Strong CP Problem Research Articles

Extended Search for the Invisible Axion with the Axion Dark Matter Experiment.

This Letter reports on a cavity haloscope search for dark matter axions in the Galactic halo in the mass range 2.81-3.31 μeV. This search utilizes the combination of a low-noise Josephson parametric amplifier and a large-cavity haloscope to achieve unprecedented sensitivity across this mass range. This search excludes the full range of axion-photon coupling values predicted in benchmark models of the invisible axion that solve the strong CP problem of quantum chromodynamics.

The axion mass from 5D small instantons

We calculate a new contribution to the axion mass that arises from gluons propagating in a 5th dimension at high energies. By uplifting the 4D instanton solution to five dimensions, the positive frequency modes of the Kaluza-Klein states generate a power-law term in the effective action that inversely grows with the instanton size. This causes 5D small instantons to enhance the axion mass in a way that does not spoil the axion solution to the strong CP problem. Moreover this enhancement can be much larger than the usual QCD contribution from large instantons, although it requires the 5D gauge theory to be near the non-perturbative limit. Thus our result suggests that the mass range of axions (or axion-like particles), which is important for ongoing experimental searches, can depend sensitively on the UV modification of QCD.

MADMAX: A new road to axion dark matter detection

The axion is a hypothetical low-mass boson predicted by the Peccei-Quinn mechanism solving the strong CP problem. It is naturally also a cold dark matter candidate if its mass is below ∼1 meV, thus simultaneously solving two major problems of nature. All existing experimental efforts to detect QCD axions focus on a range of axion masses below ∼25 μeV. The mass range above ∼40 μeV, predicted by modern models in which the Peccei-Quinn symmetry was restored after inflation, could not be explored so far. The MADMAX project is designed to be sensitive for axions with masses (40–400) μeV. The experimental design is based on the idea of enhanced axion-photon conversion in a system with several layers with alternating dielectric constants. The concept and the proposed design of the MADMAX experiment are discussed. Measurements taken with a prototype test setup are discussed. The prospects for reaching sensitivity enough to cover the parameter space predicted for QCD dark matter axions with mass in the range around 100 μeV is presented.

A holographic perspective on the axion quality problem

The axion provides a compelling solution to the strong CP problem as well as a candidate for the dark matter of the universe. However, the axion solution relies on the spontaneous breaking of a global U(1)PQ symmetry, which is also explicitly violated by quantum gravity. To preserve the axion solution, gravitational violations of the U(1)PQ symmetry must be suppressed to sufficiently high order. We present a simple, geometric solution of the axion quality problem by modelling the axion with a bulk complex scalar field in a slice of AdS5, where the U(1)PQ symmetry is spontaneously broken in the bulk but explicitly broken on the UV brane. By localising the axion field towards the IR brane, gravitational violations of the PQ symmetry on the UV brane are sufficiently sequestered. This geometric solution is holographically dual to 4D strong dynamics where the global U(1)PQ is an accidental symmetry to sufficiently high order.

WITHDRAWN

The document associated with this DOI has been withdrawn.

Modulations in Spectra of Galactic Gamma-ray sources as a result of photon-ALPs mixing.

Axion like particles (ALPs) are fundamental pseudo scalar particles with properties similar to Axions which are a well-known extensions of the standard model to solve the strong CP problem in Quantum Chromodynamics. ALPs can oscillate into photons and vice versa in the presence of an external tranversal magnetic field. This oscillation of photon and ALPs could have important implications for astronomical observations, i.e. a characteristic energy dependent attenuation in Gamma ray spectra for astrophysical sources. Here we have revisited the opportunity to search Photon-ALPs coupling in the disappearance channel. We use eight years of Fermi Pass 8 data of a selection of promising galactic Gamma-ray source candidates and study the modulation in the spectra in accordance with Photon-ALPs mixing and estimate best fit values of the parameters i.e. Photon-ALPs coupling constant (gʱγγ) and ALPs mass(mα). For the magnetic field we use large scale galactic magnetic field models based on Faraday rotation measurements and we have also studied the survival probability of photons in the Galactic plane.

