arXiv subjects

Chang Sub Shin

Publications and source records attributed to Chang Sub Shin.

At least 19 recordsLinked to original sources

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

Bipartite Solution to the Lithium Problem

The primordial lithium problem remains a persistent motivation for new-physics modifications of Big Bang nucleosynthesis, yet the precision of the observed deuterium abundance now places strong constraints on such attempts. This indicates that the challenge is not simply to reduce $^{7}\mathrm{Li}$, but to realize the correlated shifts among light-element abundances required to do so without spoiling deuterium. We investigate this issue in a concrete two-step decay scenario involving two unstable particles undergoing sequential late decays. In the first stage, a majoron with lifetime $\tau_J \sim 10\,\text{--}\,10^4\,\mathrm{sec}$ decays predominantly into neutrinos, increasing the neutron abundance and thereby reducing the primordial $^{7}\mathrm{Li}+\!{}^{7}\mathrm{Be}$ yield. This mechanism, however, simultaneously drives deuterium above the observationally allowed range. In the second stage, an axion-like particle with a longer lifetime $\tau_\phi \gtrsim 10^5\,\mathrm{sec}$ decays into photons, inducing late-time photodissociation that compensates the excess deuterium without erasing the earlier reduction of lithium, while further amplifying the depletion of $^{7}\mathrm{Li}+\!{}^{7}\mathrm{Be}$. Although the setup is model-dependent, it serves as an explicit proof of concept that the lithium abundance can be lowered consistently with current deuterium constraints. More broadly, our analysis highlights that a viable resolution may require a nontrivial combination of decay channels and decay epochs, and clarifies the pattern of abundance response that successful late-decay scenarios must achieve.

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arXiv2026

Revisiting QCD-induced little inflation with chiral density wave state and its implications on pulsar timing array gravitational-wave signals

We revisit QCD-induced little inflation in which the Universe begins with a large baryon chemical potential and undergoes a strong first-order QCD phase transition, generating an observable stochastic gravitational-wave background in the nano-Hz range relevant for pulsar timing array (PTA) observations. We point out that the conventional homogeneous transition from the quark-gluon plasma phase to the hadronic gas phase faces an unavoidable difficulty in achieving the required strength of supercooling for the observed baryon density. This motivates us to explore whether a qualitatively different phase structure at a large baryon chemical potential can alter the relation between the baryon density and the chemical potential, and thereby modify the supercooling history of the transition. Using the nucleon-meson model with isoscalar vector mesons, we determine the critical and spinodal structure of the chiral density wave (CDW) phase in the $(\mu_B, T)$ plane. We find that the CDW phase exhibits a nontrivial structure and can remain metastable down to a low baryon density in a certain region of the parameter space. Taking into account the subsequent liquid-gas transition and phase separation, however, the released latent heat is too small to realize a viable QCD-induced little inflation scenario and its associated PTA-scale gravitational-wave signal. Our analysis sharpens the conditions under which QCD phase transitions may act as cosmological sources of nano-Hz gravitational waves, while clarifying the possible cosmological relevance of inhomogeneous QCD phases.

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arXiv2026

Kinetic Isocurvature Perturbation

We formulate a new class of primordial perturbations called $\textit{kinetic isocurvature perturbations}$, where the mass density of dark matter is constant relative to the photon number density while the kinetic energy of dark matter fluctuates in space. Such perturbations naturally arise in scenarios where a nonrelativistic heavy field decays into relativistic dark matter particles with a spatially modulated rate. As dark matter cools and becomes nonrelativistic, these fluctuations in kinetic energy leave large-scale density perturbations essentially unaffected and therefore evade the Cosmic Microwave Background bounds on isocurvature perturbations, yet survive as spatial variations in the free-streaming scale, resulting in patch-by-patch variation of the matter power spectrum.

