where one Higgs doublet carries a nonzero Peccei-Quinn (PQ) charge and the other is neutral under the PQ $\text U(1)$ symmetry. We consider the most interesting and less constrained scenario of the variant axion model, where only the right-handed top quark is charged under the PQ symmetry and couples with the PQ-charged Higgs doublet. As a result, the top quark can decay to the observed standard-model-like Higgs boson $h$ and the charm…
We use the chiral Lagrangian to investigate the global properties of the $N$-flavor QCD vacuum as a function of the $θ$ parameter. In the case of exact quark degeneracy we find evidence for first order phase transitions at $θ= π\cdot ({\rm odd~ integer})$. The first order transitions are smoothed by quark mass splittings, although interesting effects remain at realistic quark masses. We emphasize the role of the $η'$ condensate in our analysis. Finally, we discuss the implications of our results on the internal hadronic structure of axion domain walls and axion cosmology.
We construct the minimal SUSY model that causes spontaneous CP violation with an abelian flavor symmetry in the context of the large extra dimensions and show that various phenomenological problems can be solved by introducing only scales below the low fundamental scale. We also realize the realistic size of the CP violation and the small masses of the neutrinos. The strong CP problem can be solved by the axion scenario and the axion can be made invisible by introducing an additional large extra dimension.
The "gauged'' Peccei-Quinn (PQ) mechanism of Fukuda, Ibe, Suzuki and Yanagida is implemented in the flavorful axion model of Carone and Merchand. This model of flavor is similar to other successful ones based on the double tetrahedral group, but the flavor symmetry includes a global U(1) factor that leads to the presence of a flavorful axion. Here we gauge that U(1) symmetry and introduce a heavy sector that includes (1) the fermions necessary to cancel anomalies and (2) a second scalar flavon field that spontaneously breaks the U(1) symmetry. The full theory has an accidental U(1)$\times$U(1)…
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 study QCD axion or cosmological axion-like particles (ALPs) in a model inspired by the recent interest in 4-dimensional clockwork models, with the global symmetry being accidentally enforced by a gauge abelian quiver with scalar bifundamental fields. For the QCD axion, we analyze the connection between the degree of protection of the axion mass against gravitational corrections, the explanation of the hierarchy $f_a \ll M_P$ and the number of colored fermions needed to generate anomalous couplings to gluons, all linked together by the underlying gauge symmetries. Based on that model and on …
