arXiv subjects

Seung-il Nam

Publications and source records attributed to Seung-il Nam.

At least 109 records · Page 6Linked to original sources

arXiv2009

Photoproduction of Theta^+(1540,1/2^+) reexamined with new theoretical information

We reinvestigate the photoproduction of the exotic pentaquark baryon Theta^+(1540,1/2^+) from the gamma N -> Kbar Theta^+ reaction process within the effective Lagrangian approach, taking into account new theoretical information on the K-N-Theta and K*-N-Theta coupling strengths from the chiral quark-soliton model (chiQSM). We also consider the crossing-symmetric hadronic form factor, satisfying the on-shell condition as well. Due to the sizable vector and tensor couplings for the vector kaon, g_{K* N Theta} and f_{K* N Theta}, which are almost the same with the vector coupling g_{K N Theta} ~ 0.8 for the pseudoscalar kaon, the K*-exchange contribution plays a critical role in the photon beam asymmetries.

hep-ph↗

arXiv2009

Scalar susceptibility from the instanton vacuum with meson-loop corrections

The scalar susceptibility (chi_s) of QCD, which represents the response of the chiral condensate to a small perturbation of explicit chiral-symmetry breaking, is investigated within the nonlocal chiral quark model (NLchiQM) based on the instanton vacuum configuration for N_f = 2. We also take into account 1/N_c meson-loop (ML) corrections including scalar and pseudoscalar mesons. It turns out that the chiral condensate is modified to a large extend by the ML corrections in the vicinity of m = 0, whereas its effect becomes weak beyond m ~ 100 MeV. As numerical results, we find that chi_s = -0.34 GeV^2 with the ML corrections and 0.18 GeV^2 without it, respectively. From these observations, we conclude that the ML corrections play an important role in the presence of finite current-quark mass.

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arXiv2008

Pion and kaon structures from the instanton vacuum

In this presentation, we briefly review recent investigations on pion and kaon structures from the instanton vacuum. Starting from the low-energy QCD partition function, we have computed the Gasser-Leutwyler low-energy constants, electromagnetic form factors of the pion and kaon, semileptonic decay form factors of the kaon, and light-cone distribution amplitues of the pion and kaon. The results are in good agreement with the experimental and empirical data.

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arXiv2008

Pion properties at finite density

In this talk, we report our recent work on the pion weak decay constant (F_pi) and pion mass (m_pi) using the nonlocal chiral quark model with the finite quark-number chemical potential (mu) taken into account. Considering the breakdown of Lorentz invariance at finite density, the time and space components are computed separately, and the corresponding results turn out to be: F^t_pi = 82.96 MeV and F^s_pi = 80.29 MeV at mu_c ~ 320 MeV, respectively. Using the in-medium Gell-Mann Oakes-Renner (GOR) relation, we show that the pion mass increases by about 15% at mu_c.

hep-ph↗

arXiv2008

Pion weak decay constant at finite density from the instanton vacuum

We investigate the pion weak decay constant (F_pi) and pion mass (m_pi) at finite density within the framework of the nonlocal chiral quark model from the instanton vacuum with the finite quark-number chemical potential (mu) taken into account. We mainly focus on the Nambu-Goldstone phase below the critical value of the chemical potential mu_c ~ 320 MeV, which is determined consistently within the present framework. The breakdown of Lorentz invariance at finite density being considered, the time (F^t_pi) and space (F^s_pi) components are computed separately, and the corresponding results turn out to be: F^t_pi = 82.96 MeV and F^s_pi = 80.29 MeV at mu_c, respectively. Using the in-medium Gell-Mann-Oakes-Renner (GOR) relation, we show that the pion mass increases by about 15% at mu_c.

hep-ph↗

arXiv2008

Electromagnetic form factors of the pion and kaon from the instanton vacuum

We investigate the pion and kaon (pi^+, K^+, K^0) electromagnetic form factors in the space-like region: Q^2 ~< 1 GeV, based on the gauged low-energy effective chiral action from the instanton vacuum in the large N_c limit. Explicit flavor SU(3) symmetry breaking is taken into account. The nonlocal contributions turn out to be crucial to reproduce the experimental data. While the pion electromagnetic form factor is in good agreement with the data, the kaon one seems underestimated. We also calculate the electromagnetic charge radii for the pion and kaon: _{pi^+}=0.455 fm^2, _{K^+}=0.486 fm^2 and _{K^0}=-0.055 fm^2 without any adjustable free parameter except for the average instanton size and inter-instanton distance, and they are compatible with the experimental data. The low-energy constant L_9 in the large N_c limit is estimated to be 8.42*10^{-3} from the pion charge radius.

hep-ph↗

arXiv2008

Magnetic susceptibility of QCD vacuum at finite density from the nonlocal chiral quark model

We present in this talk a recent investigation on the magnetic susceptibility (chi) of the QCD vacuum at finite density, utilizing the nonlocal chiral quark model from the instanton vacuum. We take into account the nonzero current-quark mass (m_q) explicitly to consider the effect of explicit flavor SU(3) symmetry breaking. It turns out that, when we turn on the current-quark mass, chi becomes smaller, indicating less response to the externally induced electromagnetic field, in comparison to that for m_q=0.

