arXiv subjects

P. Ko

Publications and source records attributed to P. Ko.

At least 73 records · Page 4Linked to original sources

arXiv2015

AMS02 positron excess from decaying fermion DM with local dark gauge symmetry

Positron excess observed by PAMELA, Fermi and AMS02 may be due to dark matter (DM) pair annihilation or decay dominantly into muons. In this paper, we consider a scenario with thermal fermionic DM ($χ$) with mass $\sim O(1-2)$ TeV decaying into a dark Higgs ($ϕ$) and an active neutrino ($ν_a$) instead of the SM Higgs boson and $ν_a$. We first present a renormalizable model for this scenario with local dark $U(1)_X$ gauge symmetry, in which the DM $χ$ can be thermalized by Higgs portal and the gauge kinetic mixing. Assuming the dark Higgs ($ϕ$) mass is in the range $2 m_μ< m_ϕ< 2 m_{π^0}$, the positron excess can be fit in a natural manner without conflict with constraints from antiproton and gamma ray fluxes or direct detection experiments. Also having such a light dark Higgs, the self-interaction of DM can be enhanced to some extent, and three puzzles in the CDM paradigm can be somewhat relaxed.

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arXiv2014

Dark Matter and Dark Force in the Type-I Inert 2HDM with Local U(1)_H Gauge Symmetry

We discuss dark matter (DM) physics in the Type-I inert two-Higgs-doublet model (2HDM) with local U(1)_H Higgs gauge symmetry. The local U(1)_H gauge symmetry is assigned to the extra Higgs doublet in order to avoid the Higgs-mediated flavor problems, and it spontaneously breaks down to its discrete subgroup. The lightest neutral scalar component H of the U(1)_H-charged Higgs doublet, which does not have Yukawa couplings with the Standard-Model (SM) fermions, is stable because of the remnant discrete symmetry, and it interacts with the SM particles through the U(1)_H gauge boson (Z_H) exchange as well as the SM boson exchange. We first investigate the constraint on the U(1)_H gauge interaction, especially through the kinetic and mass mixing between the SM gauge bosons and the extra gauge boson. Then we discuss dark matter physics in our 2HDM: thermal relic density, and direct/indirect detections of dark matter. The additional U(1)_H gauge interaction plays a crucial role in reducing the DM thermal relic density. The most important result within the inert DM model with local U(1)_H symmetry is that ~ O(10) GeV dark matter scenario, which is strongly disfavored in the usual Inert Doublet Model (IDM) with Z_2 symmetry, is revived in our model because of newly open channels, H H -> Z_H Z_H , Z_H Z. Exotic Higgs decays, h -> Z_H Z_H, Z Z_H, would be distinctive signatures of the inert 2HDM with local U(1)_H symmetry.

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arXiv2014

Renormalization group-induced phenomena of top pairs from four-quark effective operators

We study the renormalization group(RG) evolution of four-quark operators that contribute to the top pair production. In particular, we focus on the cases in which certain observables are \emph{first} induced from the one-loop RG while being absent at tree-level. From the operator mixing pattern, we classify all such RG-induced phenomena and underlying models that can induce them. We then calculate the full one-loop QCD RG evolution as the leading estimator of the effects and address the question of which RG-induced phenomena have largest and observable effects. The answer is related to the color structure of QCD. The studied topics include the RG-induction of top asymmetries, polarizations and polarization mixings as well as issues arising at this order. The RG-induction of top asymmetries is further compared with the generation of asymmetries from QCD and QED at one-loop order. We finally discuss the validity of using the RG as the proxy of one-loop effects on the top pair production. As an aside, we clarify the often-studied relations between top pair observables.

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arXiv2014

Correlation of top asymmetries: loop versus tree origins

We study the correlation of top asymmetries that are sensitive to the different origin of (a new contribution to) the total asymmetry: loop- or tree-level origins. We find that both the size and sign of the correlation between total and $t\bar{t}j$ inclusive asymmetries are inherently different depending on the origin. We demonstrate the correlation by using the color-singlet $Z^\prime$ and the pure axigluon taken as representative models of loop- and tree-induced total asymmetries. We calculate the next-to-leading order QCD corrections to the $Z^\prime$ and perform Monte-Carlo event generation. The correlation is understood in the QCD eikonal approximation using its color structure.

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arXiv2014

$νΛ$MDM: A Model for Sterile Neutrino and Dark Matter Reconciles Cosmological and Neutrino Oscillation Data after BICEP2

We propose a ultraviolet complete theory for cold dark matter(CDM) and sterile neutrino that can accommodate both cosmological data and neutrino oscillation experiments at $1σ$ level. A new $U(1)_X$ dark gauge symmetry is introduced, and is assumed to be broken at $\sim \mathcal{O}$(MeV) scale. Such a light mediator for DM's self-scattering and scattering-off sterile neutrinos can resolve three controversies for cold DM on small cosmological scales, cusp vs. core, too-big-to-fail and missing satellites problems. We can also accommodate $\sim$ eV scale sterile neutrinos as the hot dark matter(HDM) and can fit some neutrino anomalies from neutrino oscillation experiments within $1σ$. Finally the right amount of HDM can make a sizable contribution to dark radiation, and also helps to reconcile the tension between the data on the tensor-to-scalar ratio reported by Planck and BICEP2 Collaborations.

