Search

In this paper, we propose a bubble filtering-out mechanism for an asymmetric dark matter scenario during the Peccei-Quinn (PQ) phase transition. Based on a QCD axion model, extended by extra chiral neutrinos, we show that the PQ phase transition can be first order in the parameter space of the model and regarding the PQ symmetry breaking scale, the mechanism can generate PeV-scale heavy neutrinos as a dark matter candidate. Considering a CP-violating source, during the phase transition, discriminating between the neutrino and antineutrino number density, we find the observed dark matter relic abundance, such that the setup can be applied to the first order phase transition with different strengths. We then calculate effective couplings of the QCD axion addressing the strong CP problem within the model. We also study the energy density spectrum of gravitational waves generated from the first order phase transition and show that the signals can be detected by future ground-based detectors such as Einstein Telescope. In particular, for a visible heavy axion case of the model, it is shown that gravitational waves can be probed by DECIGO and BBO interferometers. Furthermore, we discuss the dark matter-standard model neutrino annihilation process as a source for the creation of PeV-scale neutrinos., 20 pages, 6 figures; matches published version

The axion is a light pseudoscalar particle postulated to solve issues with the Standard Model, including the strong CP problem and the origin of dark matter. In recent years, there has been remarkable progress in the physics of axions in several directions. An unusual type of axion-like particle termed the relaxion was proposed as a new solution to the weak scale hierarchy problem. There are also new ideas for laboratory, astrophysical, or cosmological searches for axions; such searches can probe a wide range of model parameters that were previously inaccessible. On the formal theory side, the weak gravity conjecture indicates a tension between quantum gravity and a trans-Planckian axion field excursion. Many of these developments involve axions with hierarchical couplings. In this article, we review recent progress in axion physics, with particular attention paid to hierarchies between axion couplings. We emphasize that the parameter regions of hierarchical axion couplings are the most accessible experimentally. Moreover, such regions are often where important theoretical questions in the field are addressed, and they can result from simple model-building mechanisms., Comment: 1+35 pages, 4 figures; invited review published in Annual Review of Nuclear and Particle Science; v3: sign errors in Eq. (2.26) corrected, various minor changes

Axions and axion-like particles (ALPs) are well-motivated low-energy relics of high-energy extensions of the Standard Model, which interact with the known particles through higher-dimensional operators suppressed by the mass scale $\Lambda$ of the new-physics sector. Starting from the most general dimension-5 interactions, we discuss in detail the evolution of the ALP couplings from the new-physics scale to energies at and below the scale of electroweak symmetry breaking. We derive the relevant anomalous dimensions at two-loop order in gauge couplings and one-loop order in Yukawa interactions, carefully considering the treatment of a redundant operator involving an ALP coupling to the Higgs current. We account for one-loop (and partially two-loop) matching contributions at the weak scale, including in particular flavor-changing effects. The relations between different equivalent forms of the effective Lagrangian are discussed in detail. We also construct the effective chiral Lagrangian for an ALP interacting with photons and light pseudoscalar mesons, pointing out important differences with the corresponding Lagrangian for the QCD axion., Comment: 44 pages and 9 figures

General flavor changing interactions for a Goldstone boson (GB) to fermions due to a spontaneous global $U(1)_G$ symmetry breaking are discussed. This GB may be the Axion, solving the strong QCD CP problem, if there is a QCD anomaly for the $U(1)_G$ charge assignments for quarks. Or it may be the Majoron in models from lepton number violation in producing seesaw Majorana neutrino masses if the symmetry breaking scale is much higher than the electroweak scale. It may also, in principle, play the roles of Axion and Majoron simultaneously as far as providing solution for the strong CP problem and generating a small Majorana neutrino masses are concerned. Great attentions have been focused on flavor conserving GB interactions. Recently flavor changing Axion and Majoron models have been studied in the hope to find new physics from rare decays in the intensity frontier. In this work, we will provide a systematic model building aspect study for a GB having flavor changing neutral current (FCNC) interactions in both the quark and lepton sectors, or separately in the quark, charged lepton and neutrino sectors with sources of FCNC interactions identified in detail. We provide a general proof of the equivalence of using physical GB components and GB broken generators for calculating GB couplings to two gluons and two photons, and some issues related to models having spontaneous CP violation are discussed. We will also provide some details for obtaining FCNC GB interactions in several popular models, such as the Type-I, -II, -III seesaw and Left-Right symmetric models, and point out some special features in these models., Comment: 12 pages,no figure

