Searcharxiv⌕ Search

arXiv subjects

Wan-Il Park

Publications and source records attributed to Wan-Il Park.

At least 19 recordsLinked to original sources
arXiv2026

Phase Transitions and Gravitational Wave Production at the End of Thermal Inflation

We investigate the first-order phase transition that terminates thermal inflation and evaluate the associated stochastic gravitational-wave signals. The transition is first characterized through semi-analytic calculations of the bounce action, which are compared with numerical results obtained using CosmoTransitions. We then study its real-time evolution in a three-dimensional Langevin lattice simulation that incorporates Hubble expansion and the corresponding temperature evolution throughout the transition. The lattice dynamics are consistent with the bounce-action estimates: the transition proceeds through localized bubble nucleation and subsequent bubble growth, rather than through a phase-mixing instability. Using the resulting transition parameters, we estimate the gravitational-wave spectra generated by bubble collisions and acoustic motions in the plasma. The predicted stochastic background lies within the projected sensitivity ranges of future gravitational-wave observatories, including BBO and DECIGO.

hep-ph↗
arXiv2026

Cogenesis of baryon and lepton number asymmetries matching the EMPRESS Data

We show that a simple supersymmetric $U(1)_{B-L}$ extension of the standard model can explain simultaneously the large electron neutrino asymmetry hinted by the recent EMPRESS data as well as the observed tiny baryon asymmetry via the resonant leptogenesis mechanism. The condensation of $B-L$ Higgs dominating the universe at its decay is the sole source for these generation processes. Here, the infrequent decays of the $B-L$ Higgs to heavy right-handed neutrinos and successive prompt decays of these right-handed neutrinos around the electroweak phase transition produce the observed baryon asymmetry while the complete decay of the same $B-L$ Higgs at a later epoch leads to a large lepton number asymmetry. The right amounts of both asymmetries are found to be obtained for the symmetry breaking scale $v_ϕ\sim 10^{10}~{\rm GeV}$. Moreover, in a close connection to the positivity of both asymmetries, seemingly only the normal mass hierarchy of light neutrino species works. Finally, the gravitational wave background from the topologically stable strong type-I cosmic strings, generated from the breaking of $U(1)_{B-L}$ symmetry, can be within the reach of future experiments such as ultimate DECIGO.

hep-ph↗
arXiv2023

Supersymmetric $U(1)_{B-L}$ flat direction and NANOGrav 15 year data

We show that, when connected with monopoles, the flat $D$-flat direction breaking the local $U(1)_{B-L}$ symmetry as an extension of the minimal supersymmetric standard model can be responsible for the signal of a stochastic gravitational wave background recently reported by NANOGrav collaborations, while naturally satisfying constraints at high frequency band. Thanks to the flatness of the direction, a phase of thermal inflation arises naturally. The reheating temperature is quite low, and suppresses signals at frequencies higher than the characteristic frequency set by the reheating temperature. Notably, forthcoming spaced-based experiments such as LISA can probe the cutoff frequency, providing an indirect clue of the scale of soft SUSY-breaking mass parameter.

hep-ph↗
arXiv2023

Light cold dark matter from non-thermal decay

We investigate the mass range and the corresponding free-streaming length scale of dark matter produced non-thermally from decay of heavy objects which can be either dominant or sub-dominant at the moment of decay. We show that the resulting dark matter could be very light well below keV scale with a free-streaming length satisfying the Lyman-α constraints. We demonstrate two explicit examples for such light cold dark matter.

hep-ph↗
arXiv2023

Gravitational waves from walls bounded by strings in $SO(10)$ model of pseudo-Goldstone dark matter

We explore the gravitational wave spectrum generated by string-wall structures in an $SO(10)$ ($Spin(10)$) based scenario of pseudo-Goldstone boson dark matter (pGDM) particle. This dark matter candidate is a linear combination of the Standard Model (SM) singlets present in the 126 and 16 dimensional Higgs fields. The Higgs $126$-plet vacuum expectation value (VEV) $\left<126_H\right>$ leaves unbroken the $\mathbb{Z}_2$ subgroup of $\mathbb{Z}_4$, the center of $SO(10)$. Among other things, this yields topologically stable cosmic strings with a string tension $μ\sim \left<126_H\right>^2$. The subsequent (spontaneous) breaking of $\mathbb{Z}_2$ at a significantly lower scale by the $16$-plet VEV $\left<16_H\right>$ leads to the appearance of domain walls bounded by the strings produced earlier. We display the gravitational wave spectrum for $G μ$ values varying between $10^{-15}$ and $10^{-9}$ ($\left<126_H\right>\sim 10^{11}$ - $10^{14}$ GeV), and $\left<16_H\right>\sim 0.1$ - $10^2$ TeV range ($G$ denotes Newton's constant.) These predictions can be tested, as we show, by a variety of (proposed) experiments including LISA, ET, CE and others.

