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Seung-il Nam

Publications and source records attributed to Seung-il Nam.

At least 91 records · Page 5Linked to original sources
arXiv2011

Charged pion photoproduction with the Delta(1232) baryon beyond the resonance region

We investigate the charged pion photoproduction off the proton target with the Delta(1232) baryon in the final state, i.e. gamma p -> pi^- Delta^++(1232) and gamma p -> pi^+ Delta^0(1232), based on the effective Lagrangian method, beyond the resonance region, E_cm >~ GeV. We employ the pi- and rho-meson Regge trajectories in the t-channel, in addition to the proton- and Delta-pole, in the s- and u-channels respectively, and the contact-interaction contributions. A specific scheme for the form factor which satisfies the Ward-Takahashi identity, crossing symmetry, and on-shell condition, is taken into account. To discuss the validity of the Regge approach within the Effective Lagrangian method, we also consider a smooth interpolation between the Regge- and Feynman-propagators for the t-channel meson exchanges as a function of sqrt(s). We present the numerical results for the energy and angular dependences of the cross sections, and double-polarization observable. It turns out that the present framework shows the significance of the pi-exchange and contact interaction terms to reproduce the experimental data qualitatively well. Especially, the interpolation between the two propagators plays a crucial role to reproduce the high-energy experimental data. The pi^+ decay-angle distribution is also studied using the Delta^++ decay frame, i.e. the Gottfried-Jackson frame. The present results will be a useful guide for future high-energy photon-beam experiments.

hep-ph↗
arXiv2011

Chiral restoration at finite T under the magnetic field with the meson-loop corrections

We investigate the (partial) chiral restoration at finite temperature (T) under the strong external magnetic field B_0 of the SU(2) light-flavor QCD matter. To this end, we employ the instanton-liquid QCD vacuum configuration accompanied with the linear Schwinger method for inducing the magnetic field. The Harrington-Shepard caloron solution is used to modify the instanton parameters, i.e. the average instanton size (rho) and inter-instanton distance (R), as functions of T. In addition, we include the meson-loop corrections (MLC) as the large-N_c corrections because they are critical for reproducing the universal chiral restoration pattern. We present the numerical results for the constituent-quark mass as well as chiral condensate which signal the spontaneous breakdown of chiral-symmetry SBCS, as functions of T and B_0. From our results we observe that the strengths of those chiral order parameters are enhanced with respect to B_0 due to the magnetic catalysis effect. We also find that there appears a region where the u and d-quark constituent masses coincide with each other at eB_0 = (7 ~ 9) m^2_pi, even in the presence of the explicit isospin breaking. The critical T for the chiral restoration T_c tends to shift to the higher temperature in the presence of the B_0 for the chiral limit but keeps almost stationary for the physical quark mass case. We also compute the pion weak-decay constant F_pi and pion mass m_pi below T_c, varying the strength of the magnetic field, showing correct partial chiral restoration behaviors. Besides we find that the changes for the F_pi and m_pi due to the magnetic field is relatively small, in comparison to those caused by the finite T effect.

hep-ph↗
arXiv2011

Spin structure of the pion from the instanton vacuum

We investigate the spin structure of the pion within the framework of the nonlocal chiral quark model from the instanton vacuum. We first evaluate the tensor form factors of the pion for the first and second moment (n=1,2) and compare it with the lattice data. Combining the tensor form factor of the pion with the electromagnetic one, we determine the impact-parameter dependent probability density of transversely polarized quarks inside the pion. It turns out that the present numerical results for the tensor form factor as well as those for the probability density are in good agreement with the lattice data. We also discuss the distortion of the spatial distribution of the quarks in the transverse plane inside the pion.

hep-ph↗
arXiv2011

K*Λ(1116) photoproduction and nucleon resonances

In this presentation, we report our recent studies on the $K^*Λ(1116)$ photoproduction off the proton target, using the tree-level Born approximation, via the effective Lagrangian approach. In addition, we include the nine (three- or four-star confirmed) nucleon resonances below the threshold $\sqrt{s}_\mathrm{th}\approx2008$ MeV, to interpret the discrepancy between the experiment and previous theoretical studies, in the vicinity of the threshold region. From the numerical studies, we observe that the $S_{11}(1535)$ and $S_{11}(1650)$ play an important role for the cross-section enhancement near the $\sqrt{s}_\mathrm{th}$. It also turns out that, in order to reproduce the data, we have the vector coupling constants $g_{K^*S_{11}(1535)Λ}=(7.0\sim9.0)$ and $g_{K^*S_{11}(1650)Λ}=(5.0\sim6.0)$.