A major theoretical problem of the otherwise successful standard model (SM) is the presence of an arbitrary amount of CP violation induced by the periodic vacuum structure of Quantum Chromodynamics (known as the strong CP problem). While the most popular solution to this problem is the Peccei-Quinn mechanism, it predicts a new superllight particle, the axion, which has not been found despite extensive experimental searches. An alternative solution to this problem that does not predict an axion is one based on a parity symmetric extension of SM, which also provides a framework for understanding the neutrino masses via the seesaw mechanism. In this mini-review, I describe how minimal versions of the parity solution to strong CP require supersymmetry and how a class of SO(10) theories provide a natural grand unified (GUT) embedding of these models. These approaches have the advantage that the observed CKM CP violation emerges in a simple way in contrast to some other non-axion approaches. We discuss the importance of a search for electric dipole moment of the neutron as a way to probe these solutions.

The recent tension between local and early measurements of the Hubble constant can be explained in a particle physics context. A mechanism is presented where this tension is alleviated due to the presence of a Majoron, arising from the spontaneous breaking of Lepton Number. The lightness of the active neutrinos is consistently explained. Moreover, this mechanism is shown to be embeddable in the Minimal (Lepton) Flavour Violating context, providing a correct description of fermion masses and mixings, and protecting the flavour sector from large deviations from the Standard Model predictions. A QCD axion is also present to solve the Strong CP problem. The Lepton Number and the Peccei-Quinn symmetries naturally arise in the Minimal (Lepton) Flavour Violating setup and their spontaneous breaking is due to the presence of two extra scalar singlets. The Majoron phenomenology is also studied in detail. Decays of the heavy neutrinos and the invisible Higgs decay provide the strongest constraints in the model parameter space., 4 pages, 1 figure. Contribution to the proceedings of the European Physical Society Conference on High Energy Physics (EPS-HEP2021), 26-30 July 2021

Orientador: Prof. Dr. Celso Chikahiro Nishi Dissertação (mestrado) - Universidade Federal do ABC, Programa de Pós-Graduação em Física, Santo André, 2022. A simetria de CP e violada no Modelo Padrao pela fase da matriz CKM no setor eletrofraco e, alem disso, a presenca do termo ¿Æ no setor forte do Modelo Padrao tambem violaria CP. O problema de CP forte consiste em explicar porque esta ultima violacao nao e observada experimentalmente e o valor de ¿Æ deve ser tao pequeno. Modelos do tipo Nelson-Barr sao modelos com violacao espontanea de CP que conseguem explicar simultaneamente o valor da fase da CKM e do termo ¿Æ. Modelos desse tipo necessitam da presenca de quarks vetoriais que transmitem a violacao de CP para o Modelo Padrao. Neste trabalho, vai se estudar diferentes modelos do tipo Nelson-Barr com um ou dois quarks vetoriais, com CP usual e CP4. Tambem foram abordadas as consequencias fenomenologicas e os limites experimentais do setor eletrofraco e da fisica de sabor para modelos do tipo Nelson-Bar com diferentes conteudos de quarks vetoriais. The CP symmetry is violated in the Standard Model by the phase of the CKM matrix in the electroweak sector and, in addition, the presence of the è term in the strong sector would violate CP as well. The strong CP problem consists on explaining why this last violation is not experimentally observed and the value of è should be so small. Nelson-Barr models are models with spontaneous CP violation that are capable of explaining simultaneously the value of the CKM phase and the è term. Models of this kind require the presence of vector-like quarks that transmit the CP violation to the SM. In this work, we study two models of Nelson-Barr type: the first is the minimal extension of the SM with the usual CP symmetry and the second with the non-conventional CP symmetry, CP4. We also discuss the phenomenological consequences and experimental limits from the electroweak sector and flavor physics for Nelson-Barr models with different contents of vector-like quarks.

