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Wan-Il Park

Publications and source records attributed to Wan-Il Park.

At least 37 records · Page 2Linked to original sources
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↗
arXiv2016

Gamma-ray boxes from axion-mediated dark matter

We compute the gamma-ray output of axion-mediated dark matter and derive the corresponding constraints set by recent data. In such scenarios the dark matter candidate is a Dirac fermion that pair-annihilates into axions and/or scalars. Provided that the axion decays (at least partly) into photons, these models naturally give rise to a box-shaped gamma-ray spectrum that may present two distinct phenomenological behaviours: a narrow box, resembling a line at half the dark matter mass, or a wide box, spanning an extensive energy range up to the dark matter mass. Remarkably, we find that in both cases a sizable gamma-ray flux is predicted for a thermal relic without fine-tuning the model parameters nor invoking boost factors. This large output is in line with recent Fermi-LAT observations towards the Galactic centre region and is on the verge of being excluded. We then make use of the Fermi-LAT and H.E.S.S. data to derive robust, model-independent upper limits on the dark matter annihilation cross section for the narrow and wide box scenarios. H.E.S.S. constraints, in particular, turn out to match the ones from Fermi-LAT at hundreds of GeV and extend to multi-TeV masses. Future Cherenkov telescopes will likely probe gamma-ray boxes from thermal dark matter relics in the whole multi-TeV range, a region hardly accessible to direct detection, collider searches and other indirect detection strategies.

hep-ph↗
arXiv2016

Beyond the Dark matter effective field theory and a simplified model approach at colliders

Direct detection of and LHC search for the singlet fermion dark matter (SFDM) model with Higgs portal interaction are considered in a renormalizable model where the full Standard Model (SM) gauge symmetry is imposed by introducing a singlet scalar messenger. In this model, direct detection is described by an effective operator m_q \bar{q} q \barχ χas usual, but the full amplitude for monojet + \not E_T involves two intermediate scalar propagators, which cannot be seen within the effective field theory (EFT) or in the simplified model without the full SM gauge symmetry. We derive the collider bounds from the ATLAS monojet + \not E_T as well as the CMS t\bar{t} + \not E_T data, finding out that the bounds and the interpretation of the results are completely different from those obtained within the EFT or simplified models. It is pointed out that it is important to respect unitarity, renormalizability and local gauge invariance of the SM.

hep-ph↗
arXiv2016

New- vs. chaotic-inflations

We show that "spiralized" models of new-inflation can be experimentally identified mostly by their positive spectral running in direct contrast with most chaotic-inflation models which have negative runnings typically in the range of $\mathcal{O}(10^{-4}-10^{-3})$.

astro-ph.CO↗
arXiv2015

Local $Z_2$ scalar dark matter model confronting galactic ${\mathrm GeV}$-scale $γ$-ray

We present a scalar dark matter (DM) model where DM ($X_I$) is stabilized by a local $Z_2$ symmetry originating from a spontaneously broken local dark $U(1)_X$. Compared with the usual scalar DM with a global $Z_2$ symmetry, the local $Z_2$ model possesses three new extra fields, dark photon $Z^{'}$, dark Higgs $ϕ$ and the excited partner of scalar DM ($X_R$), with the kinetic mixing and Higgs portal interactions dictated by local dark gauge invariance. The resulting model can accommodate thermal relic density of scalar DM without conflict with the invisible Higgs branching ratio and the bounds from DM direct detections, thanks to the newly opened channels, $X_I X_I \rightarrow Z^{'} Z^{'}, ϕϕ$. In particular, due to the new particles, the ${\rm GeV}$ scale $γ$-ray excess from the Galactic Center (GC) can be originated from the decay of dark Higgs boson which is produced in DM annihilations.

hep-ph↗
arXiv2015

Higgs-Portal Dark Matter for GeV Gamma-Ray Excess

We present Higgs-Portal dark matter (DM) models to explain the reported Galactic Center GeV gamma-ray excess. Naive effective theories are inconsistent with direct detection constraint for the relevant parameter range. Simple extended models with dark gauge symmetries can easily accommodate the gamma-ray excess through the Higgs-Portal coupling while satisfying various constraints.

hep-ph↗
arXiv2015

Spiral Inflation with Coleman-Weinberg Potential

We apply the idea of spiral inflation to Coleman-Weinberg potential, and show that inflation matching well observations is allowed for a symmetry-breaking scale ranging from an intermediate scale to GUT scale even if the quartic coupling $λ$ is of $\mathcal{O}(0.1)$. The tensor-to-scalar ratio can be of $\mathcal{O}(0.01)$ in case of GUT scale symmetry-breaking.

