arXiv subjects

Taewook Youn

Publications and source records attributed to Taewook Youn.

18 recordsLinked to original sources

arXiv2026

Mixing-suppressed inelastic dark matter: a minimal model for the LZ 248 keV event

We construct a minimal Majorana singlet-vector-like-doublet model for the single nuclear-recoil-like event reported by LZ near 248 keV. At the splitting inferred from the recoil energy, an electroweak-strength $Z$ transition predicts thousands of events; singlet-doublet mixing suppresses the rate without moving the recoil spectrum. Fixed-coupling interpretations instead require a larger splitting in the extreme halo tail, but solar gravitational acceleration removes this suppression, while the larger splitting shifts recoils toward LZ's empty high-energy sideband. In our model the same mixing suppresses solar capture and annihilation, while the small mass gap required for coannihilation weakens Higgs-mediated cooling, allowing the captured population to remain extended and out of equilibrium. Imposing the relic abundance and normalizing the rate to one event leaves a two-dimensional mass-splitting parameter space. A thermalized population gives the conservative IceCube limit $\delta=$300-301 keV; with our nonthermal cooling ansatz and elastic scattering treated at tree level, the nominal limit is $\delta=$331-341 keV near the 730-733 GeV relic-density endpoint, which is also favored by the recoil spectrum. We outline a gauged $U(1)_N$ origin for the parity and splitting and a candidate $R$-symmetric supersymmetric embedding.

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arXiv2026

Probing CP-violating top-quark dipole moments with tomographic observables

Quantum tomography program reconstructs the full spin density matrix of top-quark pairs from dilepton angular correlations at both electron-positron and hadron colliders. We use symmetry arguments to identify tomographic observables sensitive to CP violation in top quark couplings. As a concrete example, we study the sensitivity of these observables to (chromo-)electric dipole moment operators at the LHC and the FCC-ee. We also construct the optimal observable, which combines tomographic and kinematic information to achieve statistically optimal sensitivity for each operator and production process. The analysis is based on fast detector simulation of top pair production and decay in the dilepton channel. We project a $95\%$ CL sensitivity to new-physics scales (the inverse dipole couplings) of $10-15$ TeV for the electroweak electric dipoles at the FCC-ee ($365$ GeV, $3~{\rm ab}^{-1}$) and of 40 (200) TeV for the top chromo-electric dipole moment at the LHC Run 2 (HL-LHC). This sensitivities exceed those of the traditional observables currently used by experiments to use for these operators, indicating the power of the tomographic approach to search for CP violation.

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arXiv2026

The Holographic QCD Axion in Five Dimensions

We present a holographic construction of the QCD axion based on a warped 5D model. A key ingredient of our setup is the introduction of a bulk scalar field $\theta$, which is holographically dual to the topological operator of QCD. This makes the relation among the axion, the $\eta'$, and the anomalies transparent. We identify the bulk modes corresponding to the $\eta'$ and axion states, and show that an adjustment analogous to that of the usual 4D axion takes place. We identify the origin of the axion quality problem in this framework and show that a large degree of axion compositeness is needed to solve it. We also find that, in the limit of a high quality axion, the physical axion state is predominantly contained in the bulk gauge field.

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arXiv2026

$\theta$ Angle and Axial Anomaly in Holographic QCD

We present a bottom-up holographic description of the QCD $\theta$-vacuum and the $U(1)_A$ anomaly in five dimensions. The multi-branched $\theta$-vacuum structure emerges geometrically from a higher-dimensional gauge field, while the axial anomaly is realized through a St\"uckelberg coupling that is dual to a Chern-Simons term. In this framework, the $\eta'$ meson appears as a zero mode of bulk fluctuations, and its mass arises from the anomaly-induced St\"uckelberg term. The construction provides a transparent holographic derivation of the anomaly contribution to the $\eta'$ mass and naturally reproduces the Witten-Veneziano relation between the $\eta'$ mass and the Yang-Mills topological susceptibility.

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arXiv2026

Constraints on Loryons in a Two Higgs Doublet Model

We consider Loryons, particles beyond the Standard Model that receive a significant fraction of their masses from electroweak symmetry breaking, in the context of a two Higgs doublet model. Using scalar Loryons in the $[1,1]$, $[1,3]$ (as well as the equivalent $[3,1]$) and the $[2,2]$ representations of the custodial $SU(2)_L \times SU(2)_R$ global symmetry as benchmarks, we study the constraints on the Loryon parameter space, focusing on unitarity, Higgs decay observables, and the absence of Loryon vacuum expectation values. We find that while neutral singlet Loryons remain viable for masses up to 700 GeV, representations containing charged scalars are severely constrained by LHC data, particularly as the fraction of mass generated by symmetry breaking increases.

