Korea Institute for Advanced Study KIAS

Seoul, South Korea

Korea Institute for Advanced Study KIAS

Seoul, South Korea
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Nam S.-I.,Korea Institute for Advanced Study KIAS | Kao C.-W.,Chung Yuan Christian University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We employ the diluted instanton liquid model and the Green-Kubo formula to investigate the shear viscosity of the SU(2) light-flavor quark matter at finite temperature under an external strong magnetic field e|B|∼mπ2. We apply the Schwinger method to calculate the effect of the external magnetic field. We find that the shear viscosity increases as temperature increases even beyond the transition temperature T0=170 MeV if temperature-dependent model parameters are used. On the other hand, with temperature-independent ones the shear viscosity starts to drop when the temperature goes beyond T0. Furthermore, we find that the presence of an external magnetic field will reduce the shear viscosity. However, this effect is almost negligible in the chiral-restored phase even for a very strong magnetic field, e|B|≈1020 gauss. We also compute the ratio of the shear viscosity and the entropy density η/s and our results are well compatible with the other theoretical results for a wide temperature range. We also provide the parametrization of the temperature-dependent ratio η/s from our numerical result as η/s=0.27-0.87/t+1.19/t2-0.28/t3 with t≡T/T0 for T=(100∼350) MeV when e|B|=0. © 2013 American Physical Society.


Nam S.-I.,Korea Institute for Advanced Study KIAS | Kao C.-W.,Chung Yuan Christian University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We investigate the unpolarized pion and kaon fragmentation functions, employing the nonlocal chiral-quark model, which manifests the nonlocal interactions between the quarks and pseudoscalar mesons, considering the explicit flavor-SU(3)-symmetry breaking in terms of the current-quark masses. Moreover, we study the quark distribution functions, derived from the fragmentation ones with the Drell-Yan-Levi relation. Numerical results are evaluated to higher Q2 by the QCD evolution and compared with the empirical data. The ratios between the relevant valance-quark distribution functions are also discussed. It turns out that the present results are in relatively good agreement with available data and other theoretical estimations. © 2012 American Physical Society.


Nam S.-I.,Korea Institute for Advanced Study KIAS | Kao C.-W.,Chung Yuan Christian University
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We study the unpolarized fragmentation functions and the parton distribution functions of the pion employing the nonlocal chiral quark model. This model manifests the nonlocal interactions between the quarks and pseudoscalar mesons in the light-cone coordinate. We find that the nonlocal interactions result in substantial differences in comparison to the result of typical models with only local couplings. We also perform the high Q2-evolution for our results which is calculated at a relatively low-renormalization scale Q2 0.36GeV2 and compare them with the experimental data. It has shown that our results after evolution are in qualitatively good agreement with those experimental data. © 2012 American Physical Society.


Nam S.-I.,Korea Institute for Advanced Study KIAS
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2013

We investigate QCD magnetic susceptibility χq for light-flavor SU(2) at finite temperature (T) beyond the chiral limit, using the liquid instanton model for Nc=3, defined in Euclidean space and modified by the T-dependent caloron solution. The background electromagnetic fields are included in the QCD vacuum, employing the Schwinger method. We first compute the scalar (chiral) and tensor condensates as functions of T and the current-quark mass m, signaling the correct universal chiral restoration patterns. It turns out that χq, given by the ratio of the two condensates, is a smoothly decreasing function of T, showing about 20% reduction of its strength at the chiral transition T≡T0, in comparison to that at T=0, and decreases almost linearly beyond T0 for m≠0. We observe that the present numerical results are in qualitatively good agreement with other theoretical results, including the lattice simulations. Finally, we examine the effects of the external magnetic field on the tensor-polarization VEV, which plays the role of the chiral order parameter. © 2013 American Physical Society.


Nam S.-I.,Korea Institute for Advanced Study KIAS
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

In this work, we construct a phenomenological effective model for the heavy-light-quark systems, which consist of (u,d,c,b) quarks, i.e., extended nonlocal chiral-quark model (ExNLChQM) to compute the heavy-meson weak-decay constants f D and f B. ExNLChQM is based on the heavy-quark effective field theory as well as the dilute-instanton-vacuum configuration. In ExNLChQM, a certain portion of the heavy-meson mass is considered to be generated from the nontrivial QCD-vacuum contribution, similar to the light quarks in usual instanton approaches. Hence, the effective heavy- and light-quark masses become momentum-dependent and play the role of a smooth UV regulator. Employing a generic external-field method applied to the effective action from ExNLChQM, we obtain f D=(169.28∼234.57)MeV and f B=(165.41∼229.21)MeV from the numerical results, depending on different model parameters. These values are in relatively good agreement with experimental data and various theoretical estimations. We also discuss the heavy-quark effects on the QCD vacuum, and the decay constants f D * and fB * in terms of the heavy-quark spin symmetry. © 2012 American Physical Society.


Nam S.-I.,Korea Institute for Advanced Study KIAS
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We investigate the parton-distribution functions (PDFs) for the positively charged pion and kaon at a low renormalization scale ∼1GeV. To this end, we employ the gauge-invariant effective chiral action from the nonlocal chiral-quark model, resulting in the vector currents being conserved. All the model parameters are determined phenomenologically with the normalization condition for PDF and the empirical values for the pseudoscalar meson weak-decay constants. We consider the momentum dependence of the effective quark mass properly within the model calculations. It turns out that the leading local contribution provides about 70% of the total strength for PDF, whereas the nonlocal one, which is newly taken into account in this work for the gauge invariance, does the rest. High-Q2 evolution to 27GeV2 is performed for the valance-quark distribution function, using the Dokshitzer-Gribov-Lipatov- Altarelli-Parisi equation. The moments for the pion and kaon valance-quark distribution functions are also computed. The numerical results are compared with the empirical data and theoretical estimations, and show qualitatively agreement with them. © 2012 American Physical Society.