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arXiv2025

Probing Heavy Dark Matter in Red Giants

Red giants (RGs) provide a promising astrophysical environment for capturing dark matter (DM) via elastic scattering with stellar nuclei. Captured DM particles migrate toward the helium-rich core and accumulate into a compact configuration. As the DM population grows, it can become self-gravitating and undergo gravitational collapse, leading to adiabatic contraction through interactions with the ambient medium. The resulting energy release, through elastic scattering and, where relevant, DM annihilation during collapse, locally heats the stellar core and can trigger helium ignition earlier than that predicted by standard stellar evolution. We analyze the conditions under which DM-induced heating leads to runaway helium burning and identify the critical DM mass required for ignition. Imposing the observational constraint that helium ignition must not occur before the observed luminosity at the tip of the RG branch, we translate these conditions into bounds on DM properties. Remarkably, we find that RGs are sensitive to DM, particularly with masses around $10^{11} \,{\rm GeV}$ and spin-independent scattering cross sections near $10^{-37}\,{\rm cm}^2$, which is comparable to the reach of current terrestrial direct detection experiments. Noteworthy, observations of RG stars provide a unique probe for high-mass and large-cross-section DM, a regime that remains currently inaccessible to direct detection experiments.

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arXiv2025

Parametric Coincidence in the Baryon to Dark Matter Ratio from Affleck-Dine Baryogenesis and UV Freeze-in Dark Matter

We highlight that the observed concurrence between the baryon and dark matter relic densities can be explained via a parametric coincidence between two distinct production mechanisms: Affleck-Dine baryogenesis and dark matter UV freeze-in. In the Affleck-Dine mechanism, the baryon asymmetry is naturally proportional to the inflationary reheating temperature $T_{\rm rh}$, which also plays a critical role in setting the relic abundance of UV freeze-in dark matter. Since Affleck-Dine baryogenesis requires flat directions in the potential, the framework is inherently supersymmetric, offering compelling UV freeze-in dark matter candidates such as the gravitino. We outline scenarios in which $T_{\rm rh}$ simultaneously determines both relic abundances, resulting in a baryon-to-dark matter ratio of order unity that is largely insensitive to $T_{\rm rh}$. We also discuss the conditions required to avoid Q-ball formation or dark matter production by other mechanisms, such as NLSP decays, to preserve the parametric coincidence between baryon and dark matter abundances.

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arXiv2025

Superheavy Supersymmetric Dark Matter for the origin of KM3NeT Ultra-High Energy signal

We propose an explanation for the recently reported ultra-high-energy neutrino signal at KM3NeT, which shows no clear association with known astrophysical sources. While decaying dark matter in the Galactic Center is a natural candidate, the observed arrival direction strongly suggests an extragalactic origin. We introduce a multicomponent dark matter scenario in which the components are part of a supermultiplet, with supersymmetry ensuring a nearly degenerate mass spectrum among the fields with different spins. In this setup, a cosmologically long-lived fermionic state decays into a slightly lighter bosonic dark matter state, producing a boosted neutrino spectrum with energy $E_\nu \sim 100$ PeV, determined by the mass difference. The heavy-to-light decay occurs at a cosmological redshift of $z \sim \text{a few}$ or higher, leading to an isotropic directional distribution of the signal.

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arXiv2024

What happens when supercooling is terminated by curvature flipping of the effective potential?

We explore the nature of a certain type of supercooled phase transition, where the supercooling is guaranteed to end due to the curvature of the finite-temperature effective potential at the origin experiencing a sign flip at some temperature. In such models the potential barrier trapping the scalar field at the meta-stable origin is quickly vanishing at the temperature scale of the phase transition. It is therefore not immediately clear if critical bubbles are able to form, or whether the field will simply transition over the barrier and smoothly roll down to the true minimum. To address this question, we perform lattice simulations of a scalar potential exhibiting supercooling, with a small barrier around the origin, and qualitatively determine the fate of the phase transition. Our simulations indicate that, owing to the required flatness of the potential, the scalar field remains trapped around the origin such that the phase transition generically proceeds via the nucleation and expansion of true-vacuum bubbles. We comment on the possible gravitational wave signals one might expect in a concrete toy model and discuss the parameter space in which bubble percolation is and isn't expected. Animated versions of Figures 6 through 9 can be found at: https://www.youtube.com/playlist?list=PLhT9Np0-FMHBjkejei0bb9qbxbWKHAUXf