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arXiv2025

Dark Matter-Dark Radiation Interactions and the Hubble Tension

Models in which a subcomponent of dark matter interacts with dark radiation have been proposed as a solution to the Hubble tension. In this framework, the interacting subcomponent of dark matter is in thermal equilibrium with the dark radiation in the early universe, but decouples from it around the time of matter-radiation equality. We study this general class of models and evaluate the quality of fit to recent cosmological data on the cosmic microwave background (from Planck 2018 and ACT DR6), baryon acoustic oscillations, large-scale structure, supernovae type Ia, and Cepheid variables. We focus on three benchmark scenarios that differ in the rate at which the dark matter decouples from the dark radiation, resulting in different patterns of dark acoustic oscillations. Fitting without ACT DR6 data, we find that all three scenarios significantly reduce the Hubble tension relative to $\Lambda$CDM, with an exponentially fast decoupling being the most preferred. The tension is reduced to less than $2 \, \sigma$ in fits that don't include the SH0ES collaboration results as part of the data and to less than $1 \, \sigma$ when these are included. When ACT DR6 data is included, the fit is significantly worsened. We find that the largest $H_0$ value at the $95 \%$ confidence region is $70.1$ km/s/Mpc without the SH0ES data, leading to only a mild reduction in the tension. This increases to $72.5$ km/s/Mpc, corresponding to a reduction in the tension to less than $3 \, \sigma$, if the SH0ES results are included in the fit.

astro-ph.CO↗

arXiv2025

Collider Searches for Near-Continuum Dark Matter

We study collider constraints on the near-continuum dark matter model, in which the dark sector consists of a tower of closely spaced states with weak-scale masses coupled to the Standard Model through a $Z$-portal. To capture this structure in a model-agnostic way, we introduce a minimal parameterization that encodes the dominant geometric information with three parameters. Using a custom-built Monte-Carlo tool for near-continuous spectra, we simulate DM-pair production at $\sqrt{s}=13$ TeV and subsequent cascade decays via on/off-shell $Z$ bosons, which yield events with large missing transverse momentum and high jet multiplicity. Recasting the CMS multijet$+H_T^{\rm miss}$ analysis of Run-2 data (35.9 fb$^{-1}$), we derive bounds on the model parameter space. Extrapolating these bounds, we provide High-Luminosity LHC projections (3 ab$^{-1}$). We also project sensitivities for future electron-positron colliders at $\sqrt{s}=365$ GeV and $\sqrt{s}=500$ GeV, showing substantial improvements over the HL-LHC. The current LHC sensitivity is beginning to approach the theoretically motivated region of the parameter space, while future colliders will be able to comprehensively test this model.

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arXiv2025

Strong CP from a Hidden Chiral Condensate

Models which solve the strong CP problem by employing discrete spacetime symmetries generically suffer fine-tuning and quality problems. We demonstrate that these issues are greatly ameliorated when the only source of spontaneous CP breaking is from the chiral condensate of a strongly coupled hidden sector. This is shown explicitly in a model with the SM extended by a vector-like quark family and a complex scalar portal to QCD-like dark sector with $N_f$ families of dark fermions that confines at a high scale. The dark pions of the hidden sector are natural dark matter candidates, with the correct relic abundance obtained via freeze-in. These "confining" Nelson-Barr solutions connect phenomenological questions regarding the strong CP problem to recent developments in the understanding of confining gauge theories, and present ample room for further model building.

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arXiv2025

Dynamical Up-quark Mass Generation in QCD-like theories

We calculate the dynamically generated up quark mass in some QCD-like theories with $F=3$ light flavors, obtained from supersymmetric QCD perturbed via anomaly mediated supersymmetry breaking. We match the low-energy effective theory to the traditional chiral Lagrangian of QCD and determine the coefficients to next-to-leading order in chiral perturbation theory, while also varying the number of colors $N$. We find that the dynamically generated up quark mass vanishes in the large $N$ limit, and is small for $F<N$, however for $F=N$ there is a sizeable $O(1)$ contribution. While our results are reliable only for small supersymmetry breaking, we observe that extrapolating the $F=N$ result to large supersymmetry breaking would lead to a dynamical up quark mass that is large enough to account for its entire physical mass.