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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.

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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.

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arXiv2014

Two Higgs doublet models with local U(1)_H gauge symmetry and dark matter

We propose to implement the softly broken Z_2 symmetry in the usual two Higgs doublet model (2HDM) to spontaneously broken local U(1)_H gauge symmetry, and show that the resulting phenomenology can be very rich and is distinctly different from the usual 2HDMs. Likewise, the exact Z_2 symmetry in ordinary inert doublet model (IDM) could be a remnant of spontaneously broken local U(1)_H symmetry stabilizing dark matter (DM). In this case, new channels for DM pair annihilation into U(1)_H gauge boson(s) open up, allowing the DM mass below \lesssim 40 GeV, unlike the usual IDM.

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arXiv2014

Self-interacting scalar dark matter with local $Z_{3}$ symmetry

We construct a self-interacting scalar dark matter (DM) model with local discrete $Z_{3}$ symmetry that stabilizes a weak scale scalar dark matter $X$. The model assumes a hidden sector with a local $U(1)_X$ dark gauge symmetry, which is broken spontaneously into $Z_3$ subgroup by nonzero VEV of dark Higgs field $ϕ_X$ ($ \langle ϕ_X \rangle \neq 0$). Compared with global $Z_3$ DM models, the local $Z_3$ model has two new extra fields: a dark gauge field $Z^{'}$ and a dark Higgs field $ϕ$ (a remnant of the $U(1)_X$ breaking). After imposing various constraints including the upper bounds on the spin-independent direct detection cross section and thermal relic density, we find that the scalar DM with mass less than $125$ GeV is allowed in the local $Z_3$ model, in contrary to the global $Z_3$ model. This is due to new channels in the DM pair annihilations open into $Z^{'}$ and $ϕ$ in the local $Z_3$ model. Most parts of the newly open DM mass region can be probed by XENON1T and other similar future experiments. Also if $ϕ$ is light enough (a few MeV $\lesssim m_ϕ\lesssim$ O(100) MeV), it can generate a right size of DM self-interaction and explain the astrophysical small scale structure anomalies. This would lead to exotic decays of Higgs boson into a pair of dark Higgs bosons, which could be tested at LHC and ILC.

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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.

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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.

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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.

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arXiv2014

Higgs-dilaton(radion) system confronting the LHC Higgs data

We consider the Higgs-dilaton(radion) system using the trace of energy-momentum tensor ($T_{~μ}^μ$) with the full Standard Model (SM) gauge symmetry $G_{\rm SM} \equiv SU(3)_c \times SU(2)_L \times U(1)_Y$, and find out that the resulting phenomenology for the Higgs-dilaton(radion) system is distinctly different from the earlier studies based on the $T_{~μ}^μ$ with the unbroken subgroup $H_{\rm SM} \equiv SU(3)_c \times U(1)_{\rm em}$ of $G_{\rm SM}$. After electroweak symmetry breaking (EWSB), the SM Higgs boson and dilaton(radion) will mix with each other, and there appear two Higgs-like scalar bosons and the Higgs-dilaton mixing changes the scalar phenomenology in interesting ways. The signal strengths for the $gg$-initiated channels could be modified significantly compared with the SM predictions due to the QCD scale anomaly and the Higgs-dilaton(radion) mixing, whereas anomaly contributions are almost negligible for other channels. We also discuss the self-couplings and the signal strengths of the $126$ GeV scalar boson in various channels and possible constraints from the extra light/heavy scalar boson. The Higgs-dilaton(radion) system considered in this work has a number of distinctive features that could be tested by the upcoming LHC running and at the ILC.

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arXiv2014

Multi-Higgs doublet models with local $U(1)_H$ gauge symmetry and neutrino physics therein

Multi-Higgs doublet models appear in many interesting extensions of the standard model (SM). But they suffer from Higgs-mediated flavor changing neutral current (FCNC) problem which is very generic. In this talk, I describe that this problem can be resolved or mitigated if we introduce local $U(1)_H$ Higgs flavor gauge symmetry. As examples, I describe chiral $U(1)_{H}$ models where the right-handed up-type quarks also carry $U(1)_H$ charges and discuss the top forward-backward asymmetry (FBA) and $B\rightarrow D^{(*)} τν$ puzzle. Next I describe the two-Higgs doublet models where the usual $Z_2$ symmetry is implemented to $U(1)_H$ and show how the Type-I and Type-II models are extended. One possible extension of Type-II has the same fermion contents with the leptophobic $E_6$ $Z^{'}$ model by Rosner, and I discuss the neutrino sector in this model briefly.