hep-ph↗
arXiv2011

An effective thermodynamic potential from the instanton vacuum with the Polyakov loop

In this talk, we report our recent studies on an effective thermodynamic potential (Omega_eff) at finite temperature (T>0) and zero quark-chemical potential (mu_R=0), using the singular-gauge instanton solution and Matsubara formula for N_c=3 and N_f=2 in the chiral limit, i.e. m_q=0. The momentum-dependent constituent-quark mass is computed as a function of T, together with the Harrington-Shepard caloron solution in the large-N_c limit. In addition, we take into account the imaginary quark-chemical potential mu_I = A_4, indentified as the traced Polayakov-loop (Phi) as an order parameter for the Z(N_c) symmetry, characterizing the confinement (intact) and deconfinement (spontaneously broken) phases. As a consequence, we observe the crossover of the chiral (chi) order parameter sigma^2 and Phi. It also turns out that the critical temperature for the deconfinement phase transition, T^Z_c is lowered by about (5~10) % in comparison to the case with the constant constituent-quark mass. This behavior can be understood by considerable effects from the partial chiral restoration and nontrivial QCD vacuum on the Phi. Numerical results show that the crossover transitions occur at (T^chi_c,T^Z_c) ~ (216,227) MeV.

hep-ph↗
arXiv2011

Lambda(1520) photoprodcution with Regge contribution

In this talk, we report our recent progresses on the Lambda(1520) photoproduction using the effective Lagrangian approach. In addition to the tree-level Born diagrams, we take into account the Regge-trajectories for the possible strange-meson exchanges in the t channel. We compute the angular and energy dependences of the production process, including polarization observables, such as the photon-beam asymmetry and the polarization-transfer coefficients, resulting in good qualitative agreement with current experimental data. We also compute the K^- angle distribution function in the Gottfried-Jackson frame, using the polarization-transfer coefficients in the z direction.

hep-ph↗
arXiv2010

QCD under magnetic field: chiral magnetic effect at heavy ion collisions

In this talk, we report our recent studies for the chiral magnetic effect, which signals the P- and CP-violations at the heavy ion collisions. We compute the electric current and its correlations, induced by the external magnetic field, inside the hot and dense QCD matter created in the heavy ion collisions. For this purpose, we employ the instanton-liquid model, modified by the Harrington-Shepard caloron at finite T. We observe that the chiral magnetic effect current and its correlations increase with respect to the magnetic field, whereas they decrease as functions of T, due to the diluting instanton ensemble. It turns out that the numerical results are in good agreement with those from the model-independent analyses and lattice QCD simulations. We also reproduce the charge separations, observed in the STAR experiment, qualitatively well, considering the simplified Lienard-Wiechert potential, screening and size effects.

hep-ph↗
arXiv2010

Chiral restoration at finite temperature with meson loop corrections

We investigate chiral-restoration patterns of QCD for N_{c}=3 and N_{f}=2 at finite temperature (T) and zero quark-chemical potential beyond the chiral limit, indicating the explicit chiral-symmetry breaking. To this end, we employ the instanton-vacuum configuration for the flavor SU(2) sector and the Harrington-Shepard caloron for modifying relevant instanton parameters as functions of T. The meson-loop corrections (MLC), which correspond to 1/N_{c} corrections, are also taken into account to reproduce appropriate m_{q} dependences of chiral order parameters. We compute chiral condensate as a function of T and/or m_{q}. From the numerical calculations, we observe that MLC play an important role to have a correct universality-class behavior of chiral-restoration patterns in this framework, depending on m_{q}: Second-order phase transition in the chiral limit, m_{q}=0 and crossover for m_{q}>0. Without MLC, all the restoration patterns are crossover, due to simple saddle-point approximations. It turns out that T^χ_{c}=159 MeV in the chiral limit and T^χ_{c}=(177,186,196) MeV for m_{q}=(5,10,15) MeV, using the phenomenological choices for the instanton parameters at T=0.