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arXiv2024

Spontaneous CP Breaking in a QCD-like Theory

We examine the phase structure of a QCD-like theory at $\barθ=π$ obtained from supersymmetric $SU(N)$ QCD perturbed by a small amount of supersymmetry breaking via anomaly mediation (AMSB QCD). The spectrum of this theory matches that of QCD at the massless level, though the superpartners are not decoupled. In this theory it is possible to nail down the phase structure at $\barθ=π$ as a function of the quark masses and the number of flavors $F$. For one flavor we find that there is a critical quark mass, below which CP is unbroken, while above the critical mass CP is spontaneously broken. At the critical mass there is a second-order phase transition along with a massless $η'$. We are able to analytically solve for the minima and the critical mass for $N=2,3$ as well as for the large $N$ limit, while for other $N$ one can find numerical results. For two flavors, we find that CP is always broken as long as the quark masses are equal and non-zero, however there is a non-trivial phase boundary for unequal quark masses, which we find numerically. For $F\geq 3$ we obtain an intricate phase boundary which reproduces the various quark mass limits. All our results are in agreement with the predictions for ordinary QCD that were based on anomaly matching arguments for generalized symmetries and the effective chiral Lagrangian. We also briefly comment on the domain wall solutions first discussed by Draper, and are able to present analytic results for the simplest case of $SU(2)$ with one flavor.

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arXiv2024

Conformal Freeze-In from Neutrino Portal

We study a scenario where a dark sector, described by a Conformal Field Theory (CFT), interacts with the Standard Model through the neutrino portal. In this setup, conformal invariance breaks below the electroweak scale, causing the theory to transition into a confined (hadronic) phase. One of the hadronic excitations in this phase can act as dark matter. In the "Conformal Freeze-In" cosmological framework, the dark sector is populated through interactions with the Standard Model at temperatures where it retains approximate conformal symmetry. The dark matter relic density depends on the CFT parameters, such as the dimension of the operator coupled to the Standard Model. We demonstrate that this model can reproduce the DM relic density and meet all observational constraints. The same neutrino portal interaction may also generate masses for the active neutrinos. The dark matter candidate could either be a pseudo-Goldstone boson (PGB) or a composite fermion with the quantum numbers of a sterile neutrino. In the latter case, the model is consistent with the current X-ray constraints, and may be detectable with future X-ray observations.

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arXiv2024

Atomic Dark Matter, Interacting Dark Radiation, and the Hubble Tension

We present a new class of interacting dark sector models that can address the Hubble tension. Interacting dark radiation (DR) has previously been put forward as a solution to the problem, but this proposal is disfavored by the high-$\ell$ cosmic microwave background (CMB) data. We modify this basic framework by introducing a subcomponent of dark matter (DM) that interacts strongly with the DR, so that together they constitute a tightly coupled fluid at early times. We show that if this subcomponent decouples from the interacting DR during the CMB epoch, the $\ell$ modes of the CMB that entered the horizon before decoupling are impacted differently from those that entered after, allowing a solution to the problem. We present a model that realizes this framework, which we dub "New Atomic Dark Matter", or nuADaM, in which the interacting dark matter (iDM) subcomponent is composed of dark atoms, and dark "neutrinos" with long-range interactions contribute to the DR, hence the name of the model. This iDM subcomponent is acoustic at early times but decouples from the DR following dark recombination. In contrast to conventional atomic dark matter (ADM) models, the dark photon is part of a richer DR sector, which ensures that it continues to be self-interacting even after recombination. We show that this model admits a fit to the available cosmological data that is significantly better than both $Λ$CDM and conventional ADM.

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arXiv2024

A Generative Modeling Approach to Reconstructing 21-cm Tomographic Data

Analyses of the cosmic 21-cm signal are hampered by astrophysical foregrounds that are far stronger than the signal itself. These foregrounds, typically confined to a wedge-shaped region in Fourier space, often necessitate the removal of a vast majority of modes, thereby degrading the quality of the data anisotropically. To address this challenge, we introduce a novel deep generative model based on stochastic interpolants to reconstruct the 21-cm data lost to wedge filtering. Our method leverages the non-Gaussian nature of the 21-cm signal to effectively map wedge-filtered 3D lightcones to samples from the conditional distribution of wedge-recovered lightcones. We demonstrate how our method is able to restore spatial information effectively, considering both varying cosmological initial conditions and astrophysics. Furthermore, we discuss a number of future avenues where this approach could be applied in analyses of the 21-cm signal, potentially offering new opportunities to improve our understanding of the Universe during the epochs of cosmic dawn and reionization.

astro-ph.CO↗

arXiv2023

Stepped Partially Acoustic Dark Matter: Likelihood Analysis and Cosmological Tensions

