arXiv subjects

Seung-il Nam

Publications and source records attributed to Seung-il Nam.

At least 55 records · Page 3Linked to original sources

arXiv2019

Empirical estimate of the unpolarized dihadron fragmentation functions

We estimate the unpolarized dihadron fragmentation functions (uDiFFs) within the framework of the single-cascade jet algorithm. We first obtain the elementary fragmentation functions generating the single hadron fragmentation functions by using the single-cascade jet algorithm. These results are very close to the empirical parametrizations of the single hadron fragmentation functions which are extracted from the experimental data of the $e^+e^-$ annihiliation and semi-inclusive deeply inelastic scatterings (SIDIS). We then use those elementary fragmentation functions to generate the dihadron fragmentation functions by the help of the single-cascade jet algorithm again. The comparison between our empirical results with the results of the Nambu-Jona-Lasinio model and the non-local chiral quark model (NL$χ$QM) is also presented.

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arXiv2019

Hyperon production in $Λ_c^+\to K^-pπ^+$ and $Λ_c^+\to K^0_Spπ^0 $

We investigate $S=-1$ hyperon production from the $Λ_c^+\to K^-pπ^+$ and $Λ_c^+\to K^0_Spπ^0$ decays within the effective Lagrangian approach. We consider the $Σ/Λ$ ground states, $Λ(1520)$, $Λ(1670)(J^p=1/2^-)$, $Λ(1890)(J^p=3/2^+)$; $Λ/Σ$-pole contributions from the combined resonances between 1800 MeV and 2100 MeV; and $N/Δ$-pole and $K^\ast$-pole contributions, which include the proton, $Δ(1232)$, and $K(892)$. We calculate the Dalitz plot density $(d^2Γ/dM_{K^-p}dM_{K^-π^+}$) for the $Λ_c^+\to K^-pπ^+$ decay. The calculated result is in good agreement with experimental data from the Belle Collaboration. Using the parameters from the fit, we present the Dalitz plot density for the $Λ_c^+\to K^0_Spπ^0$ decay. In our calculation, a sharp peak-like structure near 1665 MeV is predicted in the $Λ_c^+\to K^-pπ^+$ decay because of the interference effects between the $Λ(1670)$ resonance and $η$-$Λ$ loop channels. We also demonstrate that we can access direct information regarding the weak couplings of $Λ(1670)$ and $Σ(1670)$ from the $Λ_c^+\to K^0_Spπ^0$ decay. Finally, a possible interpretation for the 1665 MeV structure beyond our prediction is briefly discussed.

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arXiv2019

Studies on photo- and electro-productions of $Λ(1405)$ via $γ^{()} p\to K^{+}π^0Σ^0$

We study the photo- and electro-productions of the vector kaon off the proton, i.e., $γ^{(*)}p\to K^{*+}π^0Σ^0$, and investigate the line shape of the $π^0Σ^0$ invariant mass in an effective Lagrangian approach with the inclusion of a $K^*NΛ^*$ interaction. Relevant electromagnetic form factors for the neutral hyperons and charged strange mesons are constructed by considering experimental and theoretical information. We find that the $Λ^*$ peak is clearly observed for the photo- and electro-productions with the finite $K^*NΛ^*$ interaction, whereas the clear peak signals survive only for the electro-production, when we ignore the interaction. These different behaviors can be understood by different $Q^2$ dependences in the $K^*$ electromagnetic and $K^*\toγK$ transition form factors. We suggest a photon-polarization asymmetry $Σ$ to extract information of the $K^*NΛ^*$ interaction. It turns out that $Σ$ near the $Λ^*$ peak region becomes negative with a finite $K^*NΛ^*$ interaction while positive without it for $Q^2 = 0$, due to the different naturalities of $K$ and $K^*$ exchanges. For $Q^2\ne 0$, we observe more obvious signals in the peak region due to the additional contribution of the longitudinal virtual photon for $Λ^*$.