Actinide and lanthanide molecules to search for strong CP-violation.

The existence of the fundamental CP-violating interactions inside the nucleus leads to the existence of a nuclear Schiff moment. The Schiff moment potential corresponds to the electric field localized inside the nucleus and directed along its spin. This field can interact with electrons of an atom and induce the permanent electric dipole moment (EDM) of the whole system. The Schiff moment and the corresponding electric field are enhanced in the nuclei with octupole deformation leading to an enhanced atomic EDM. There is also a few-order enhancement of the T,P-violating effects in molecules due to the existence of energetically close levels of opposite parity. We study the Schiff moment enhancement in the class of diatomic molecules with octupole-deformed lanthanide and actinide nuclei: 227AcF, 227AcN, 227AcO+, 229ThO, 153EuO+ and 153EuN. Projecting the existing experimental achievements to measure the EDM in diamagnetic molecules with a spherical nucleus (205TlF) to the considered systems one can expect very high sensitivity to the quantum chromodynamics parameter [small theta, Greek, macron] and other hadronic CP-violation parameters surpassing the current best limits by several orders of magnitude. It can have a dramatic impact on the modern understanding of the nature of CP-violating fundamental interactions.

Erratum: Axions and the strong CP problem [Rev. Mod. Phys. 82 , 557 (2010)

Erratum: Axions and the strong <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>C</mml:mi><mml:mi>P</mml:mi></mml:math> problem [Rev. Mod. Phys. <b>82</b> , 557 (2010)

Baryon asymmetry from left-right phase transition

We extend the standard model fermions by a mirror copy to realize a left-right symmetry. During a strongly first order phase transition of the spontaneous left-right symmetry breaking, the CP-violating reflections of the mirror fermions off the mirror Higgs bubbles can generate a mirror lepton asymmetry and an equal mirror baryon asymmetry. We then can obtain an ordinary baryon asymmetry through the mirror fermion decays where a dark matter scalar plays an essential role. Benefitted from a parity symmetry for solving the strong CP problem, the cosmic baryon asymmetry can be well described by the ordinary lepton mass matrices up to an overall factor. In this scenario, the Dirac CP phase in the Majorana neutrino mass matrix can provide a unique source for the required CP violation. Furthermore, the Higgs triplet for type-II seesaw as well as the first generation of mirror charged fermions can be allowed at the TeV scale.

Strong CP problem with low-energy emergent QCD: The 4321 case

We analyze the strong CP problem and the implications for axion physics in the context of $U_1$ vector leptoquark models, recently put forward as an elegant solution to the hints of lepton flavor universality violation in B-meson decays. It is shown that in minimal gauge models containing the $U_1$ as a gauge boson, the Peccei-Quinn solution of the strong CP problem requires the introduction of two axions. Characteristic predictions for the associated axions can be deduced from the model parameter space hinted by B-physics, allowing the new axion sector to account for the dark matter of the Universe. We also provide a specific ultraviolet completion of the axion sector that connects the Peccei-Quinn mechanism to the generation of neutrino masses.

Asymptotic safety, the Higgs boson mass, and beyond the standard model physics

There are many hints that gravity is asymptotically safe. The inclusion of gravitational corrections can result in the ultraviolet fundamental Standard Model and constrain the Higgs mass to take the smallest value such that electroweak vacuum is stable. Taking into account the current top quark mass measurements this value is $m_H \approx 130$ GeV.\\ This article considers the predictions of the Higgs mass in two minimal Beyond Standard Model scenarios, where the stability is improved. One is the sterile quark axion model, while the other is the $U(1)_{B-L}$ gauge symmetry model introducing a new massive $Z'$ gauge boson. The inclusion of $Z'$ boson gives the correct prediction for this mass, while inclusion of sterile quark(s) gives only a slight effect.\\ Also a new, gravitational solution to the strong CP problem is discussed.

