Peccei-Quinn Charge Research Articles

Axion species scale and axion weak gravity conjecture-like bound

As a cutoff scale of quantum gravity, the species scale can be defined by the scale at which the perturbativity of the non-renormalizable gravitational interaction begins to break down. Since it is determined by the number of species in the effective field theory, we can find the close connection to the distance conjecture, which predicts the lowering of the cutoff at the asymptotic limit of the moduli space caused by the descent of a tower of states from UV. Meanwhile, the same kind of the cutoff scale can be obtained from any non-renormalizable interaction, in particular the interaction between the axion and the gauge field through the θF ∧ F term. Demanding this ‘axion species scale’ not to exceed the gravitational species scale, we obtain the bound (8π2/g2)f ≲ MPl. This is quite similar to the axion weak gravity conjecture bound, but can be applied to any gauge as well as the string interactions which are relevant to towers of states. We also investigate the implications of the (axion) species scale and the axion weak gravity conjecture-like bound by considering the Peccei-Quinn charge reduction of black hole through the interaction between black hole and the string or wormhole.

Peccei-Quinn inflation at the pole and axion kinetic misalignment

We propose a minimal extension of the Standard Model with the Peccei-Quinn (PQ) scalar field and explain the relic density of the QCD axion through the kinetic misalignment with a relatively small axion decay constant. To this purpose, we consider a slow-roll inflation from the radial component of the PQ field with the PQ conserving potential near the pole of its kinetic term and investigate the post-inflationary dynamics of the PQ field for reheating. The angular mode of the PQ field, identified with the QCD axion, receives a nonzero velocity during inflation due to the PQ violating potential, evolving with an approximately conserved Noether PQ charge. We determine the reheating temperature from the perturbative decays and scattering processes of the inflaton and obtain dark radiation from the axions produced from the inflaton scattering at a testable level in the future Cosmic Microwave Background experiments. We show the correlation between the reheating temperature, the initial velocity of the axion and the axion decay constant, realizing the axion kinetic misalignment for the correct relic density.

The quark flavor-violating ALPs in light of B mesons and hadron colliders

The axion-like particle (ALP) may induce flavor-changing neutral currents (FCNCs) when the fermions’ Peccei-Quinn charges are not generation universal. The search for flavor-violating ALP couplings with a bottom quark so far focused on FCNC processes of B mesons at low energies. The recent measurements of B → K + X rare decays place stringent bounds on the quark flavor violations of a light ALP in different decay modes. In this work we propose a novel direct search for bottom flavor-violating interaction of a heavy ALP at the LHC and its upgrades, namely QCD production of an ALP associated with one b jet and one light jet p p → b j a. We consider the decay of the ALP to photons, muons and invisible ALP decays. The Boosted Decision Tree (BDT) algorithm is used to analyze the events and we train the BDT classifier by feeding in the kinematic observables of signal and backgrounds. Finally, we show the complementarity between the search prospects of hadron colliders and the low-energy B meson constraints from B meson mixing and B meson decays to a light ALP.

Hadrophobic axion from a GUT

We propose a new kind of axion model derived from the grand unified theory (GUT) based on SU(5)×U(1)PQ. We demonstrate that, given a certain charge assignment and potential flavor models, the axion is naturally hadrophobic and provides a novel explanation for the required condition using isospin symmetry. This axion can be the QCD axion that solves the strong CP problem. Furthermore, to satisfy the limit on the axion-electron coupling from the tip of the red giant branch, we impose the condition of electrophobia to determine a possible Peccei-Quinn charge assignment consistent with GUT. We then discuss the possibility that the hadrophobic and electrophobic axion serves as an inflaton and dark matter, as in the axionlike particle miracle scenario. Interestingly, in the viable parameter region, the strong CP phase must be suppressed, providing another solution to the strong CP problem. This scenario is intimately linked to flavor physics, dark matter searches, and stellar cooling. Detecting such an axion with peculiar couplings in various experiments would serve as a probe for GUT and the origin of flavor. Published by the American Physical Society 2024

Naturally astrophobic QCD axion

We present a QCD axion model where the couplings of the axion to nucleons, electrons, and muons are naturally suppressed because of the appropriate choice of the Peccei-Quinn charges of the Standard Model fermions. We reexamine next-to-leading order corrections to the couplings of the axion with nucleons and photons and show that the axion decay constant may be as small as 107 GeV. It is also possible to suppress the coupling with the photon so that the decay constant is even smaller and minimal axiogenesis works. In this scenario, the axion has a mass above 1 eV and may be directly detected via absorption of axion dark matter. Flavor-violating axion couplings are generically predicted in our model, but we show that they may be naturally and sufficiently suppressed. We discuss the implications of the hints for anomalous cooling in several stellar environments to our model.