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arXiv2019

$Ξ(1690)^-$ resonance production via $K^-p \to K^+K^-Λ$

In this talk, we investigate $Ξ(1690)^-$ production from the $K^-p\to K^+K^-Λ$ reaction wit the effective Lagrangian method and consider the $s$- and $u$-channel $Σ/Λ$ ground states and resonances for the $Ξ$-pole contributions, in addition to the $s$-channel $Λ$, $u$-channel nucleon pole, and $t$-channel $K^-$-exchange for the $ϕ$-pole contributions. The $Ξ$-pole includes $Ξ(1320)$, $Ξ(1535)$, $Ξ(1690)(J^p=1/2^-)$, and $Ξ(1820)(J^p=3/2^-)$. We compute the Dalitz plot density of $(d^2σ/dM_{K^+K^-}dM_{K^-Λ}$ at 4.2 GeV$/c$) and the total cross sections for the $K^-p\to K^+K^-Λ$. Employing the parameters from the fit, we present the cross sections for the two-body $K^-p\to K^+Ξ(1690)^-$ reaction near the threshold. We also demonstrate that the Dalitz plot analysis for $p_{K^-}=1.915 \sim2.065$ GeV/c makes us to explore direct information for $Ξ(1690)^-$ production, which can be done by future $K^-$ beam experiments.

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arXiv2018

$Ξ(1690)^-$ production in the $K^-p\to K^+K^-Λ$ reaction process near threshold

We investigate $Ξ(1690)^-$ production from the $K^-p\to K^+K^-Λ$ reaction within the effective Lagrangian approach at the tree-level Born approximation. We consider the $s$- and $u$-channel $Σ/Λ$ ground states and resonances for the $Ξ$-pole contributions, in addition to the $s$-channel $Λ$, $u$-channel nucleon pole, and $t$-channel $K^-$-exchange for the $ϕ$-pole contributions. The $Ξ$-pole includes $Ξ(1320)$, $Ξ(1535)$, $Ξ(1690)(J^p=1/2^-)$,and $Ξ(1820)(J^p=3/2^-)$. We calculate the Dalitz plot density of $(d^2σ/dM_{K^+K^-}dM_{K^-Λ}$ at 4.2 GeV$/c$) and the total cross sections for the $K^-p\to K^+K^-Λ$ reaction near the threshold. The calculation results are in good agreement with previously acquired experimental data. Using the parameters from the fit, we present the total and differential cross sections for the two-body $K^-p\to K^+Ξ(1690)^-$ reaction near the threshold. In our calculation, a strong enhancement at backward $K^+$ angles is predicted because of the dominant $u$-channel contribution. We also demonstrate that the Dalitz plot analysis for $p_{K^-}=1.915 -- 2.065$ GeV/c enables us to access direct information regarding $Ξ(1690)^-$ production, which can be tested by future $K^-$ beam experiments. The possible spin-parity states of $Ξ(1690)^-$ are briefly discussed as well.

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arXiv2017

Quasi-distribution amplitudes for pion and kaon via the nonlocal chiral-quark model

We investigate the pseudoscalar (PS) meson ($π$ and $K$) quasi-distribution amplitude (QDA), which is supposed to be an asymptotic analog to the meson distribution amplitude (DA) $ϕ_{π,K}(x)$ in the limit of the large longitudinal PS-meson momentum, i.e. $p_3\to\infty$, in the nonperturbative region. For this purpose, we employ the nonlocal chiral-quark model (NLChQM) in the light-front formalism with a minimal Fock-state for the mesons $\sim q\bar{q}$ at the low-energy scale parameter of the model $Λ\sim1$ GeV. As a trial, we extract the transverse-momentum distribution amplitude (TMDA) from the light-front wave function within the model, and convert it to QDA with help of the virtuality-distribution amplitude. By doing that, we derive an analytical expression for the nonperturbative QDA with the current-quark mass correction up to $\mathcal{O}(Δm_q)$. Numerically, we confirm that the obtained TMDA reproduces the experimental data for the photon-pion transition form factor $F_{γγ^*π^0}(Q^2)$ at the low-$Q^2$ qualitatively well. We also observe that the obtained QDA approaches to DA as $p_3$ increases, showing the symmetric and asymmetric curves with respect to $x$ for the pion and kaon, respectively, due to the current-quark mass difference $m_{u,d}\ll m_s$. Assigning $ξ\equiv2x-1$, the moments $\langleξ^n\rangle_{π,K}$ are computed, using the pion and kaon QDAs, and there appear only a few percent deviations in the moments for $p_3\gtrsim30Λ$ in comparison to the values calculated directly from DAs. It turns out that the higher moments are more sensitive to the change of $p_3$, whereas the lower ones depend less on it.