Solving the strong CP problem with horizontal gauge symmetry

We present a solution to the strong CP problem, which relies on the horizontal gauge symmetry and CP invariance in a full theory. Similar to other Nelson-Barr type solutions, CP violation in both the strong and weak sectors in the Standard model (SM) is attributed to the condensation of complex scalars $\Phi$ in the model. The model is differentiated by others in that it explains the hierarchy in quark-Higgs Yukawa coupling in the SM based on a series of sequential breaking of the horizontal $SU(3)_{f}$ gauge symmetry. The experimental constraint $\overline{\theta}\lesssim10^{-10}$ requires $<\!\!\Phi\!\!>\,\lesssim\,10^{13}-10^{14}{\rm GeV}$ (vacuum expectation value of complex scalars) and $\lambda\,\lesssim\,10^{-6}$ (scalar quartic coupling). We show that this small coupling is natural in the sense of 'tHooft naturalness. Compared to other models of Nelson-Barr type with CP breaking scale $\Lambda_{CP}\lesssim10^{8}{\rm GeV}$, our model is more advantageous in terms of consistency with the thermal leptogenesis.

ν solution to the strong CP problem

We present a new solution to the strong CP problem in which the imaginary component of the up quark mass, $\mathcal{I}[m_u]$, acquires a tiny, but non-vanishing value. This is achieved via a Dirac seesaw mechanism, which is also responsible for the generation of the small neutrino masses. Consistency with the observed value of the up quark mass is achieved via instanton contributions arising from QCD-like interactions. In this framework, the value of the neutron electric dipole moment is directly related to $\mathcal{I}[m_u]$, which, due to its common origin with the neutrino masses, implies that the neutron electric dipole moment is likely to be measured in the next round of experiments. We also present a supersymmetric extension of this Dirac seesaw model to stabilize the hierarchy among the scalar mass scales involved in this new mechanism.

SMASH model and atmospheric neutrino mass splitting

Five fundamental problems - neutrino mass, baryogenesis, dark matter, inflation, strong CP problem - are solved at one stroke in a model, dubbed as “SM-A-S-H” (Standard Model-Axion-Seesaw-Higgs portal inflation) by Andreas Ringwald et. al. The Standard Model (SM) particle content is extended by three right-handed SM-singlet neutrinos Ni, a vector-like color triplet quark Q, a complex SM-singlet scalar field σ that stabilises the Higgs potential, all of them being charged under Peccei-Quinn (PQ) U (1) symmetry, the vacuum expectation value vσ ∼ 1011 GeV breaks the lepton number and the Peccei-Quinn symmetry simultaneously. We found that numerically SMASH model not only solves five fundamental problems but also the sixth problem “Vacuum Metastability” through the extended scalar sector and can predict approximately correct atmospheric neutrino mass splitting around 0.05 eV and the solar neutrino mass splitting around 0.009 eV.

Higgs Parity grand unification

The vanishing of the Higgs quartic coupling of the Standard Model at high energies may be explained by spontaneous breaking of Higgs Parity. Taking Higgs Parity to originate from the Left-Right symmetry of the SO(10) gauge group, leads to a new scheme for precision gauge coupling unification that is consistent with proton decay. We compute the relevant running of couplings and threshold corrections to allow a precise correlation among Standard Model parameters. The scheme has a built-in solution for obtaining a realistic value for mb/mτ , which further improves the precision from gauge coupling unification, allowing the QCD coupling constant to be predicted to the level of 1% or, alternatively, the top quark mass to 0.2%. Future measurements of these parameters may significantly constrain the detailed structure of the theory. We also study an SO(10) embedding of quark and lepton masses, showing how large neutrino mixing is compatible with small quark mixing, and predict a normal neutrino mass hierarchy. The strong CP problem may be explained by combining Higgs Parity with space-time parity.

Quintessential inflation with a trap and axionic dark matter

We study a new model of quintessential inflation which is inspired by supergravity and string theory. The model features a kinetic pole, which gives rise to the inflationary plateau, and a runaway quintessential tail. We envisage a coupling between the inflaton and the Peccei-Quinn (PQ) field which terminates the roll of the runaway inflaton and traps the latter at an enhanced symmetry point (ESP), thereby breaking the PQ symmetry. The kinetic density of the inflaton is transferred to the newly created thermal bath of the hot big bang due to the decay of PQ particles. The model successfully accounts for the observations of inflation and dark energy without any fine-tuning, while also resolving the strong CP problem of QCD and generating axionic dark matter, without isocurvature perturbations. Trapping the inflaton at the ESP ensures that the model does not suffer from the infamous 5th force problem, which typically plagues quintessence.