hep-ph↗
arXiv2022

Addendum to "Invisible Higgs decay width versus dark matter direct detection cross section in Higgs portal dark matter models"

This article is an addendum to Ref.~\cite{Baek:2014jga}. Here, we discuss the invisible Higgs decay width $Γ_{h}^{\rm inv}$ in the Higgs portal vector dark matter (VDM) model in the limit $m_V \rightarrow 0^+$. In the effective field theory (EFT) approach where the VDM mass is attributed to the Stückelberg mechanism, $( Γ_{h}^{\rm inv} )_{\rm EFT}$ is divergent, which is unphysical and puzzling. On the other hand $( Γ_{h}^{\rm inv} )_{\rm UV}$ becomes finite in a UV completion, where the VDM mass is generated by the dark Higgs mechanism. Then we can take the limit $m_V \rightarrow 0^+$ by taking either {\it (i)} the dark gauge coupling $g_X \rightarrow 0^+$ with a fixed dark Higgs vacuum expectation value $v_Φ$, or {\it (ii)} $v_Φ\to 0^+$ with a fixed $g_X$. Such a difference in the behavior of $Γ_{h}^{\rm inv}$ in the massless VDM limit demonstrates another limitation of EFT for the Higgs portal VDM, and the importance of gauge-invariant and renormalizable models for the Higgs portal VDM.

hep-ph↗
arXiv2021

Conformal Portal to Dark Matter

We propose a new portal coupling to dark matter by taking advantage of the nonminimally coupled portal sector to the Ricci scalar. Such a portal sector conformally induces couplings to the trace of the energy-momentum tensor of matters including highly secluded dark matter particles. The portal coupling is so feeble that dark matter is produced by freeze-in processes of scatterings and/or the decay of the mediator. We consider two concrete realizations of the portal: conformally induced Higgs portal and conformally induced mediator portal. The former case is compatible with the Higgs inflation, while the latter case can be tested by dark matter direct detection experiments.

hep-ph↗
arXiv2019

Spontaneous baryogenesis in spiral inflation

We examined the possibility of spontaneous baryogenesis driven by the inflaton in the scenario of \textit{spiral inflation}, and found the parametric dependence of the late-time baryon number asymmetry. As a result, it is shown that, depending on the effective coupling of baryon/lepton number violating operators, it is possible to obtain the right amount of asymmetry even in the presence of a matter-domination era as long as such era is relatively short. In a part of the parameter space, the required expansion rate during inflation is close to the current upper-bound, and hence can be probed in the near future experiments.

hep-ph↗
arXiv2018

A double-running inflaton mass for a flat potential and an assisted hilltop inflation

We propose a scenario to flatten inflaton potential for a small-field slow-roll inflation from the origin. In the scenario, inflaton mass-square gets through a renormalization group running which depends on not only inflaton field but also an assisting field. Thanks to the assisting field participating in the running, initial condition for a slow-roll inflation can be set naturally, and inflaton potential can be flattened by the vacuum expectation value of the field. Applying this idea, we propose a scenario of inflation, dubbed here as \textit{assisted hilltop inflation}, which is free from the initial condition and the flatness problems of inflation.

hep-ph↗
arXiv2018

Lepton number asymmetries and the lower bound on the reheating temperature

We show that the reheating temperature of a matter-domination era in the early universe can be pushed down to the neutrino decoupling temperature at around $2 \ {\rm MeV}$ if the reheating takes place through non-hadronic decays of the dominant matter and neutrino-antineutrino asymmetries are still large enough, $|L| \gtrsim \mathcal{O}(10^{-2})$ (depending on the neutrino flavor) at the end of reheating.

astro-ph.CO↗
arXiv2017

Flavor versus mass eigenstates in neutrino asymmetries: implications for cosmology

We show that, if they exist, lepton number asymmetries ($L_α$) of neutrino flavors should be distinguished from the ones ($L_i$) of mass eigenstates, since Big Bang Nucleosynthesis (BBN) bounds on the flavor eigenstates cannot be directly applied to the mass eigenstates. Similarly, Cosmic Microwave Background (CMB) constraints on mass eigenstates do not directly constrain flavor asymmetries. Due to the difference of mass and flavor eigenstates, the cosmological constraint on the asymmetries of neutrino flavors can be much stronger than conventional expectation, but not uniquely determined unless at least the asymmetry of the heaviest neutrino is well constrained. Cosmological constraint on $L_i$ for a specific case is presented as an illustration.