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arXiv2017

Photo- and electroproduction of $Λ(1405)$ via $γ^{(*)}p\to K^+π^+Σ^-$

In this work, we perform a \textit{toy-model} analysis for the unpolarized photo- and electroproduction of $Λ(1405)\equivΛ^*$ via $γ^{(*)}p\to K^+π^+Σ^-$ by employing the effective Lagrangian approach at the tree level only. We consider that $Λ^*$ consists of the high-mass ($H$) and low-mass ($L$) poles as suggested by the chiral unitary model (ChUM). We determine all the model parameters including coupling constants and cutoff parameters for the phenomenological strong form factors by using the theoretical information from ChUM and available experimental data. The electromagnetic (EM) form factors for the two poles are parameterized appropriately being similar to that for the neutron by employing the ChUM estimates for the EM root-mean-squared (rms) radii for the two poles. We observe from the numerical calculations that, for the photoproduction of $Λ^*$, the interference between the two poles turns out to be destructive, resulting in the single-peak line at $M_{π^+Σ^-}\approx1405$ MeV in the invariant-mass distribution. On the contrary, there appear two peaks in the distribution for the electroproduction as observed in the CLAS/Jlab electroproduction data, due to the constructive interference, which is mainly caused by the Dirac form factors for the two poles. From this observation, we conclude that, in order to explain the photo- and electroproduction data simultaneously, i.e., \textit{single} and \textit{double} peaks, respectively, in the invariant-mass distribution, the interference patterns for the two poles should follow those suggested by ChUM. In turn, it is a strong theoretical and experimental supports for the two-pole structure scenario for $Λ^*$.

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arXiv2017

Photoproduction of $Λ(1405)$ with the $N^*$ and $t$-channel Regge contributions

We investigate the photoproduction of $Λ(1405)\equivΛ^*$ hyperon resonance, i.e., $γp\to K^+Λ^*$, employing the effective Lagrangian approach with the $t$-channel Regge trajectories at tree level. We extensively explore the effects from the nucleon resonances in the vicinity of the threshold $\sqrt{s}_\mathrm{th}\approx1900$ MeV, i.e., $N(2000)$, $N(2030)$, $N(2055)$, $N(2095)$, and $N(2100)$, and observe that they are of great importance to reproduce the recent CLAS/Jlab experimental data. Total and differential cross sections are given as numerical results and compared with the experimental data, in addition to the $t$-channel momentum transfer and photon-beam asymmetry. The invariant-mass plots for $γp\to K^+π^0Σ^0$ via $Λ^*$ are also extracted from the two-body process results, showing a qualitative agreement with the data. We also discuss the constituent-counting rule for the internal structure of $Λ^*$, resulting in that $Λ^*$ appears to be different from a simple three-quark $(uds)$ state.