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.

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 AdS$_5$, 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., Comment: 20 pages, 5 figures

We analyze the Nelson-Barr approach to the Strong CP Problem. We derive the necessary conditions in order to simultaneously reproduce the CKM phase and the quark masses. Then we quantify the irreducible contributions to the QCD topological angle, namely the corrections arising from loops of the colored fermion mediators that characterize these models. Corrections analytic in the couplings first arise at 3-loop order and are safely below current bounds; non-analytic effects are 2-loop order and decouple as the mediators exceed a few TeV. We discuss collider, electroweak, and flavor bounds and argue that most of the parameter space above the TeV scale is still allowed in models with down-type mediators, whereas other scenarios are more severely constrained. With two or more families of mediators the dominant experimental bound is due to the neutron electric dipole moment.

We propose UV-completions of Froggatt-Nielsen-Peccei-Quinn models of fermion masses and mixings with flavored axions, by incorporating heavy fields. Here, the $U(1)$ Froggatt-Nielsen symmetry is identified with the Peccei-Quinn symmetry to solve the strong CP problem along with the mass hierarchies of the Standard Model fermions. We take into account leading order contributions to the fermion mass matrices giving rise to Nearest-Neighbour-Interaction structure in the quark sector and $A_2$ texture in the neutrino sector. A comprehensive numerical analysis has been performed for the fermion mass matrices. Subsequently, we investigate the resulting axion flavor violating couplings and the axion-photon coupling arising from the model., 24 pages, 4 figures, 5 tables. Version to appear in the EPJC

Solutions of the Strong CP Problem based on the spontaneous breaking of CP must feature a non-generic structure and simultaneously explain a coincidence between a priori unrelated CP-even and CP-odd mass scales. We show that these properties can emerge from gauge invariance and a CP-conserving, but otherwise generic, physics at the Planck scale. In our scenarios no fundamental scalar is introduced beyond the Standard Model Higgs doublet, and CP is broken at naturally small scales by a confining non-abelian dynamics. This approach is remarkably predictive: robustness against uncontrollable UV corrections to the QCD topological angle requires one or more families of vector-like quarks below a few $10$'s of TeV, hence potentially accessible at colliders. Because CP violation is communicated to the SM at these super-soft scales, our solution of the Strong CP Problem is not spoiled by the presence of heavy new states motivated by other puzzles in physics beyond the Standard Model. In addition, these models generically predict a dark sector that may lead to interesting cosmological signatures., Comment: 19 pages. v2: matches the published version

The axion, a pseudoscalar particle originally introduced by Peccei, Quinn, Weinberg, and Wilczek to solve the "strong CP problem", is a well motivated dark-matter candidate with a mass lying in a broad range from peV to few meV. The last decade witnessed an increasing interest in axions and axion-like particles with many theoretical works published and many new experimental proposals. A major challenge for cosmological-axion discovery is the detection of the faint signal expected in detectors with power as low as a fraction of yoctowatt corresponding to a single microwave photon per minute. Early experiments used GaAs field-effect-transistors as well as pioneering technology such as Rydberg-atom single-photon detection. In the attempt of reducing the noise temperature superconductive devices were soon introduced. Microstrip SQUID Amplifiers allowed SQUIDs to operate at frequencies of a few GHz with a noise temperature reaching the standard quantum limit. Josephson Parametric Amplifiers have been recently employed extending the search to higher frequency while boradband Traveling Wave Parametric Amplifiers are now under study. The ultimate sensitivity, beyond the quantum limit, is however expected from single microwave-photon detectors. Solutions for quantum sensing include quantum non-demolition measurements and switching detectors based on superconducting qubits and hot-electron detectors. A further improvement in signal sensitivity and noise reduction is expected exploiting arrays of superconducting qubits as proposed by the SUPERGALAX project.

