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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. Source


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. Source


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. Source


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. Source


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. Source

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