arXiv subjects

Kiwoon Choi

Publications and source records attributed to Kiwoon Choi.

At least 19 recordsLinked to original sources

arXiv2026

The EDM inverse problem: Identifying the sources of CP violation and PQ breaking with electric dipole moments

Many extensions of the Standard Model (SM) generically introduce new sources of CP violation, which can induce observable $P$-odd and $T$-odd permanent electric dipole moments (EDMs) of nuclei, atoms, and molecules. A future observation of nonvanishing EDMs would therefore provide a sensitive probe of physics beyond the SM, while also posing a nontrivial inverse problem: identifying their underlying ultraviolet origin. In this work, we identify six representative classes of CP-violating effective operators near the QCD scale, including the QCD $\theta$-term, that are particularly relevant for low-energy EDMs and can arise in a broad range of SM extensions. We show that these operator classes lead to distinct EDM patterns across different systems, thereby enabling discrimination among them through experimentally measured EDMs. We further emphasize that EDM measurements can shed light on the origin of the vacuum expectation value of the QCD axion. In particular, they may help distinguish whether a nonzero axion vacuum expectation value is predominantly induced by high-scale Peccei--Quinn symmetry-breaking effects, such as those associated with quantum gravity, or by the interplay between beyond-the-SM CP violation and the QCD anomaly.

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arXiv2026

Axion Quality in Warped Extra-Dimension

We investigate the axion quality problem in warped extra-dimensional models in which the QCD axion arises as the Wilson-line mode of a five-dimensional $U(1)$ gauge field compactified on an $S^1/\mathbb{Z}_2$ orbifold. Higher-dimensional gauge invariance severely constrains possible sources of Peccei--Quinn symmetry breaking, implying that non-QCD contributions to the axion potential are predominantly generated by nonlocal effects mediated by $U(1)$-charged fields propagating along the compact dimension. We systematically compute these contributions and examine how both the warped geometry and the orbifold fixed points (branes) affect the resulting axion quality. Finally, we classify the parametric suppression of the induced axion potential, thereby identifying the conditions under which warped extra-dimensional axions can achieve sufficiently high quality.

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arXiv2026

The COSMIC WISPers White Paper: The physics case for Weakly Interacting Slim Particles

Axions and other very weakly interacting slim particles (WISPs), with masses below 1 GeV, arise naturally in many extensions of the Standard Model of particle physics. In particular, they could offer a new framework to explain the nature of dark matter and may help address a range of puzzling observations in astrophysics and particle physics. This review provides an overview of ongoing WISP searches and outlines the prospects for the next decade, spanning their theoretical motivation, indirect signatures in astrophysical observations, and dedicated laboratory experiments. It is based on the work carried on by the EU-funded COST Action ``Cosmic WISPers in the Dark Universe: Theory, astrophysics, and experiments'' (CA21106, https://www.cost.eu/actions/CA21106). This network plays a key role in coordinating and supporting WISP searches across Europe, while also contributing to the development of a roadmap aimed at securing European leadership in this research area. It is emphasized that Europe is currently pursuing a rich, diverse, and cost-effective experimental program, with the potential to deliver one or more transformative discoveries.

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arXiv2024

Axion Theory and Model Building

Axions are light pseudoscalar bosons postulated with many motivations in particle physics and cosmology, including the strong CP problem and the dark matter in our Universe. In this lecture notes, we discuss a variety of known ultraviolet (UV) theories for axions and their low energy properties. We are primarily concerned with the quantum chromodynamics axion solving the strong CP problem, as well as lighter axion-like particles. In regard to their UV origin, such light axions may arise from the spontaneous breakdown of a linearly realized global Peccei-Quinn U(1) symmetry in the context of 4-dimensional effective field theories, or they may originate from a gauge field in higher dimensional theories. It is noted that different UV models for these axions predict a distinctive pattern of low energy axion couplings, which may have interesting implications for laboratory, astrophysical, or cosmological studies of axions. We also provide an introductory discussion of the effective field theory for axions from p-form gauge fields in string theory with concrete examples.