astro-ph.CO↗
arXiv2017

On large lepton number asymmetries of the Universe

A large lepton number asymmetry of $\mathcal{O}(0.1-1)$ at present universe might not only be allowed but also necessary for consistency among cosmological data. We show that, if a sizeable lepton number asymmetry were produced before the electroweak phase transition, the requirement for not producing too much baryon number asymmetry through sphalerons processes, forces the high scale lepton number asymmetry to be larger than about $30$. Therefore a mild entropy release causing $\mathcal{O}(10-100)$ suppression of pre-existing particle density should take place, when the background temperature of the universe is around $T = \mathcal{O}(10^{-2} - 10^2) {\rm GeV}$ for a large but experimentally consistent asymmetry to be present today. We also show that such a mild entropy production can be obtained by the late-time decays of the saxion, constraining the parameters of the Peccei-Quinn sector such as the mass and the vacuum expectation value of the saxion field to be $m_ϕ\gtrsim \mathcal{O}(10) {\rm TeV}$ and $ϕ_0 \gtrsim \mathcal{O}(10^{14}) {\rm GeV}$, respectively.

hep-ph↗
arXiv2017

Higgs-portal assisted Higgs inflation with a sizeable tensor-to-scalar ratio

We show that the Higgs portal interactions involving extra dark Higgs field can save generically the original Higgs inflation of the standard model (SM) from the problem of a deep non-SM vacuum in the SM Higgs potential. Specifically, we show that such interactions disconnect the top quark pole mass from inflationary observables and allow multi-dimensional parameter space to save the Higgs inflation, thanks to the additional parameters (the dark Higgs boson mass $m_ϕ$, the mixing angle $α$ between the SM Higgs $H$ and dark Higgs $Φ$, and the mixed quartic coupling) affecting RG-running of the Higgs quartic coupling. The effect of Higgs portal interactions may lead to a larger tensor-to-scalar ratio, $0.08 \lesssim r \lesssim 0.1$, by adjusting relevant parameters in wide ranges of $α$ and $m_ϕ$, some region of which can be probed at future colliders. Performing a numerical analysis we find an allowed region of parameters, matching the latest Planck data.

hep-ph↗
arXiv2016

Impact of CP-violation on neutrino lepton number asymmetries revisited

We revisit the effect of the (Dirac) CP-violating phase on neutrino lepton number asymmetries in both mass- and flavor-basis. We found that, even if there are sizable effects on muon- and tau-neutrino asymmetries, the effect on the asymmetry of electron-neutrinos is at most similar to the upper bound set by BBN for initial neutrino degeneracy parameters smaller than order unity. We also found that, for the asymmetries in mass-basis, the changes caused by CP-violation is of sub-\% level which is unlikely to be accesible neither in the current nor in the forthcoming experiments.

astro-ph.CO↗
arXiv2016

Peccei-Quinn field for inflation, baryogenesis, dark matter, and much more

We propose a scenario of brane cosmology in which the Peccei-Quinn field plays the role of the inflaton and solves simultaneously many cosmological and phenomenological issues such as the generation of a heavy Majorana mass for the right-handed neutrinos needed for seesaw mechanism, MSSM $μ$-parameter, the right amount of baryon number asymmetry and dark matter relic density at the present universe, together with an axion solution to the strong CP problem without the domain wall obstacle. Interestingly, the scales of the soft SUSY-breaking mass parameter and that of the breaking of $U(1)_{\rm PQ}$ symmetry are lower bounded at $\mathcal{O}(10) {\mathrm TeV}$ and $\mathcal{O}(10^{11}) {\mathrm GeV}$, respectively.

hep-ph↗
arXiv2016

Gravitational waves from first order phase transitions as a probe of an early matter domination era and its inverse problem

We investigate the gravitational wave background from a first order phase transition in a matter-dominated universe, and show that it has a unique feature from which important information about the properties of the phase transition and thermal history of the universe can be easily extracted. Also, we discuss the inverse problem of such a gravitational wave background in view of the degeneracy among macroscopic parameters governing the signal.

astro-ph.CO↗
arXiv2016

Eternal Hilltop Inflation

We consider eternal inflation in hilltop-type inflation models, favored by current data, in which the scalar field in inflation rolls off of a local maximum of the potential. Unlike chaotic or plateau-type inflation models, in hilltop inflation the region of field space which supports eternal inflation is finite, and the expansion rate $H_{EI}$ during eternal inflation is almost exactly the same as the expansion rate $H_*$ during slow roll inflation. Therefore, in any given Hubble volume, there is a finite and calculable expectation value for the lifetime of the "eternal" inflation phase, during which quantum flucutations dominate over classical field evolution. We show that despite this, inflation in hilltop models is nonetheless eternal in the sense that the volume of the spacetime at any finite time is exponentially dominated by regions which continue to inflate. This is true regardless of the energy scale of inflation, and eternal inflation is supported for inflation at arbitrarily low energy scale.

astro-ph.CO↗