hep-ph↗
arXiv2010

Vector-current correlation and charge separation via chiral-magnetic effect

We investigate the vector-current correlation Pi_{mu nu} (VCC) in the presence of a strong external magnetic field (B_0 in the z direction) at low temperature (T 0), i.e. the chiral-magnetic effect (CME). For this purpose, we employ the instanton-vacuum configuration at finite T with nonzero topological charge (Q_t>0). From the numerical calculations, it turns out that the longitudinal component of the connected VCC is liner in B_0 and shows a bump, representing a corresponding vector meson at |Q|=(300~400) MeV for T=0. The bump becomes enhanced as T increases and the bump position shifts to a lager |Q| value. In the limit of |Q|->0, the transverse component of the connected VCC disappears, whereas the longitudinal one remains finite and gets insensitive to B_0 with respect to T, due to diluting instanton contributions. Considering a simple collision geometry of HIC and some assumptions on the induced magnetic field and screening effect, we can estimate the charge separation (ChS) as a function of centrality using the present results for VCC. The numerical results show a qualitative agreement with experiments for the Au+Au and Cu+Cu collisions. These results are almost independent on the source of CME, instanton or sphaleron, as long as the CME current is linear in B_0.

hep-ph↗
arXiv2010

An effective thermodynamic potential from the instanton with Polyakov-loop contributions

We derive an effective thermodynamic potential (Omega_eff) at finite temperature (T>0) and zero quark-chemical potential (mu_R=0), using the singular-gauge instanton solution and Matsubara formula for N_c=3 and N_f=2 in the chiral limit. The momentum-dependent constituent-quark mass is also obtained as a function of T, employing the Harrington-Shepard caloron solution in the large-N_c limit. In addition, we take into account the imaginary quark chemical potential mu_I = A_4, translated as the traced Polayakov-loop (Phi) as an order parameter for the Z(N_c) symmsetry, characterizing the confinement (intact) and deconfinement (spontaneously broken) phases. As a result, we observe the crossover of the chiral (chi) order parameter sigma^2 and Phi. It also turns out that the critical temperature for the deconfinment phase transition, T^Z_c is lowered by about (5-10)% in comparison to the case with a constant constituent-quark mass. This behavior can be understood by considerable effects from the partial chiral restoration and nontrivial QCD vacuum on Phi. Numerical calculations show that the crossover transitions occur at (T^chi_c,T^Z_c) ~ (216,227) MeV.

hep-ph↗
arXiv2009

Chiral magnetic effect (CME) at low temperature from instanton vacuum

In this talk, we report our present work on the chiral magnetic effect (CME) under a strong magnetic field at low temperature. To this end, we use the instanton vacuum with the finite instanton-number fluctuation Delta, which relates to the nontrivial topological charge Q_t. We compute the vacuum expectation values of the local chiral density , chiral charge density and induced electromagnetic current . We observed that the longitudinal EM current is much larger than the transverse one, |j_perp/j_parallel| ~ Q_t, and the equals to the | |. It also turns out that the CME becomes insensitive to the magnetic field as T increases, since the instanton effect decreases.

hep-ph↗
arXiv2009

Chiral magnetic effect at low temperature

We investigate the chiral magnetic effect (CME) under a strong magnetic field B = B_0 x_3 at low temperature T < T^chi_c. For this purpose, we employ the instanton vacuum configuration with the finite instanton-number fluctuation Delta, which relates to the nontrivial topological charge Q_t. We compute the vacuum expectation values of the local chiral density , chiral charge density and induced electromagnetic current , which signal the CME, as functions of T and B_0. We observed that the longitudinal EM current is much larger than the transverse one, |j_perp/j_parallel| ~ Q_t, and the equals to the | |. It also turns out that the CME becomes insensitive to the magnetic field as T increases, according to the decreasing instanton, i.e. tunneling effect. Within our framework, the instanton contribution to the CME becomes almost negligible beyond T~300 MeV.

hep-ph↗
arXiv2009

Studies on the polarization transfer coefficients C_x and C_z in the gamma p -> K^+ Lambda(1520,3/2^-) reaction process