We generalize the recently proposed Stepped Partially Acoustic Dark Matter (SPartAcous) model by including additional massless degrees of freedom in the dark radiation sector. We fit SPartAcous and its generalization against cosmological precision data from the cosmic microwave background, baryon acoustic oscillations, large-scale structure, supernovae type Ia, and Cepheid variables. We find that SPartAcous significantly reduces the $H_0$ tension but does not provide any meaningful improvement of the $S_8$ tension, while the generalized model succeeds in addressing both tensions, and provides a better fit than $Λ\mathrm{CDM}$ and other dark sector models proposed to address the same tensions. In the generalized model, $H_0$ can be raised to $71.4~\mathrm{km/s/Mpc}$ (the 95% upper limit) if the fitted data does not include the direct measurement from the SH0ES collaboration, and to $73.7~\mathrm{km/s/Mpc}$ (95% upper limit) if it does. A version of $\texttt{CLASS}$ that has been modified to analyze this model is publicly available at https://github.com/ManuelBuenAbad/class_spartacous

astro-ph.CO↗

arXiv2023

Stepped Partially Acoustic Dark Matter, Large Scale Structure, and the Hubble Tension

We propose a new interacting dark sector model, Stepped Partially Acoustic Dark Matter (SPartAcous), that can simultaneously address the two most important tensions in current cosmological data, the $H_0$ and $S_8$ problems. As in the Partially Acoustic Dark Matter (PAcDM) scenario, this model features a subcomponent of dark matter that interacts with dark radiation at high temperatures, suppressing the growth of structure at small scales and thereby addressing the $S_8$ problem. However, in the SPartAcous model, the dark radiation includes a component with a light mass that becomes non-relativistic close to the time of matter-radiation equality. As this light component annihilates away, the remaining dark radiation heats up and its interactions with dark matter decouple. The heating up of the dark sector results in a step-like increase in the relative energy density in dark radiation, significantly reducing the $H_0$ tension, while the decoupling of dark matter and dark radiation ensures that the power spectrum at larger scales is identical to $Λ$CDM.

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arXiv2023

Optimizing pixel tracklet searches for shorter lifetimes

Pixel tracklets, disappearing tracks reconstructed with only pixel hits, have proven to be a promising technique in LHC analyses to search for dark matter candidates at the LHC that belong to a nearly-degenerate electroweak multiplet. However, a Pseudo-Dirac electroweak doublet fermion, arguably the most interesting such possibility, has a shorter lifetime and therefore existing tracklet searches are less sensitive in this case. We assess the performance of a tracklet search optimized for shorter lifetimes by requiring only three pixel hits for the tracklet reconstruction, and by demanding an accompanying soft track for suppressing backgrounds. We estimate how far the sensitivity of existing searches can be extended into the region of parameter space with this optimized search.

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arXiv2022

Twin Quark Dark Matter From Cogenesis

We extend the fraternal twin Higgs scenario to include a novel dark matter candidate as well as a mechanism for generating a matter/antimatter asymmetry in both sectors. A spontaneous breaking of twin color results in quark degrees of freedom that are singlets under the residual twin color group. These twin-color-singlet quarks, along with a subdominant component of twin leptons, constitute the asymmetric dark matter. The asymmetry between matter in antimatter in both sectors is co-generated from the decay of singlet fermions that provide an additional portal between the visible and twin sectors. We discuss the phenomenological aspects of this model, evaluating constraints on the parameter space and highlighting promising discovery channels in future experiments. We briefly discuss how the discovery of signals in multiple experiments may help establish the connection between the mechanisms that address the naturalness, dark matter and matter/antimatter asymmetry puzzles.

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arXiv2020

Suppressed flavor violation in Lepton Flavored Dark Matter from an extra dimension

Phenomenological studies of Flavored Dark Matter (FDM) models often have to assume a near-diagonal flavor structure in the coupling matrix in order to remain consistent with bounds from flavor violating processes. In this paper we show that for Lepton FDM, such a structure can naturally arise from an extra dimensional setup. The extra dimension is taken to be flat, with the dark matter and mediator fields confined to a brane on one end of the extra dimension, and the Higgs field to a brane on the other end. The Standard Model fermion and gauge fields are the zero modes of corresponding bulk fields with appropriate boundary conditions. Global flavor symmetries exist in the bulk and on the FDM brane, while they are broken on the Higgs brane. Flavor violating processes arise due to the misalignment of bases for which the interactions on the two branes are diagonalized, and their size can be controlled by a choice of the lepton profiles along the extra dimension. By studying the parameter space for the model, we show that when relic abundance and indirect detection constraints are satisfied, the rates for flavor violating processes such as $μ\to eγ$ remain far below the experimental limits.

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