Journal of high energy physics 2021(6), 123 (1-22) (2021). doi:10.1007/JHEP06(2021)123, 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 $f_a > 10^{12}$ GeV., Published by SISSA, [Trieste]

We perform a global fit using results of searches for electric dipole moments (EDM) of diamagnetic systems within the context of the minimal left-right symmetric model. In this way, we disentangle the new "left-right" electroweak and $\bar{\theta}$ contributions that cannot be separated using a single EDM system. Although the fit is done for a specific model, the approach can be applied to any particle physics model. Finally, we revisit the constraint on the $D$ coefficient in $\beta$-decay and find that current EDM bounds do not preclude observation of this T-violating effect in a possible next generation $\beta$-decay experiment., Comment: 8 pages, 5 figures

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 $10^{15\text{-}16}$GeV. The scenario works in the high-scale inflation models., 32pages, 8figures

The Nelson-Barr (NB) mechanism to solve the strong CP problem assumes CP conservation, arranges vanishing $\bar{\theta}$ at tree-level and requires vector-like quarks (VLQs) to transmit the CP breaking to the SM. We analyze the flavor constraints coming from the presence of one such down type VLQ of NB type by performing a global fit on the relevant flavor observables. A comparison is made to the case of one generic VLQ. We find that the allowed parameter space for the VLQ Yukawa couplings and the mixing to the SM are confined to a region much smaller than in the generic case, making the NB case falsifiable in principle., Comment: Appendices added

We reconsider entropy arguments which have been previously argued to support the idea that the dark matter constituents are primordial black holes with many solar masses. It has recently been shown that QCD axions which solve the strong CP problem may have masses $m_a$ in the extended range $10^{-3}eV > m_a > 10^{-33} eV$. Ultralight axions provide so many degrees of freedom that their entropy can exceed that of primordial black holes. This suggests that ultralight axions are more suited than primordial black holes to be constituents of dark matter., 12 pages

We discuss a possibility that a superconformal dynamics induces the emergence of a global $U(1)_{\rm PQ}$ symmetry to solve the strong CP problem through the axion. Fields spontaneously breaking the $U(1)_{\rm PQ}$ symmetry couple to new quarks charged under the ordinary color $SU(3)_C$ and a new $SU(N)$ gauge group. The theory flows into an IR fixed point where the $U(1)_{\rm PQ}$ breaking fields hold a large anomalous dimension leading to the suppression of $U(1)_{\rm PQ}$-violating higher dimensional operators. The spontaneous breaking of the $U(1)_{\rm PQ}$ makes the new quarks massive. The $U(1)_{\rm PQ}$ symmetry is anomalous under the $SU(3)_C$ but not under the $SU(N)$ so that the axion couples to only the color $SU(3)_C$ and the usual axion potential is generated. We also comment on a model that the $U(1)_{\rm PQ}$ breaking fields are realized as meson superfields in a new supersymmetric QCD., 8 pages, 3 figures, published version

We discuss composite UV completions of the Nelson-Barr(NB) solution to the strong CP problem. In our construction, the CP symmetry is broken spontaneously by the dynamics of a hidden QCD at $\theta=\pi$. We focus on the minimal implementation of the NB construction where the visible sector contains one extra pair of vector-like up/down quarks. We show that the minimal NB theory suffers from a quality problem, and discuss how composite UV completions may resolve it. We present a simple calculable scheme, free of a quality problem, where dynamical CP violation in the hidden sector is mediated through a scalar portal to the visible sector, which successfully realizes the NB construction., Comment: 28 pages, 6 Figures