hep-ph↗
arXiv2015

Spiral Inflation

We propose a novel scenario of primordial inflation in which the inflaton goes through a spiral motion starting from around the top of a symmetry breaking potential. We show that, even though inflation takes place for a field value much smaller than Planck scale, it is possible to obtain relatively large tensor to scalar ratio ($r \sim 0.1$) without fine tuning. The inflationary observables perfectly match Planck data.

hep-ph↗
arXiv2014

Cluster X-ray line at $3.5\,{\rm keV}$ from axion-like dark matter

The recently reported X-ray line signal at $E_γ\simeq 3.5\, {\rm keV}$ from a stacked spectrum of various galaxy clusters and the Andromeda galaxy may be originating from a decaying dark matter particle of the mass $2 E_γ$. A light axion-like scalar is suggested as a natural candidate for dark matter and its production mechanisms are closely examined. We show that the right amount of axion relic density with the preferred parameters, $m_a \simeq 7 \,{\rm keV}$ and $f_a \simeq 4\times 10^{14}\, {\rm GeV}$, can be naturally obtainable from the decay of inflaton. If the axions were produced from the saxion decay, it could not have constituted the total relic density due to the bound from structure formation. Nonetheless, the saxion decay is an interesting possibility, because the $3.5\, {\rm keV}$ line and dark radiation can be addressed simultaneously, being consistent with the Planck data. Small misalignment angles of the axion, ranging between $θ_a\sim 10^{-4} -10^{-1}$ depending on the reheating temperature, can also be the source of axion production. The model with axion misalignment can satisfy the constraints for structure formation and iso-curvature perturbation.

astro-ph.CO↗
arXiv2014

Indirect and direct signatures of Higgs portal decaying vector dark matter for positron excess in cosmic rays

We investigate the indirect signatures of the Higgs portal $U(1)_X$ vector dark matter (VDM) $X_μ$ from both its pair annihilation and decay. The VDM is stable at renormalizable level by $Z_2$ symmetry, and thermalized by Higgs-portal interactions. It can also decay by some nonrenormalizable operators with very long lifetime at cosmological time scale. If dim-6 operators for VDM decays are suppressed by $10^{16}$ GeV scale, the lifetime of VDM with mass $\sim$ 2 TeV is just right for explaining the positron excess in cosmic ray recent observed by PAMELA and AMS02 Collaborations. The VDM decaying into $μ^+ μ^-$ can fit the data, evading various constraints on cosmic rays. We give one UV-complete model as an example. This scenario for Higgs portal decaying VDM with mass around $\sim2$ TeV can be tested by DM direct search at XENON1T and at the future colliders by measuring the Higgs self-couplings.

hep-ph↗
arXiv2014

Hidden sector monopole, vector dark matter and dark radiation with Higgs portal

We show that the 't Hooft-Polyakov monopole model in the hidden sector with Higgs portal interaction makes a viable dark matter model, where monopole and massive vector dark matter (VDM) are stable due to topological conservation and the unbroken subgroup $U(1)_X$. We show that, even though observed CMB data requires the dark gauge coupling to be quite small, a right amount of VDM thermal relic can be obtained via $s$-channel resonant annihilation for the mass of VDM close to or smaller than the half of SM higgs mass, thanks to Higgs portal interaction. Monopole relic density turns out to be several orders of magnitude smaller than observed dark matter relic density. Direct detection experiments, particularly, the projected XENON1T experiment, may probe the parameter space where the dark Higgs is lighter than $\lesssim 60 {\rm GeV}$. In addition, the dark photon associated with unbroken $U(1)_X$ contributes to the radiation energy density at present, giving $ΔN_{\rm eff}^ν\sim 0.1$ as the extra relativistic neutrino species.

hep-ph↗
arXiv2014

The 3.5 keV X-ray line signature from annihilating and decaying dark matter in Weinberg model