Heating up Peccei-Quinn scale

We discuss production of QCD axion dark matter in a novel scenario, which assumes time-varying scale of Peccei-Quinn symmetry breaking. The latter decreases as the Universe's temperature at early times and eventually stabilises at a large constant value. Such behavior is caused by the portal interaction between the complex field carrying Peccei-Quinn charge and a Higgs-like scalar, which is in thermal equilibrium with primordial plasma. In this scenario, axions are efficiently produced during the parametric resonance decay of thecomplex Peccei-Quinn field, relaxing to the minimum of its potential in the radiation-dominated stage. Notably, this process is not affected by the Universe's expansion rate and allows to generate the required abundance of dark matter independently of an axion mass. Phenomenological constraints on the model parameter space dependon the number density of radial field fluctuations, which are also generically excited along with axions, and the rate of their thermalization in the primordial plasma.For the ratio of radial field and axion particles number densities larger than ∼ 0.01 at the end of parametric resonance decay, the combination of cosmological and astrophysical observations with the CAST limit confinesthe Peccei-Quinn scale to a narrow range of values ∼ 108 GeV, —this paves the way for ruling out our scenario with the near future searches for axions.

Flavored axions and the flavor problem

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 the parameter space obtained from the measurements of branching ratios of semileptonic meson decays.

DFSZ axion couplings revisited

Among many possibilities, solar axion has been proposed to explain the electronic recoil events excess observed by Xenon1T collaboration, although it has tension with astrophysical observations. The axion couplings, to photon gaγ and to electron gae play important roles. These couplings are related to the Peccei-Quinn (PQ) charges Xf for fermions. In most of the calculations, gaγ is obtained by normalizing to the ratio of electromagnetic anomaly factor E=TrXfQf2Nc (Nc is 3 and 1 for quarks and charged leptons respectively) and QCD anomaly factor N=TrXqT(q) (T(q) is quarks' SU(3)c index). The broken PQ symmetry generator is used in the calculation which does not seem to extract out the components of broken generator in the axion which are “eaten” by the Z boson. However, using the physical components of axion or the ratio of anomaly factors should obtain the same results in the DFSZ for gaγ. When going beyond the standard DFSZ models, such as variant DFSZ models, where more Higgs doublets and fermions have different PQ charges, one may wonder if the results are different. We show that the two methods obtain the same results as expected, but the axion couplings to quarks and leptons gaf (here f indicates one of the fermions in the SM) are more conveniently calculated in the physical axion basis. The result depends on the values of the vacuum expectation values leading to a wider parameter space for gaf in beyond the standard DFSZ axion. We also show explicitly how flavor conserving gaf couplings can be maintained when there are more than one Higgs doublets couple to the up and down fermion sectors in variant DFSZ models at tree level, and how flavor violating couplings can arise.

Quark flavor phenomenology of the QCD axion

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 also discuss in detail baryonic decays and three-body meson decays, which can lead to the best search strategies for some of the couplings. For instance, a strong limit on the $\Lambda\to n a$ transition can be derived from the supernova SN 1987A. In the near future, dedicated searches for $q\to q' a$ decays at ongoing experiments could potentially test Peccei-Quinn breaking scales up to $10^{12}$ GeV at NA62 or KOTO, and up to $10^{9}$ GeV at Belle II or BES III.

Axiogenesis.

We propose a mechanism called axiogenesis where the cosmological excess of baryons over antibaryons is generated from the rotation of the QCD axion. The Peccei-Quinn (PQ) symmetry may be explicitly broken in the early Universe, inducing the rotation of a PQ charged scalar field. The rotation corresponds to the asymmetry of the PQ charge, which is converted into the baryon asymmetry via QCD and electroweak sphaleron transitions. In the concrete model we explore, interesting phenomenology arises due to the prediction of a small decay constant and the connections with new physics at the LHC and future colliders and with axion dark matter.

Stellar cooling anomalies and variant axion models

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

Axion-electron decoupling in nucleophobic axion models

The strongest upper bounds on the axion mass come from astrophysical observations like the neutrino burst duration of SN1987A, which depends on the axion couplings to nucleons, or the white-dwarf cooling rates and red-giant evolution, which involve the axion-electron coupling. It has been recently argued that in variants of DFSZ models with generation-dependent Peccei-Quinn charges an approximate axion-nucleon decoupling can occur, strongly relaxing the SN1987A bound. However, as in standard DFSZ models, the axion remains in general coupled to electrons, unless an ad hoc cancellation is engineered. Here we show that axion-electron decoupling can be implemented without extra tunings in DFSZ-like models with three Higgs doublets. Remarkably, the numerical value of the quark mass ratio $m_u/m_d\sim 1/2$ is crucial to open up this possibility.

Axion cooling of neutron stars. II. Beyond hadronic axions

We study the axion cooling of neutron stars within the Dine-Fischler-Srednicki-Zhitnitsky (DFSZ) model, which allows for tree level coupling of electrons to the axion {and locks the Peccei-Quinn charges of fermions via an angle parameter}. This extends our previous study [Phys. Rev. D 93, 065044 (2016)] limited to hadronic models of axions. We explore the two-dimensional space of axion parameters within the DFSZ model by comparing the theoretical cooling models with the surface temperatures of a few stars with measured surface temperatures. It is found that axions masses $m_a\ge 0.06$ to 0.12 eV can be excluded by x-ray observations of thermal emission of neutron stars (in particular by those of Cas A), the precise limiting value depending on the angle parameter of the DFSZ model. It is also found that axion emission by electron bremsstrahlung in neutron star crusts is negligible except for the special case where neutron Peccei-Quinn charge is small enough, so that the coupling of neutrons to axions can be neglected.