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arXiv2008

Pion electromagnetic charge radii and rho-meson mass shift at finite density

We investigate the pion electromagnetic charge radii and mass dropping of the rho meson at finite density. We first calculate the pion charge radii within the framework of the nonlocal chiral quark model from the instanton vacuum both at zero and finite densities. In order to relate the change of the pion charge radius to that of the rho-meson mass at finite density, we employ the vector-meson dominance for the pion electromagnetic form factor. It turns out that the pion charge radius is getting larger as the quark chemical potential increases. As a result, the rho-meson mass falls off as the quark chemical potential grows and is found to be dropped by about 10% at normal nuclear matter density: m*_rho/m_rho ~ 1 - 0.12 mu_B/mu_0, which is compatible to the results of the measurement at the KEK recently.

hep-ph↗

arXiv2007

Pion electromagnetic form factor at finite density

In this talk, we introduce an effective chiral action, derived from the instanton QCD vacuum configuration, in order to study the modification of mesonic properties in medium. We present our recent theoretical results on the electromagnetic from factor and rho-meson mass shift at finite density. We observe that the rho-meson mass is dropped by about 10%: m*_rho/m_rho ~ 1-0.12 mu_B/mu_0, which is compatible with that estimated by the QCD sum rule as well as by Brown and Rho.

hep-ph↗

arXiv2007

A new nucleon resonance in eta photoproduction

We present in this talk recent investigations on the nucleon-like resonance N*(1675) newly found in eta photoproduction by the GRAAL, Tohoku LNS-gamma and CB-ELSA collaborations. We focus on the production mechanism of the N*(1675), examining its spin and parity theoretically within the framework of the effective Lagrangian method. We expliucitly consider D_{13}(1520), S_{11}(1535), S_{11}(1650), D_{15}(1675), P_{11}(1710), P_{13}(1720) as well as possible background contributions. We calculate the differential cross sections and beam asymmetries for the neutron and proton targets. It turns out that there is manifest isospin asymmetry in eta photoproduction, which can be explained by the asymmetry in the transition magnetic moments: mu_{gamma p p*} =<mu_{gamma n n*}. Moreover, we find that the spin-1/2 state is preferred and this observation implies that the new resonance may be identified as a non-strangeness member of the baryon antidecuplet.

hep-ph↗

arXiv2007

Kaon semileptonic decay (K_{l3}) form factor in the nonlocal chiral quark model

We investigate the kaon semileptonic decay (K_{l3}) form factors within the framework of the nonlocal chiral quark model from the instanton vacuum, taking into account the effects of flavor SU(3) symmetry breaking. All theoretical calculations are carried out without any adjustable parameter. We also show that the present results satisfy the Callan-Treiman low-energy theorem as well as the Ademollo-Gatto theorem. It turns out that the effects of flavor SU(3) symmetry breaking are essential in reproducing the kaon semileptonic form factors. The present results are in a good agreement with experiments, and are compatible with other model calculations.

hep-ph↗

arXiv2007

A new candidate for non-strangeness pentaquarks: N*(1675)

We study a new nucleon resonance from eta photoproduction, which was observed at sqrt{s}=1675 MeV with a narrow decay width (~10 MeV) by the Tohoku LNS group as well as the GRAAL collaboration. Using an effective Lagrangian approach, we compute differential cross sections for the eta photoproduction. In addition to N*(1675,1/2+-,3/2+-), we employ six other nucleon resonances, i.e. N*(1520,1535,1650,1675,1710,1720) and vector meson exchanges which are the most relevant ones to this reaction process. As a result, we can reproduce the GRAAL data qualitatively well and observe obvious isospin asymmetry between the transition magnetic moments of N*(1675): mu_{gamma nn*} >> mu_{gamma pp*}, which indicates that the newly found nucleon resonance may be identified as a non-strange pentaquark state.