Nam S.-I.,Korea Institute for Advanced Study KIAS
Physical Review D - Particles, Fields, Gravitation and Cosmology | Year: 2012

We investigate the electrical conductivity (σ) of quark matter via the Kubo formula at finite temperature and zero quark density (T0, μ=0) in the presence of an external strong magnetic field. For this purpose, we employ the dilute instanton-liquid model, taking into account its temperature modification with the trivial-holonomy caloron distribution. By doing that, the momentum and temperature dependences for the effective quark mass and model renormalization scale are carefully evaluated. From the numerical results, it turns out that σ(0.02-0.15)fm -1 for T=(0-400)MeV with the relaxation time τ=(0.3-0.9)fm. In addition, we also parametrize the electrical conductivity as σ/T(0.46,0.77,1.08,1.39)C EM for τ=(0.3,0.5,0.7,0.9)fm, respectively. These results are compatible with other theoretical estimations, including those from the lattice QCD simulations. It also turns out that the external magnetic field plays only a minor role for σ even for the very strong one B 0∼mπ2×10 and becomes relatively effective for T200MeV. Moreover, we compute the soft photon emission rate from the quark-gluon plasma, using the electrical conductivity calculated. © 2012 American Physical Society.


Nam S.-I.,Korea Institute for Advanced Study KIAS | Nam S.-I.,Pukyong National University
Journal of Physics G: Nuclear and Particle Physics | Year: 2013

We investigate the unpolarized electroproduction of Λ(1520) ≡ Λ* off the nucleon target, using the effective Lagrangian method at the tree-level Born approximation with nucleon-resonance contributions from S11(2090), D13(2080) and D15(2200). First, we compute various physical quantities for the proton target case, such as the total and differential cross sections, t-momentum transfer distribution and K- decay-angle (φ) distribution. It turns out that D13 plays an important role in reproducing the electroproduction data properly. The numerical results for the φ distribution show obvious different structures for the photoproduction, due to the enhancement of the kaon exchange by the longitudinal polarization of the virtual photon, as expected. Numerically, we observe that the kaon-exchange contribution in the t channel is about half that from the contact-term one that dominates the photoproduction of Λ*. We also provide theoretical estimations for the Λ* electroproduction off the neutron target, showing that its production rate is almost saturated by resonance contributions. Finally, the contact-term dominance, which is the key ingredient for Λ* electromagnetic productions, is briefly discussed. © 2013 IOP Publishing Ltd.


Batell B.,University of Chicago | Jung S.,Korea Institute for Advanced Study KIAS | Lee H.M.,Korea Institute for Advanced Study KIAS
Journal of High Energy Physics | Year: 2013

Light, electrically charged vector-like 'leptons' with O(1) Yukawa couplings can enhance the h → γγ rate, as is suggested by measurements of the signal strength μ γγ by ATLAS and CMS. However, the large Yukawa interactions tend to drive the Higgs quartic coupling negative at a low scale Λ â‰2; 10 TeV, and as such new physics is required to stabilize the electroweak vacuum. A plausible option, which does not rely on supersymmetry, is that the Higgs in fact has a much larger tree level quartic coupling than in the Standard Model, which is possible with an extended scalar sector. We investigate in detail the minimal model with a new real singlet scalar which condenses and mixes with the Higgs. The mechanism is very efficient when the singlet is heavier than the weak scale m s â‰3; TeV, in which case the vacuum can be stable and the couplings can be perturbative up to scales Λ ∼ 109 GeV while μ γγ ∼ 1.7. On the otherhand, for a singlet at the weak scale, m s ∼ v, a substantial shift in the Higgs quartic coupling requires sizable Higgs-singlet mixing. Such mixing can be utilized to further enhance μ γγ through Yukawa couplings of the singlet to the vector-like leptons. In this case, for an enhancement μ γγ ∼ 1.7 it is possible to obtain a stable vacuum up to scales Λ ∼ 1000 TeV. The singlet can have significant mixing with the Higgs particle, allowing for its resonant production via gluon fusion at the LHC, and can be searched for through Higgs like decays and decays to vector-like leptons, which can lead to multi-lepton final states. We also comment on UV extensions of the minimal model. © 2013 SISSA, Trieste, Italy.


Boukhvalov D.W.,Korea Institute for Advanced Study KIAS | Boukhvalov D.W.,Japan National Institute of Materials Science | Katsnelson M.I.,Radboud University Nijmegen
ACS Nano | Year: 2011

Motivated by recent experimental data (Sepioni, M.; et al. Phys. Rev. Lett. 2010, 105, 207-205), we have studied the possibility of forming magnetic clusters with spin S > 1/ 2 on graphene by adsorption of hydrogen atoms or hydroxyl groups. Migration of hydrogen atoms and hydroxyl groups on the surface of graphene during the delamination of HOPG led to the formation of seven atom or seven OH-group clusters with S = 5/ 2 that were of a special interest. The coincidence of symmetry of the clusters with the graphene lattice strengthens the stability of the cluster. For (OH) 7 clusters that were situated greater than 3 nm from one another, the reconstruction barrier to a nonmagnetic configuration was approximately 0.4 eV, whereas for H 7 clusters, there was no barrier and the high-spin state was unstable. Stability of the high-spin clusters increased if they were formed on top of ripples. Exchange interactions between the clusters were studied and we have shown that the ferromagnetic state is improbable. The role of the chemical composition of the solvent used for the delamination of graphite is discussed. © 2011 American Chemical Society.

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