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arXiv2023

Exploring CP Violation beyond the Standard Model and the PQ Quality with Electric Dipole Moments

In some models of physics beyond the Standard Model (SM), one of the leading low energy consequences of the model appears in the form of the chromo-electric dipole moments (CEDMs) of the gluons and light quarks. We examine if these CEDMs can be distinguished from the QCD $\theta$-term through the experimentally measurable nuclear and atomic electric dipole moments (EDMs) in both cases with and without the Peccei-Quinn (PQ) mechanism solving the strong CP problem. We find that the nucleon EDMs show a distinctive pattern when the EDMs are dominantly induced by the light quark CEDMs without the PQ mechanism. In the presence of the PQ mechanism, the QCD $\theta$-parameter corresponds to the vacuum value of the axion field, which might be induced either by CEDMs or by UV-originated PQ breaking other than the QCD anomaly, for instance the PQ breaking by quantum gravity effects. We find that in case with the PQ mechanism the nucleon EDMs have a similar pattern regardless of what is the dominant source of EDMs among the CEDMs and $\theta$-term, unless there is a significant cancellation between the contributions from different sources. In contrast, some nuclei or atomic EDMs can have characteristic patterns significantly depending on the dominant source of EDMs, which may allow identifying the dominant source among the CEDMs and $\theta$-term. Yet, discriminating the gluon CEDM from the QCD $\theta$-parameter necessitates additional knowledge of low energy parameters induced by the gluon CEDM, which is not available at the moment. Our results imply that EDMs can reveal unambiguous sign of CEDMs while identifying the origin of the axion vacuum value, however it requires further knowledge of low energy parameters induced by the gluon CEDM.

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arXiv2023

Axion dark matter with thermal friction

Cosmological evolution of axion field in the early universe might be significantly affected by a thermal friction induced by the axion coupling to thermalized hidden sector. We examine the effects of such a thermal friction on axion dark matter density and its perturbation when the thermal friction dominates over the Hubble friction until when the axion field begins to oscillate around the potential minimum. We show that in the presence of sizable thermal friction there can be an exponential decay phase of the axion field before the oscillation phase, during which the axion energy density is efficiently dissipated into hidden thermal bath. Consequently, the previously excluded parameter region due to overclosing relic axion density becomes cosmologically viable with thermal friction. In particular, a QCD axion much lighter than $μ$eV is viable without tuning the initial misalignment angle. We also find that thermal friction can affect the density perturbation of axion dark matter in various ways. For instance, it can alleviate the large-scale isocurvature bound on axion dark matter in the pre-inflationary PQ breaking scenario, which would make the pre-inflationary axion dark matter compatible with high scale inflation over a wide range of model parameters. In the post-inflationary PQ breaking scenario, thermal friction can also significantly change the scaling behavior of axionic strings, and therefore the typical size of the resultant axion miniclusters.

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arXiv2022

Axion emission from supernova with axion-pion-nucleon contact interaction

We examine the axion emission from supernovae with a complete set of relevant axion couplings including the axion-pion-nucleon contact interaction which was ignored in the previous studies. Two processes are affected by the axion-pion-nucleon contact interaction, $π^-+p \rightarrow n + a$ and $n+p\rightarrow n+p+a$, and these processes can be the dominant source of axions for some region in the axion parameter space or in astrophysical conditions encountered inside supernovae. We find that the contact interaction can enhance the axion emissivity of $π^-+p \rightarrow n + a$ by a factor of $2-4$, while the effect on $n+p\rightarrow n+p+a$ is not significant. We also discuss the relative importance of other pion-induced processes such as $π^0+n\rightarrow n+a$ and $π^-+π^0\rightarrow π^-+a$.

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arXiv2021

Precision axion physics with running axion couplings

We study the renormalization group running of axion couplings while taking into account that the Standard Model can be extended to its supersymmetric extension at a certain energy scale below the axion decay constant. We then apply our results to three different classes of axion models, i.e. KSVZ-like, DFSZ-like, and string-theoretic axions, and examine if string-theoretic axions can be distinguished from others by having a different pattern of low energy couplings to the photon, nucleons and electron. We find that the low energy couplings of string-theoretic axions have a similar pattern as those of KSVZ-like axions but yet reveal a sizable difference which might be testable in future axion search experiments. We also note that the coupling of KSVZ-like QCD axions to the electron is dominated by a three-loop contribution involving the exotic heavy quark, gluons, top quark and Higgs field.

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arXiv2021

Recent Progress in the Physics of Axions and Axion-Like Particles

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.