Journal of cosmology and astroparticle physics 2021(10), 001 (2021). doi:10.1088/1475-7516/2021/10/001, We show that dark matter can be accounted for by an axion that solves the strong CP problem, but is much lighter than usual due to a $Z_\mathcal{N}$ symmetry. The whole mass range from the canonical QCD axion down to the ultra-light regime is allowed, with $3\le\mathcal{N}\lesssim65$. This includes the first proposal of a 'fuzzy dark matter' QCD axion with $m_a\sim10^{-22}$ eV. A novel misalignment mechanism occurs - trapped misalignment - due to the peculiar temperature dependence of the $Z_{\mathcal{N}}$ axion potential. The dark matter relic density is enhanced because the axion field undergoes two stages of oscillations: it is first trapped in the wrong minimum, which effectively delays the onset of true oscillations. Trapped misalignment is more general than the setup discussed here, and may hold whenever an extra source of Peccei-Quinn breaking appears at high temperatures. Furthermore, it will be shown that trapped misalignment can dynamically source the recently proposed kinetic misalignment mechanism. All the parameter space is within tantalizing reach of the experimental projects for the next decades. For instance, even Phase I of CASPEr-Electric could discover this axion., Published by IOP, London

An axion-like particle (ALP) with mass mϕ ∼ 10−15 eV oscillates with frequency ∼1 Hz. This mass scale lies in an open window of astrophysical constraints, and appears naturally as a consequence of grand unification (GUT) in string/M-theory. However, with a GUT-scale decay constant such an ALP overcloses the Universe, and cannot solve the strong CP problem. In this paper, we present a two axion model in which the 1 Hz ALP constitutes the entirety of the dark matter (DM) while the QCD axion solves the strong CP problem but contributes negligibly to the DM relic density. The mechanism to achieve the correct relic densities relies on low-scale inflation (mϕ ≲ Hinf ≲ 1 MeV), and we present explicit realisations of such a model. The scale in the axion potential leading to the 1 Hz axion generates a value for the strong CP phase which oscillates around $$ {\overline{\theta}}_{\mathrm{QCD}}\sim {10}^{-12} $$ θ ¯ QCD ∼ 10 − 12 , within reach of the proton storage ring electric dipole moment experiment. The 1 Hz axion is also in reach of near future laboratory and astrophysical searches.

We perform a global analysis of the low-energy phenomenology of the minimal left-right symmetric model (mLRSM) with parity symmetry. We match the mLRSM to the Standard Model Effective Field Theory Lagrangian at the left-right-symmetry breaking scale and perform a comprehensive fit to low-energy data including mesonic, neutron, and nuclear $\beta$-decay processes, $\Delta F=1$ and $\Delta F=2$ CP-even and -odd processes in the bottom and strange sectors, and electric dipole moments (EDMs) of nucleons, nuclei, and atoms. We fit the Cabibbo-Kobayashi-Maskawa and mLRSM parameters simultaneously and determine a lower bound on the mass of the right-handed $W_R$ boson. In models where a Peccei-Quinn mechanism provides a solution to the strong CP problem, we obtain $M_{W_R} \gtrsim 5.5$ TeV at $95\%$ C.L. which can be significantly improved with next-generation EDM experiments. In the $P$-symmetric mLRSM without a Peccei-Quinn mechanism we obtain a more stringent constraint $M_{W_R} \gtrsim 17$ TeV at $95\%$ C.L., which is difficult to improve with low-energy measurements alone. In all cases, the additional scalar fields of the mLRSM are required to be a few times heavier than the right-handed gauge bosons. We consider a recent discrepancy in tests of first-row unitarity of the CKM matrix. We find that, while TeV-scale $W_R$ bosons can alleviate some of the tension found in the $V_{ud,us}$ determinations, a solution to the discrepancy is disfavored when taking into account other low-energy observables within the mLRSM., Comment: 42 pages plus appendices. Published version

The novel PQ mechanism replaces the strong CP problem with some challenges in a model building. In particular, the challenges arise regarding i) the origin of an anomalous global symmetry called a PQ symmetry, ii) the scale of the PQ symmetry breaking, and iii) the quality of the PQ symmetry. In this letter, we provide a natural and simple UV completed model that addresses these challenges. Extra quarks and anti-quarks are separated by two branes in the Randall-Sundrum ${\bf R}^4 \times S^1 / {\bf Z}_2$ spacetime while a hidden SU($N_H$) gauge field condensates in the bulk. The brane separation is the origin of the PQ symmetry and its breaking scale is given by the dynamical scale of the SU($N_H$) gauge interaction. The (generalized) Casimir force of SU($N_H$) condensation stabilizes the 5th dimension, which guarantees the quality of the PQ symmetry., Comment: 6 pages, 1 figure