We investigate the polarization transfer coefficients C_(x,z) for the gamma p -> K^+ Lambda(1520,3/2^-) reaction process, in which the photon is polarized circularly and the Lambda(1520,3/2^-) along the x- or z-axis. To this end, we employ the effective Lagrangian method at tree level and the gauge-invariant form factor scheme. In addition to the Born terms, (s,u,t_K,t_K^*)-channels and contact term, we include the nucleon resonance D_{13}(2080) in the s-channel. We compute the C_(x,z) as functions of theta_K as well as E_cm. It turns out that the K^*-exchange and D_{13} contributions are negligible within available experimental and theoretical inputs for them. In contrast, we observe that the contact and K-exchange contributions play dominant roles for determining the C_(x,z). Especially, the $K$-exchange enhances the transverse polarization transfer C_x considerably.

hep-ph↗
arXiv2009

Photoproduction of the Theta^+ and its vector and axial-vector structure

We present recent investigations on the vector and axial-vector transitions of the baryon antidecuplet within the framework of the self-consistent SU(3) chiral quark-soliton model, taking into account the 1/N_c rotational and linear m_s corrections. The main contribution to the electric-like transition form factor comes from the wave-function corrections. This is a consequence of the generalized Ademollo-Gatto theorem. It is also found that in general the leading-order contributions are almost canceled by the rotational 1/N_c corrections. The results are summarized as follows: the vector and tensor K^*-N-Theta coupling constants, g_{K^*-N-Theta}=0.74 - 0.87 and f_{K^*-N-Theta}=0.53 - 1.16, respectively, and Gamma_{Theta->KN}=0.71 MeV, based on the result of the K-N-Theta coupling constant g_{K-n-Theta}=0.83. We also show the differential cross sections and beam asymmetries, based on the present results. We also discuss the connection of present results with the original work by Diakonov, Petrov, and Polyakov.

hep-ph↗
arXiv2009

Magnetic susceptibility of the QCD vacuum at finite quark-chemical potential

We investigate the QCD magnetic susceptibility chi at the finite quark-chemical potential (mu>0) and at zero temperature (T=0) to explore the pattern of the magnetic phase transition of the QCD vacuum. For this purpose, we employ the nonlocal chiral quark model derived from the instanton vacuum in the presence of the chemical potential in the chiral limit. Focusing on the Nambu-Goldstone phase, we find that the magnetic susceptibility remains almost stable to mu~200 MeV, and falls down drastically until the the quark-chemical potential reaches the critical point mu_c~320 MeV. Then, the strength of the chi is reduced to be about a half of that at mu=0, and the first-order magnetic phase transition takes place, corresponding to the chiral restoration. From these observations, we conclude that the response of the QCD vacuum becomes weak and unstable to the external electromagnetic field near the critical point, in comparison to that for vacuum. It is also shown that the breakdown of Lorentz invariance for the magnetic susceptibility, caused by the finite chemical potential, turns out to be small.

hep-ph↗
arXiv2009

Lambda(1405) photoprodcution with p-wave contributions

In this report, we present our recent studies on the Lambda(1405,1/2-)=Lambda* photoproduction via the gamma p -> K+ pi Sigma scattering process, employing a coupled-channel formalism, i.e., the chiral unitary model, which respects chiral dynamics in terms of the Weinberg-Tomozawa meson baryon interaction. In addition to the dynamically generated s-wave resonance, Lambda*, we include the p-wave resonance contribution from Sigma(1385,3/2+). We observe that the p-wave contribution provides considerable effects on the pi-Sigma invariant-mass spectrum. It also turns out that, within the present level of calculations, we can reproduce hardly the recent Lambda* photoproduction experiment by LEPS collaboration at SPring-8.

hep-ph↗
arXiv2009

eta photoproduction and N* resonances

We investigate the eta photoproduction using the effective Lagrangian approach at the tree level. We focus on the new nucleon resonance N*(1675), which was reported by the GRAAL, CB-ELSA and Tohoku LNS, testing its possible spin and parity states theoretically (J^P=1/2^+-,3/2^+-). In addition, we include six nucleon resonances, D_13(1520), S_11(1535), S_11(1650), D_15(1675), P_11(1710), P_13(1720) as well as the possible background contributions. We calculate various cross sections including beam asymmetries for the neutron and proton targets. We find noticeable isospin asymmetry in transition amplitudes for photon and neutron targets. This observation may indicate that the new resonance can be identified as a non-strangeness member of the baryon antidecuplet.

hep-ph↗