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arXiv2020

Gamma-ray spectral modulations induced by photon-ALP-dark photon oscillations

Recently it has been noticed that the Fermi-LAT data of gamma-rays from some galactic pulsars and supernova remnants reveal spectral modulations that might be explained by the conversion of photons to ALPs (axion-like particles) induced by the conventional ALP coupling to photon in the presence of galactic magnetic fields. However the corresponding ALP mass and coupling are in a severe tension with the observational constraints from CAST, SN1987A, and other gamma-ray observations. Motivated by this, we examine an alternative possibility that those spectral modulations are explained by other type of ALP coupling involving both the ordinary photon and a massless dark photon, when nonzero background dark photon gauge fields are assumed. We find that our scheme results in oscillations among the photon, ALP, and dark photon, which can explain the gamma-ray spectral modulations of galactic pulsars or supernova remnants, while satisfying the known observational constraints.

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arXiv2019

Light Higgs bosons in the general NMSSM

Physics beyond the Standard Model (SM) may manifest itself as small deviations from the SM predictions for Higgs signal strengths at 125 GeV. Then, a plausible and interesting possibility is that the Higgs sector is extended and at the weak scale there appears an additional Higgs boson weakly coupled to the SM sector. Combined with the LEP excess in $e^+e^-\to Z(h\to b\bar b)$, the diphoton excess around 96 GeV recently reported by CMS may suggest such a possibility. We examine if those LEP and CMS excesses can be explained simultaneously by a singlet-like Higgs boson in the general next-to-minimal supersymmetric Standard Model (NMSSM). Higgs mixing in the NMSSM relies on the singlet coupling to the MSSM Higgs doublets and the higgsino mass parameter, and thus is subject to the constraints on these supersymmetric parameters. We find that the NMSSM can account for both the LEP and CMS excesses at 96 GeV while accommodating the observed 125 GeV SM-like Higgs boson. Interestingly, the required mixing angles constrain the heavy doublet Higgs boson to be heavier than about 500 GeV. We also show that the viable region of mixing parameter space is considerably modified if the higgsino mass parameter is around the weak scale, mainly because of the Higgs coupling to photons induced by the charged higgsinos.

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arXiv2019

Axion scales and couplings with Stückelberg mixing

We study the axion field range and low energy couplings in models with Stückelberg mixing between axions and $U(1)$ gauge bosons. It is noted that the gauge-invariant axion combination $ξ$ in the model is periodic $modulo$ an appropriate shift of gauge-variant axions eaten by the massive $U(1)$ gauge bosons, which in some cases makes the connection between the field range and the low energy couplings less transparent. We derive the field range of $ξ$ for generic forms of the axion kinetic metric and $U(1)$ charges, and identify the field basis for which all non-derivative couplings of $ξ$ are quantized in a manner manifestly consistent with the periodicity of $ξ$. Generically Stückelberg mixing reduces the axion field range. In particular, the mixings between $N$ axions and $(N-1)$ $U(1)$ gauge bosons typically result in an exponentially reduced field range $M_ξ={\cal O}\left({k^{-(N-1)} f}/{\sqrt{N!}}\right)$ for the residual gauge-invariant axion $ξ$ in the limit $N\gg 1$, where $f$ and $k$ denote the typical decay constant and the root mean square of the $U(1)$ gauge charges of the original $N$ axions. Using simple examples, we study also the reparameterization-invariant physical quantities such as the axion effective potential and 1PI couplings to gauge bosons, which are determined by the reparameterization-dependent axion couplings in the model.

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arXiv2019

Axion-photon-dark photon oscillation and its implication for 21 cm observation

We examine the resonant conversion of axion-like particle (ALP) or dark photon to the electromagnetic photon in the early Universe, which takes place due to the ALP-photon-dark photon oscillations in background dark photon gauge fields. It is noted that the corresponding conversion probability can have an unusual spectral feature which allows strong conversion at low frequency domain, but has negligible conversion at high frequencies above certain critical frequency which is determined by the ALP coupling to dark photon and the strength of background dark photon gauge field. We apply this scheme to heat up the 21 cm photons without affecting the Cosmic Microwave Background, which can explain the tentative absorption signal of 21 cm photons detected recently by the EDGES experiment.