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arXiv2024

Revisiting Reactor Anti-Neutrino 5 MeV Bump with $^{13}$C Neutral-Current Interaction

For the first time, we comprehensively examine the potential of a neutral-current interaction of reactor neutrino with $^{13}$C emitting a 3.685 MeV photon to identify the origin of the 5 MeV bump in reactor antineutrino spectra observed through the inverse beta decay (IBD) process. This anomaly may be due to new physics, reactor antineutrino flux inaccuracies, or IBD systematics. The 3.685 MeV photon released during the de-excitation of $^{13}$C$^\ast$ to its ground state is observable in liquid scintillator detectors. Remarkably, we confirm the powerfulness of our proposal by completely ruling out a new physics scenario explaining the bump from the existing NEOS data. We also explore the potential of current and forthcoming experiments, including solar neutrino studies at JUNO, pion and muon decay-at-rest experiments at OscSNS, and isotope decay-at-rest studies at Yemilab, to measure the cross-section precisely enough to distinguish the expected bump and the theoretical flux models via our channel. Additionally, we propose a novel method to track the time evolution of reactor isotopes by analyzing the $^{13}$C signal, which yields critical insights into the contributions of $^{235}$U and $^{239}$Pu to the bump, acting as a robust tool.

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arXiv2024

Dynamical Generation of the Baryon Asymmetry from a Scale Hierarchy

We propose a novel baryogenesis scenario where the baryon asymmetry originates directly from a hierarchy between two fundamental mass scales: the electroweak scale $v$ and the Planck scale $M_P$, in the form of \begin{equation} Y_B \sim \sqrt{\frac{v}{M_P}} \, . \nonumber \end{equation} This relation straightforwardly gives the observed baryon yield today $Y_B$, which can be a hint for underlying fundamental physics. We provide an example of baryogenesis models that yield this relation. Our model is based on the neutrino-portal Affleck-Dine mechanism, which generates the asymmetry of the Affleck-Dine sector during the radiation-dominated era and subsequently transfers it to the baryon number before the electroweak phase transition. The observed baryon asymmetry is then a natural outcome of this scenario. The model is testable as it predicts the existence of a Majoron with a keV mass and an electroweak scale decay constant. The impact of the relic Majoron on the effective number of neutrinos ($\Delta N_{\rm eff}$) can be measured through near-future cosmic microwave background observations.

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arXiv2024

Constraining MeV to 10 GeV majoron by Big Bang Nucleosynthesis

We estimate the Big Bang nucleosynthesis (BBN) constraint on the majoron in the mass range between $1\,{\rm MeV}$ to $10\,{\rm GeV}$ which dominantly decays into the standard model neutrinos. When the majoron lifetime is shorter than $1\,{\rm sec}$, the injected neutrinos mainly heat up background plasma, which alters the relation between photon temperature and background neutrino temperature. For a lifetime longer than $1\,{\rm sec}$, most of the injected neutrinos directly contribute to the protons-to-neutrons conversion. In both cases, deuterium and helium abundances are enhanced, while the constraint from the deuterium is stronger than that from the helium. $^7{\rm Li}$ abundance gets decreased as a consequence of additional neutrons, but the parameter range that fits the observed $^7{\rm Li}$ abundance is excluded by the deuterium constraint. We also estimate other cosmological constraints and compare them with the BBN bound.

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arXiv2023

Effective Theory Approach for Axion Wormholes

We employ the effective field theory approach to analyze the characteristics of Euclidean wormholes within axion theories. Using this approach, we obtain non-perturbative instantons in various complex scalar models with and without a non-minimal coupling to gravity, as well as models featuring the $R^2$ term for a range of coupling values. This yields a series of analytical expressions for the axion wormhole action, shedding light on the model parameters and field dependencies of contributions in both the ultraviolet and infrared domains. Consequently, model-dependent local operators that disrupt axion shift symmetries are generated at lower energy levels. This, in turn, provides crucial insights into the gravitational influences on the axion quality problem.