Recently two groups independently observed unidentified X-ray line signal at the energy 3.55 keV from the galaxy clusters and Andromeda galaxy. We show that this anomalous signal can be explained in annihilating dark matter model, for example, fermionic dark matter model in hidden sector with global $U(1)_X$ symmetry proposed by Weinberg. There are two scenarios for the production of the annihilating dark matters. In the first scenario the dark matters with mass 3.55 keV decouple from the interaction with Goldstone bosons and go out of thermal equilibrium at high temperature ($>$ 1 TeV) when they are still relativistic, their number density per comoving volume being essentially fixed to be the current value. The correct relic abundance of this warm dark matter is obtained by assuming that about ${\cal O}(10^3)$ relativistic degrees of freedom were present at the decoupling temperature or alternatively large entropy production occurred at high temperature. In the other scenario, the dark matters were absent at high temperature, and as the universe cools down, the SM particles annihilate or decay to produce the dark matters non-thermally as in `freeze-in' scenario. It turns out that the DM production from Higgs decay is the dominant one. In the model we considered, only the first scenario can explain both X-ray signal and relic abundance. The X-ray signal arises through $p$-wave annihilation of dark matter pair into two photons through the scalar resonance without violating the constraints from big bang nucleosynthesis, cosmic microwave background, and astrophysical objects such as red giants or white dwarfs. We also discuss the possibility that the signal may result from a decaying dark matter in a simple extension of Weinberg model.

hep-ph↗
arXiv2014

Invisible Higgs Decay Width vs. Dark Matter Direct Detection Cross Section in Higgs Portal Dark Matter Models

The correlation between the invisible Higgs branching ratio ($B_h^{\rm inv} $) vs. dark matter (DM) direct detection ($σ_p^{\rm SI}$) in Higgs portal DM models is usually presented in the effective field theory (EFT) framework. This is fine for singlet scalar DM, but not in the singlet fermion DM (SFDM) or vector DM (VDM) models. In this paper, we derive the explicit expressions for this correlation within UV completions of SFDM and VDM models with Higgs portals, and discuss the limitation of the EFT approach. We show that there are at least two additional hidden parameter in $σ_p^{\rm SI}$ in the UV completions: the singlet-like scalar mass $m_2$ and its mixing angle $α$ with the SM Higgs boson ($h$). In particular, if the singlet-like scalar is lighter than the SM Higgs boson ($m_2 < m_h \cos α/ \sqrt{1 + \cos^2 α}$), the collider bound becomes weaker than the one based on EFT.

hep-ph↗
arXiv2014

Higgs portal vector dark matter for $\mathinner{\mathrm{GeV}}$ scale $γ$-ray excess from galactic center

We show that the $\mathinner{\mathrm{GeV}}$ scale $γ$-ray excess from the direction of the Galactic Center can be naturally explained by the pair annihilation of Abelian vector dark matter (VDM) into a pair of dark Higgs bosons $VV \rightarrow ϕϕ$, followed by the subsequent decay of $ϕ$ into $ϕ\rightarrow b\bar{b} , τ\barτ $. All the processes are described by a renormalizable VDM model with the Higgs portal, which is naturally flavor-dependent. Some parameter space of this scenario can be tested at the near future direct dark matter search experiments such as LUX and XENON1T.

hep-ph↗
arXiv2014

PQ-symmetry for a small Dirac neutrino mass, dark radiation and cosmic neutrinos

We propose a supersymmetric scenario in which the small Yukawa couplings for the Dirac neutrino mass term are generated by the spontaneous-breaking of Pecci-Quinn symmetry. In this scenario, a right amount of dark matter relic density can be obtained by either right-handed sneutrino or axino LSP, and a sizable amount of axion dark radiation can be obtained. Interestingly, the decay of right-handed sneutrino NLSP to axino LSP is delayed to around the present epoch, and can leave an observable cosmological background of neutrinos at the energy scale of $\mathcal{O}(10-100) {\rm GeV}$.

hep-ph↗
arXiv2013

An alternative to the standard model

We present an extension of the standard model to dark sector with an unbroken local dark $U(1)_X$ symmetry. Including various singlet portal interactions provided by the standard model Higgs, right-handed neutrinos and kinetic mixing, we show that the model can address most of phenomenological issues (inflation, neutrino mass and mixing, baryon number asymmetry, dark matter, direct/indirect dark matter searches, some scale scale puzzles of the standard collisionless cold dark matter, vacuum stability of the standard model Higgs potential, dark radiation) and be regarded as an alternative to the standard model. The Higgs signal strength is equal to one as in the standard model for unbroken $U(1)_X$ case with a scalar dark matter, but it could be less than one independent of decay channels if the dark matter is a dark sector fermion or if $U(1)_X$ is spontaneously broken, because of a mixing with a new neutral scalar boson in the models.

hep-ph↗