The axion is much lighter than all other degrees of freedom introduced by the Peccei-Quinn mechanism to solve the strong CP problem. It is therefore natural to use an effective field theory (EFT) to describe its interactions. Loop processes calculated in the EFT may, however, explicitly depend on the ultraviolet cutoff. In general the UV cutoff is not uniquely defined, but the dimensionful couplings suggest to identify it with the Peccei-Quinn symmetry-breaking scale. An example are $K \rightarrow \pi + a$ decays that will soon be tested to improved precision in NA62 and KOTO and whose amplitude is dominated by the term logarithmically dependent on the cutoff. In this paper, we critically examine the adequacy of using such a naive EFT approach to study loop processes by comparing EFT calculations with ones performed in complete QCD axion models. In DFSZ models, for example, the cutoff is found to be set by additional Higgs degrees of freedom and to therefore be much closer to the electroweak scale than to the Peccei-Quinn scale. In fact, there are non-trivial requirements on axion models where the cutoff scale of loop processes is close to the Peccei-Quinn scale, such that the naive EFT result is reproduced. This suggests that the existence of a suitable UV embedding may impose restrictions on axion EFTs. We provide an explicit construction of a model with suitable fermion couplings and find promising prospects for NA62 and IAXO., Comment: 28 pages + appendices. v3: revised text, main results unchanged

We propose a framework where a phase transition associated with a gauge symmetry breaking that occurs (not far) above the electroweak scale sets a stage for baryogenesis similar to the electroweak baryogenesis in the Standard Model. A concrete realization utilizes the breaking of $SU(2)_R \times U(1)_X \rightarrow U(1)_Y$. New chiral fermions charged under the extended gauge symmetry have nonzero lepton numbers, which makes the $B-L$ symmetry anomalous. The new lepton sector contains a large flavor-dependent CP violation, similar to the Cabibbo-Kobayashi-Maskawa phase, without inducing sizable electric dipole moments of the Standard Model particles. A bubble wall dynamics associated with the first-order phase transition and $SU(2)_R$ sphaleron processes generate a lepton asymmetry, which is transferred into a baryon asymmetry via the ordinary electroweak sphaleron process. Unlike the Standard Model electroweak baryogenesis, the new phase transition can be of the strong first order and the new CP violation is not significantly suppressed by Yukawa couplings, so that the observed asymmetry can be produced. The model can be probed by collider searches for new particles and the observation of gravitational waves. One of the new leptons becomes a dark matter candidate. The model can be also embedded into a left-right symmetric theory to solve the strong CP problem., Comment: 61 pages, 9 figures

The axion is a dark matter candidate arising from the spontaneous breaking of the Peccei–Quinn symmetry, introduced to solve the strong CP problem. It has been shown that radio/microwave radiation sent out to space is backscattered in the presence of axion dark matter due to stimulated axion decay. This backscattering is a feeble and narrow echo signal centered at an angular frequency very close to one-half of the axion mass. In this article, we summarize all the relevant results found so far, including analytical formulas for the echo signal, as well as sensitivity prospects for possible near-future experiments.

The experimental non-observation of charge-parity (CP) symmetry violation in quantum chromodynamics is commonly referred to as the strong CP problem. The axion, a hypothetical elementary pseudoscalar, is expected to solve the strong CP problem and is also a promising candidate for dark matter. The most sensitive axion search experiments operate at millikelvin temperatures and hence rely on instrumentation that carries signals from a system at cryogenic temperatures to room temperature instrumentation. One of the biggest limiting factors affecting the parameter scanning speed of these detectors is the noise added by the components in the signal detection chain. Since the first amplifier in the chain limits the minimum noise, low-noise amplification is of paramount importance. This paper reports on the operation of a flux-driven Josephson parametric amplifier (JPA) operating at around 2.3 GHz with added noise approaching the quantum limit. The JPA was employed as a first stage amplifier in an experimental setting similar to the ones used in haloscope axion detectors. By operating the JPA at a gain of 19 dB and cascading it with two cryogenic amplifiers operating at 4 K, noise temperatures as low as 120 mK were achieved for the whole signal detection chain.