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arXiv2013

Higgs phenomenology in Type-I 2HDM with U(1)_H Higgs gauge symmetry

It is well known that generic two-Higgs-doublet models (2HDMs) suffer from potentially large Higgs-mediated flavor-changing neutral current (FCNC) problem, unless additional symmetries are imposed on the Higgs fields thereby respecting the Natural Flavor Conservation Criterion (NFC) by Glashow and Weinberg. A common way to respect the NFC is to impose Z_2 symmetry which is softly broken by a dim-2 operator. Another new way is to introduce local U(1)_H Higgs flavor symmetry that distinguishes one Higgs doublet from the other. In this paper, we consider the Higgs phenomenology in Type-I 2HDMs with the U(1)_H symmetry with the simplest U(1)_H assignments that the SM fermions are all neutral under U(1)_H, and we make detailed comparison with the ordinary Type-I 2HDM. After imposing various constraints such as vacuum stability and perturbativity as well as the electroweak precision observables and collider search bounds on charged Higgs boson, we find that the allowed Higgs signal strengths in our model are much broader than those in the ordinary Type-I 2HDM, because of newly introduced U(1)_H-charged singlet scalar and U(1)_H gauge boson. Still the ATLAS data on gg to h to gamma gamma cannot be accommodated. Our model could be distinguished from the ordinary 2HDM with the Z_2 symmetry in a certain parameter region and some channels. If the couplings of the new boson turn out to be close to those in the SM, it would be essential to search for extra U(1)_H gauge boson and/or one more neutral scalar boson to distinguish two models.

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arXiv2013

Singlet portal extensions of the standard seesaw models to dark sector with local dark symmetry: An alternative to the new minimal standard model

Assuming dark matter is absolutely stable due to unbroken dark gauge symmetry and singlet operators are portals to the dark sector, we present a simple extension of the standard seesaw model that can accommodate all the cosmological observations as well as terrestrial experiments available as of now, including leptogenesis, extra dark radiation of $\sim 0.08$ (resulting in $N_{\rm eff} = 3.130$ the effective number of neutrino species), Higgs inflation, small and large scale structure formation, and current relic density of scalar DM ($X$). The Higgs signal strength is equal to one as in the SM 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.

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arXiv2013

Implications of LHC data on 125GeV Higgs-like boson for the Standard Model and its various extensions

Recent data on 125 GeV Higgs-like boson at the LHC starts to constrain the electroweak symmetry breaking sector of the SM and its various extensions. If one imposes the local gauge symmetry of the Standard Model (SM) ($SU(3)_c \times SU(2)_L \times U(1)_Y$) to the SM and any possible new physics scenarios, the SM Higgs properties will be modified by intrinsically two different ways: by new physics either coupling directly to the SM Higgs boson $h$, or affecting indirectly the SM Higgs properties through the mixing of $h$ with a SM singlet scalar $s$. The models of two Higgs doublet, extra sequential and mirror fermions belong to the first category, whereas the models with a hidden sector dark matter, extra vector-like fermions and new charged vector bosons, which can enhance the diphoton rate of the SM Higgs-like resonance, belong to the second category. We perform a global fit to data in terms of the effective Lagrangian description of two interaction eigenstates of scalar bosons, a SM Higgs and a singlet scalar, and their mixing. This framework is more suitable to study singlet-extended scenarios discussed above compared to other approaches based on the Lagrangian of mass eigenstates. With fairly model-independent assumptions, the effective Lagrangian contains at most four free parameters still encompassing the majority of models in the literature. Interestingly, the SM gives the best fit if all data from ATLAS and CMS are used, whereas various singlet extensions can fit better to individual ATLAS or CMS data. Without further assumptions, an upper bound on the total width (or, non-standard branching ratio) is generically obtained. Furthermore, global fit based on our parameterization can be used to probe interactions of the singlet scalar if the singlet resides below $2m_W$.

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arXiv2013

Singlet Portal Extensions of the Standard Seesaw Models to a Dark Sector with Local Dark Symmetry

Assuming ({\it i}) the seesaw mechanism works for neutrino masses and mixings, ({\it ii}) dark matter is absolutely stable due to unbroken $U(1)_X$ dark gauge symmetry, and ({\it iii}) the singlet fields ($H^\dagger H$ and $N_R$) are portal to the dark sector, we construct a simple model which is consistent with all the cosmological observations as well as terrestrial experiments available as of now, including leptogenesis, extra dark radiation of $\sim 8 %$ (resulting in $N_{\rm eff} = 3.130$ the effective number of neutrino species), Higgs inflation, small and large scale structure formation, and current relic density of scalar dark matter ($X$). The electroweak vacuum of this model is stable up to Planck scale for $m_H = 125$ GeV without any other new physics. 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. Detailed study of Higgs properties at the LHC would shed light on the models described in this work.

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