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arXiv2023

Dark gauge boson emission from supernova pions

The hot, neutron-rich, and dense circumstance in core-collapse supernovae provides a source of negatively charged pions that may make up a significant portion of the matter. These abundant thermal pions can play a role to populate light and hidden hypothetical particles. We discuss the dark gauge boson production via reactions involving supernova pions, the rate of which is determined by the isovector nucleon coupling. We take into account the two toy models, the dark photon and the gauged $B-L$ models, that carry the typical distinct isovector nucleon coupling structure in the medium. Pion-induced dark gauge bosons leave an imprint on several observational consequences associated with supernova. Their sizable emissivity and characteristic hard spectral distribution result in the stringent constraints on the dark gauge boson models, in particular at masses above the two electron mass.

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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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arXiv2022

Dark gauge boson production from neutron stars via nucleon-nucleon bremsstrahlung

We discuss the dark gauge boson emission from neutron stars via nucleon-nucleon bremsstrahlung. Through the rigorous treatment of the effective field theory prescription and the thermal effect, we derive the relevant couplings of dark gauge bosons to hadrons in medium. As a specific example, the $U(1)_{\rm B-L}$ gauge boson scenario is chosen to investigate dark gauge boson emissivities during supernovae and cooling of young neutron stars. From the stellar cooling argument, we obtain the constraints on the $\rm B-L$ gauge coupling for given gauge boson masses in two observations: the duration of the supernova neutrino signal of SN1987A, and the inferred x-ray luminosity of the compact object in the remnant of SN1987A (NS1987A). In particular, the constraint from SN1987A on the $U(1)_{\rm B-L}$ gauge boson scenario is revisited. The excluded gauge coupling due to the emission of transverse polarizations is an order of magnitude enhanced compared to the previous derivation. There is also a newly excluded parameter space due to the emission of longitudinal polarizations.

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arXiv2022

Massless Preheating and Electroweak Vacuum Metastability

Current measurements of Standard Model parameters suggest that the electroweak vacuum is metastable. This metastability has important cosmological implications, because large fluctuations in the Higgs field could trigger vacuum decay in the early universe. For the false vacuum to survive, interactions which stabilize the Higgs during inflation -- e.g., inflaton-Higgs interactions or non-minimal couplings to gravity -- are typically necessary. However, the post-inflationary preheating dynamics of these same interactions could also trigger vacuum decay, thereby recreating the problem we sought to avoid. This dynamics is often assumed catastrophic for models exhibiting scale invariance since these generically allow for unimpeded growth of fluctuations. In this paper, we examine the dynamics of such "massless preheating" scenarios and show that the competing threats to metastability can nonetheless be balanced to ensure viability. We find that fully accounting for both the backreaction from particle production and the effects of perturbative decays reveals a large number of disjoint "islands of (meta)stability" over the parameter space of couplings. Ultimately, the interplay among Higgs-stabilizing interactions plays a significant role, leading to a sequence of dynamical phases that effectively extend the metastable regions to large Higgs-curvature couplings.

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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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arXiv2021

Aligned Natural Inflation in the Large Volume Scenario

We embed natural inflation in an explict string theory model and derive observables in cosmology. We achieve this by compactifying the type IIB string on a Calabi-Yau orientifold, stabilizing moduli via the Large Volume Scenario, and configuring axions using D7-brane stacks. In order to obtain a large effective decay constant, we employ the Kim-Nilles-Peloso alignment mechanism, with the required multiple axions arising naturally from anisotropic bulk geometries. The bulk volumes, and hence the axion decay constants, are stabilized by generalized one-loop corrections and subject to various conditions: the Kähler cone condition on the string geometry; the convex hull condition of the weak gravity conjecture; and the constraint from the power spectrum of scalar perturbations. We find that all constraints can be satisfied in a geometry with relatively small volume and thus heavy bulk axion mass. We also covariantize the convex hull condition for the axion-dilaton-instanton system and verify the normalization of the extremal bound.

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