Parity solutions to the strong CP problem are a compelling alternative to approaches based on Peccei-Quinn symmetry, particularly given the expected violation of global symmetries in a theory of quantum gravity. The most natural of these solutions break parity at a low scale, giving rise to a host of experimentally accessible signals. We assess the status of the simplest parity-based solution in light of LHC data and flavor constraints, highlighting the prospects for near-future tests at colliders, tabletop experiments, and gravitational wave observatories. The origin of parity breaking and associated gravitational effects play crucial roles, providing new avenues for discovery through EDMs and gravity waves. These experimental opportunities underline the promise of generalized parity, rather than Peccei-Quinn symmetry, as a robust and testable solution to the strong CP problem.

To address five fundamental shortcomings of the Standard Model (SM) of particle physics and cosmology, we propose a SMART U(1)$_X$ model which is a $U(1)_X \times U(1)_{PQ}$ extension of the SM. The $U(1)_X$ gauge symmetry is a generalization of the well-known $U(1)_{B-L}$ symmetry and $U(1)_{PQ}$ is the global Peccie-Quinn (PQ) symmetry. Three right handed neutrinos are added to cancel $U(1)_X$ related anomalies, and they play a crucial role in understanding the observed neutrino oscillations and explaining the observed baryon asymmetry in the universe via leptogenesis. The PQ symmetry helps resolve the strong CP problem and also provides axion as a compelling dark matter (DM) candidate. The $U(1)_X$ gauge symmetry enables us to implement the inflection-point inflation scenario with $H_{inf} \lesssim 2 \times 10^{7}$ GeV, where $H_{inf}$ is the value of Hubble parameter during inflation. This allows us to overcome a potential axion domain wall problem as well as the axion isocurvature problem. The SMART U(1)$_X$ model can be merged with $SU(5)$ as we briefly show., 18 pages, 2 figures

We propose two models where a U(1) Peccei-Quinn global symmetry arises accidentally and is respected up to high-dimensional operators, so that the axion solution to the strong CP problem is successful even in the presence of Planck-suppressed operators. One model is SU($$ \mathcal{N} $$ N ) gauge interactions with fermions in the fundamental and a scalar in the symmetric. The axion arises from spontaneous symmetry breaking to SO($$ \mathcal{N} $$ N ), that confines at a lower energy scale. Axion quality in the model needs $$ \mathcal{N} $$ N ≳ 10. SO bound states and possibly monopoles provide extra Dark Matter candidates beyond the axion. In the second model the scalar is in the anti-symmetric: SU($$ \mathcal{N} $$ N ) broken to Sp($$ \mathcal{N} $$ N ) needs even $$ \mathcal{N} $$ N ≳ 20. The cosmological DM abundance, consisting of axions and/or super-heavy relics, can be reproduced if the PQ symmetry is broken before inflation (Boltzmann-suppressed production of super-heavy relics) or after (super-heavy relics in thermal equilibrium get partially diluted by dark glue-ball decays).

The Nelson-Barr mechanism to solve the strong CP problem requires vector-like quarks (VLQs) to transmit the spontaneous CP breaking to the SM. We study the scenario where only these VLQs are within reach at the TeV scale while the spontaneous CP breaking sector is inaccessible. We investigate how these VLQs of Nelson-Barr type differ from generic VLQs and find from parameter counting that one less parameter is needed. In particular, for one VLQ of Nelson-Barr type, there is only one CP odd quantity that is responsible for all CP violation. In this case, we solve the technical problem of parametrizing only the new physics parameters while keeping the SM parameters as independent inputs. For one down-type VLQ, the model is largely flavor safe because the VLQ couplings to the SM up quarks and the $W$ are hierarchically smaller for lighter quarks., Slightly updates; references and appendix added. Comply with published version