Emergent Solution to the Strong CP Problem.

We construct a theory in which the solution to the strong CP problem is an emergent property of the background of the dark matter in the Universe. The role of the axion degree of freedom is played by multibody collective excitations similar to spin waves in the medium of the dark matter of the Galactic halo. The dark matter is a vector particle whose low energy interactions with the standard model take the form of its spin density coupled to GG[over ˜], which induces a potential on the average spin density inducing it to compensate θ[over ¯], effectively removing CP violation in the strong sector in regions of the Universe with sufficient dark matter density. We discuss the viable parameter space, finding that light dark matter masses within a few orders of magnitude of the fuzzy limit are preferred, and discuss the associated signals with this type of solution to the strong CP problem.

Cooling binary neutron star remnants via nucleon-nucleon-axion bremsstrahlung

The QCD axion is a hypothetical particle motivated by the Strong CP problem of particle physics. One of the primary ways in which its existence can be inferred is via its function as an additional cooling channel in stars, with some of the strongest constraints coming from the supernova observation SN1987A. Multimessenger observations of binary neutron star mergers (such as those of GW170817, AT2017gfo, and GRB170817A) may provide another scenario in which such constraints could be obtained. In particular, the axion could potentially alter the lifetime, the ejection of material, and the emitted gravitational wave signal of the postmerger remnant. In this article, we perform numerical relativity simulations of a binary neutron star merger, including a phenomenological description of the nucleon-nucleon-axion bremsstrahlung to quantify the effects of such a cooling channel on the dynamical evolution. While our simulations show a difference in the temperature profile of the merger remnant, the imprint of the axion via nucleon-nucleon-axion bremsstrahlung on the emitted gravitational wave signal and the ejecta mass is too small to improve constraints on the axion mass with current or future planned detectors. Whilst we consider a limited number of cases, and a simplified cooling model, these broadly represent the "best case" scenario, thus, a more thorough investigation is unlikely to change the conclusions, at least for this particular interaction channel.

Large gauge transformations and the strong CP problem

According to the Standard Model of particle physics, some gauge transformations are physical symmetries. That is, they are mathematical transformations that relate representatives of distinct physical states of affairs. This is at odds with the standard philosophical position according to which gauge transformations are an eliminable redundancy in a gauge theory's representational framework. In this paper I defend the Standard Model's treatment of gauge from an objection due to Richard Healey. If we follow the Standard Model in taking some gauge transformations to be physical symmetries then we face the “strong CP problem”, but if we adopt the standard philosophical position on gauge then the strong CP problem dissolves. Healey offers this as a reason in favor of the standard philosophical view. However, as I argue here, following Healey's recommendation gives a theory that makes bad empirical predictions.

Kaon oscillations and baryon asymmetry of the Universe

Baryon asymmetry of the universe (BAU) is naturally explained with $K^0-K^{0'}$ oscillations of a newly developed mirror-matter model and new understanding of quantum chromodynamics (QCD) phase transitions. A consistent picture for the origin of both BAU and dark matter is presented with the aid of $n-n'$ oscillations of the new model. The global symmetry breaking transitions in QCD are proposed to be staged depending on condensation temperatures of strange, charm, bottom, and top quarks in the early universe. The long-standing BAU puzzle can then be understood with $K^0-K^{0'}$ oscillations that occur at the stage of strange quark condensation and baryon number violation via a non-perturbative sphaleron-like (coined "quarkiton") process. Similar processes at charm, bottom, and top quark condensation stages are also discussed including an interesting idea for top quark condensation to break both the QCD global $U_t(1)_A$ symmetry and the electroweak gauge symmetry at the same time. Meanwhile, the $U(1)_A$ or strong CP problem of particle physics is simply solved under the same framework.