hep-ph↗

arXiv2007

Kaon semileptonic decay (K_{l3}) form factors from the instanton vacuum

We investigate the kaon semileptonic decay (K_{l3}) form factors within the framework of the nonlocal chiral quark model from the instanton vacuum, taking into account the effects of flavor SU(3) symmetry breaking. We also consider the problem of gauge invariance arising from the momentum-dependent quark mass in the present work. All theoretical calculations are carried out without any adjustable parameter, the average instanton size (rho ~ 1/3 fm) and the inter-instanton distance (R ~ 1 fm) having been fixed. We also show that the present results satisfy the Callan-Treiman low-energy theorem as well as the Ademollo-Gatto theorem. Using the K_{l3} form factors, we evaluate relevant physical quantities. It turns out that the effects of flavor SU(3) symmetry breaking are essential in reproducing the kaon semileptonic form factors. The present results are in a good agreement with experiments, and are compatible with other model calculations.

hep-ph↗

arXiv2007

QCD condensates with flavor SU(3) symmetry breaking from the instanton vacuum

We investigate the effects of flavor SU(3)-symmetry breaking on the quark, gluon, and mixed quark-gluon condensates, based on the nonlocal effective chiral action from the instanton vacuum. We employ two different parameterizations for the dependence of the momentum-dependent dynamical quark mass on the strange current quark mass. With a proper choice of the current-quark mass dependence of the dynamical quark mass, we show that the gluon condensate is independent of the current quark mass. We obtain the ratios of the quark and mixed condensates, respectively: $[<\bar{s}s>/< \bar{u}u>]^{1/3}=0.75$ and $[<\bar{s}σ_{μν}G^{μν}s > <\bar{u}σ_{μν}G^{μν}u>]^{1/5}=0.87$. It turns out that the dimensional parameter $m^2_0=<\bar{q} σ_{μν}G^{μν}q>/ <\bar{q}q> =1.60\sim 1.92 {\rm GeV}^2$.

hep-ph↗

arXiv2007

Test of the reaction mechanism for gamma N -> K Lambda(1520) using the polarized photon

We study the reaction mechanism for the photoproduction of the Lambda(1520), based on an effective Lagrangian approach. We investigate each contribution of the s-, u-, t-channel processes and contact term, separately. One of the most characteristic features of this reaction is the contact-term dominance which governs the photoproduction from the proton, when the K*-exchange contribution is possibly not too large. We suggest several different ways of the polarizations and arrangement of the beam and target to make to understand the role of each contribution separately in future experiments.

hep-ph↗

arXiv2006

Twist-3 pion and kaon distribution amplitudes from the instanton vacuum with flavor SU(3) symmetry breaking

We investigate the twist-3 pion and kaon distribution amplitudes of the pseudoscalar (phi^p_{pi,K}) and pseudotensor (phi^{sigma}_{pi,K}) types, based on the effective chiral action from the instanton vacuum. Effects of flavor SU(3) symmetry breaking are explicitly taken into account. The Gegenbauer moments and the moments of the distribution amplitudes ( ) are also computed. Our results are summarized as follows: a^p_{2,pi} ~ 0.4, a^p_{1,K} ~ 0.02 and a^p_{2,K} ~ 0.14, and a^{sigma}_{2,pi} ~ 0.02 and a^{sigma}_{1,K} ~ a^{sigma}_{2,K} ~ 0; ^p_π ~ ^p_K ~ 0.37 and ^p_K ~ 0, and ^{sigma}_{pi} ~ ^{sigma}_K ~ 0.20 and ^{sigma}_K ~ 0. We compare our results with those from the QCD sum rules. We also discuss the relevant Wilson coefficients which were analyzed recently in chiral perturbation theory.

hep-ph↗

arXiv2006

Leading-twist pion and kaon distribution amplitudes in the gauge-invariant nonlocal chiral quark model from the instanton vacuum

We investigate the leading-twist light-cone distribution amplitudes for the pion and kaon based on the gauge-invariant nonlocal chiral quark model from the instanton vacuum in the presence of external axial-vector currents. We find that the nonlocal contribution from the gauge invariance has much effects on the pion distribution amplitudes, while it changes mildly the kaon ones. We also study the Gegenbauer moments of the distribution amplitudes and compare them with the empirical analysis of the CLEO data.

hep-ph↗

arXiv2006

The leading-twist pion and kaon distribution amplitudes from the QCD instanton vacuum

We investigate the leading-twist light-cone distribution amplitudes for the pion and kaon, based on the nonlocal chiral quark model from the instanton vacuum. Effects of explicit flavor SU(3)-symmetry breaking are taken into account. The Gegenbauer moments are computed, analyzed and compared with those of other models. The one-loop QCD evolution of the moments is briefly discussed. The transverse momentum distributions are also discussed for the pion and kaon light-cone wave functions.

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