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arXiv2018

The dS swampland conjecture with the electroweak symmetry and QCD chiral symmetry breaking

The dS swampland conjecture $|\nabla V|/V \geq c$, where $c$ is presumed to be a positive constant of order unity, implies that the dark energy density of our Universe can not be a cosmological constant, but mostly the potential energy of an evolving quintessence scalar field. As the dark energy includes the effects of the electroweak symmetry breaking and the QCD chiral symmetry breaking, if the dS swampland conjecture is applicable for the low energy quintessence potential, it can be applied for the Higgs and pion potential also. On the other hand, the Higgs and pion potential has the well-known dS extrema, and applying the dS swampland conjecture to those dS extrema may provide stringent constraints on the viable quintessence, as well as on the conjecture itself. We examine this issue and find that the pion dS extremum at $\cos(π_0/f_π)=-1$ implies $c\lesssim {\cal O}(10^{-2}-10^{-5})$ for $arbitrary$ form of the quintessence potential and couplings, where the weaker bound ($10^{-2}$) is available $only$ for a specific type of quintessence whose couplings respect the equivalence principle, while the stronger bound ($10^{-5}$) applies for generic quintessence violating the equivalence principle. We also discuss the possibility to relax this bound with an additional scalar field, e.g. a light modulus which has a runaway behavior at the pion dS extremum. We argue that such possibility is severely constrained by a variety of observational constraints which do not leave a room to significantly relax the bound. We make a similar analysis for the Higgs dS extremum at $H=0$, which results in a weaker bound on $c$.

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arXiv2018

General Continuum Clockwork

The continuum clockwork is an extra-dimensional set-up to realize certain features of the clockwork mechanism generating exponentially suppressed or hierarchical couplings of light particles. We study the continuum clockwork in a general scheme in which large volume, warped geometry, and localization of zero modes in extra dimension are described by independent parameters. For this, we propose a generalized 5-dimensional linear dilaton model which can realize such set-up as a solution of the model, and examine the KK spectrum and the couplings of zero modes and massive KK modes to boundary-localized operators for the bulk graviton, Abelian gauge bosons and periodic scalar fields. We discuss how those KK spectra and couplings vary as a function of the volume, warping and localization parameters, and highlight the behavior in the parameter region corresponding to the clockwork limit. We discuss also the field range of 4-dimensional axions originating from either 5-dimensional periodic scalar field or the 5-th component of an Abelian gauge field, and comment on the limitations of continuum clockwork compared to the discrete clockwork.

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arXiv2018

Late-time magnetogenesis driven by ALP dark matter and dark photon

We propose a mechanism generating primordial magnetic fields after the $e^+e^-$ annihilations. Our mechanism involves an ultra-light axion-like particle (ALP) which constitutes the dark matter, and a dark $U(1)_X$ gauge boson introduced to bypass the obstacle placed by the conductivity of cosmic plasma. In our scheme, a coherently oscillating ALP amplifies the dark photon field, and part of the amplified dark photon field is concurrently converted to the ordinary magnetic field through the ALP-induced magnetic mixing. For the relevant ALP mass range $10^{-21} {\rm eV}\lesssim m_ϕ\lesssim 10^{-17}{\rm eV}$, our mechanism can generate $B\sim 10^{-24} \,{\rm G} \,(m_ϕ/10^{-17} {\rm eV})^{5/4}$ with a coherent length $λ\sim (m_ϕ/10^{-17} {\rm eV})^{-1/2}$ kpc, which is large enough to provide a seed of the galactic magnetic fields. The mechanism also predicts a dark $U(1)_X$ electromagnetic field $E_X \sim B_X\sim 80\,{\rm nG}\, (m_ϕ/10^{-17}{\rm eV})^{-1/4}$, which can result in interesting astrophysical/cosmological phenomena by inducing the mixings between the ALP, ordinary photon, and dark photon states.

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arXiv2017

Minimal Flavor Violation with Axion-like Particles

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 discuss also the implication of our result for string theoretic ALP originating from higher-dimensional $p$-form gauge fields, for instance for ALP in large volume string compactification scenario.

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arXiv2017

Dynamics of the cosmological relaxation after reheating

We examine if the cosmological relaxation mechanism, which was proposed recently as a new solution to the hierarchy problem, can be compatible with high reheating temperature well above the weak scale. As the barrier potential disappears at high temperature, the relaxion rolls down further after the reheating, which may ruin the successful implementation of the relaxation mechanism. It is noted that if the relaxion is coupled to a dark gauge boson, the new frictional force arising from dark gauge boson production can efficiently slow down the relaxion motion, which allows the relaxion to be stabilized after the electroweak phase transition for a wide range of model parameters, while satisfying the known observational constraints.

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