Astrophobic Axions

We propose a class of axion models with generation dependent Peccei-Quinn charges for the known fermions that allow to suppress the axion couplings to nucleons and electrons. Astrophysical limits are thus relaxed, allowing for axion masses up to ${\cal O}(0.1)$ eV. The axion-photon coupling remains instead sizeable, so that next generation helioscopes will be able to probe this scenario. Astrophobia unavoidably implies flavor violating axion couplings, so that experimental limits on flavour-violating processes can provide complementary probes. The astrophobic axion can be a viable dark matter candidate in the heavy mass window, and can also account for anomalous energy loss in stars.

Possible roles of Peccei-Quinn symmetry in an effective low energy model

The strong $CP$ problem is known to be solved by imposing Peccei-Quinn (PQ) symmetry. However, the domain wall problem caused by the spontaneous breaking of its remnant discrete subgroup could make models invalid in many cases. We propose a model in which the PQ charge is assigned quarks so as to escape this problem without introducing any extra colored fermions. In the low energy effective model resulting after the PQ symmetry breaking, both the quark mass hierarchy and the CKM mixing could be explained through Froggatt-Nielsen mechanism. If the model is combined with the lepton sector supplemented by an inert doublet scalar and right-handed neutrinos, the effective model reduces to the scotogenic neutrino mass model in which both the origin of neutrino masses and dark matter are closely related. The strong $CP$ problem could be related to the quark mass hierarchy, neutrino masses, and dark matter through the PQ symmetry.

Flavor-changing neutral-current decays in top-specific variant axion model

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 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 or up quark, t ! h c=u, which is testable soon at the LHC Run-II. Moreover, we propose a method to probe the chiral nature of the Higgs avor-changing interaction using the angular distribution of t! ch decays if a sucient number of such events are observed. We also show that our model has the capacity to explain the h ! decay reported by the CMS Collaboration, if the right-handed tau lepton also carries a PQ charge and couples to the PQ-charged Higgs boson.

Axion mass limit from observations of the neutron star in Cassiopeia A

Direct Chandra observations of a surface temperature of isolated neutron star in Cassiopeia A (Cas A NS) and its cooling scenario which has been recently simultaneously suggested by several scientific teams put stringent constraints on poorly known properties of the superfluid neutron star core. It was found also that the thermal energy losses from Cas A NS are approximately twice more intensive than it can be explained by the neutrino emission. We use these unique data and well-defined cooling scenario to estimate the strength of KSVZ axion interactions with neutrons. We speculate that enlarged energy losses occur owing to emission of axions from superfluid core of the neutron star. If the axion and neutrino losses are comparable we find cn2ma2∼ 5.7× 10-6 eV2, where ma is the axion mass, and cn is the effective Peccei-Quinn charge of the neutron. (Given the QCD uncertainties of the hadronic axion models, the dimensionless constant cn could range from -0.05 to 0.14.)

Mixing of axino and goldstino, and axino mass

Axino, related to the SUSY transformation of axion, can mix with goldstino in principle. This case is realized when some superfields carrying nonvanishing Peccei–Quinn charges develop both scalar VEVs and F-terms. In this case, we present a proper definition of axion and axino. With this definition, we present the QCD axino mass in the most general framework, including nonminimal Kähler potential. The axino mass is known to have a hierarchical mass structure depending on accidental symmetries. With only one axino, if GA=0 where G=K+ln|W|2, we obtain ma˜=m3/2. For GA≠0, the axino mass depends on the details of the Kähler potential. In the gauge mediation scenario, the gaugino mass is the dominant axino mass parameter. Therefore, we can take the theoretical QCD axino mass as a free parameter in the study of its cosmological effects, ranging from eV to multi-TeV scales, without a present knowledge on its ultraviolet completion.

Probing variant axion models at LHC

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

Cosmology and broken discrete symmetry

It is argued that under various circumstances spontaneously broken discrete symmetry, which is commonly thought to lead to grave cosmological problems, need not do so. A particularly interesting example is the two-doublet version of the standard model. This extended model avoids unacceptable flavor-changing neutral currents in a natural way only if an appropriate discrete symmetry is postulated. Here it is shown that this discrete symmetry is anomalous. As a result, when it is spontaneously broken it does not produce stable domain walls (which would be problematic for cosmology). Instead an interesting cosmological scenario results, which incorporates substantial deviations from thermal equilibrium at the QCD scale. If axions are used to address the strong CP problem, then the analysis is altered. Depending on the Peccei-Quinn charge assignments, domain walls may be either stable or nonexistent.