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arXiv2016

QCD topological susceptibility from the nonlocal chiral quark model

We investigate the QCD topological susceptibility $χ_t$ by using the nonlocal chiral quark model (NL$χ$QM). This model is based on the liquid instanton QCD-vacuum configuration in which $\mathrm{SU}(3)$ flavor symmetry is explicitly broken by the current quark mass $(m_{u,d},m_s)\approx(5,135)$ MeV. To compute $χ_t$, the local topological charge density operator $Q_t(x)$ is derived from the effective partition function of NL$χ$QM. We take into account the contributions from the leading-order (LO) ones $\sim\mathcal{O}(N_c)$ in the $1/N_c$ expansion. We also verify that the analytical expression of $χ_t$ in NL$χ$QM satisfy the Witten-Veneziano (WV) and the Leutwyler-Smilga (LS) formulae. Once the average instanton size and inter-instanton distance are fixed with $\barρ=1/3$ fm and $\bar{R}=1$ fm, respectively, all the associated model parameters are all determined self-consistently within the model, including the $η$ and $η'$ weak decay constants. We obtain the results such as $F_η=96.77$ MeV and $F_{η'}=102.53$ MeV for instance. Numerically we observe that $χ_{t}=(165.57\,\mathrm{MeV})^4$ in our full calculation. This value is comparable with its empirical one $χ_t=(175\pm5\,\mathrm{MeV})^4$. We also find that our $χ^\mathrm{WV}_t=χ^\mathrm{QL}_t=(194.30,\mathrm{MeV})^4$ in the quenched limit and $χ^\mathrm{LS}_t=(162.54\,\mathrm{MeV})^4$ in the chiral limit. Consequently, we conclude that $χ_{t}<χ^\mathrm{QL}_t$. Our result also implies that the $(10\sim20)\,\%$ decrease with the dynamical quark contributions.

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arXiv2015

Consistency check of charged hadron multiplicities and fragmentation functions in SIDIS

We derived the conditions on certain combinations of integrals of the fragmentation functions of pion using HERMES data of the sum for the charged pion multiplicities from semi-inclusive deep-inelastic scattering (SIDIS) off the deuteron target. In our derivation the nucleon parton distribution functions (PDFs) are assumed to be isospin SU(2) symmetric. Similar conditions have also been obtained for the fragmentation functions (FFs) of kaon by the sum of charged kaon multiplicities as well. We have chosen several FFs to study the impact of those conditions we have derived. Among those FFs, only that produced in the nonlocal chiral-quark model (NL$χ$QM) constantly satisfy the conditions. Furthermore, the ratios of the strange PDFs $S(x)$ and the nonstrange PDFs $Q(x,Q^2)$ extracted from the charged pion and kaon multiplicities differ from each other significantly. Finally, we demonstrate that the HERMES pion multiplicity data is unlikely to be compatible with the two widely-used PDFs, namely CTEQ6M and NNPDF3.0.

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arXiv2015

Weak $K\to π$ generalized form factors and transverse transition quark-spin density from the instanton vacuum

We investigate the generalized $K\to π$ transition vector and tensor form factors, from which we derive the transverse quark spin density in the course of the $K\toπ$ transition, based on the nonlocal chiral quark model from the instanton vacuum. The results of the transition tensor form factor are in good agreement with recent data of lattice QCD. The behavior of the transverse quark spin density of the $K\toπ$ transition turn out to be very similar to those of the pion and the kaon.

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arXiv2015

Thermal conductivity of the quark matter for the SU(2) light-flavor sector

We investigate the thermal conductivity ($κ$) of the quark matter at finite quark chemical potential $(μ)$ and temperature $(T)$, employing the Green-Kubo formula, for the SU(2) light-flavor sector with the finite current-quark mass $m=5$ MeV. As a theoretical framework, we construct an effective thermodynamic potential from the $(μ,T)$-modified liquid-instanton model (mLIM). Note that all the relevant model parameters are designated as functions of $T$, using the trivial-holonomy caloron solution. By solving the self-consistent equation of mLIM, we acquire the constituent-quark mass $M_0$ as a function of $T$ and $μ$, satisfying the universal-class patterns of the chiral phase transition. From the numerical results for $κ$, we observe that there emerges a peak at $μ\approx200$ MeV for the low-$T$ region, i.e. $T\lesssim100$ MeV. As $T$ increase over $T\approx100$ MeV, the curve for $κ$ is almost saturated as a function of $T$ in the order of $\sim10^{-1}\,\mathrm{GeV}^2$, and grows with respect to $μ$ smoothly. At the normal nuclear-matter density $ρ_0=0.17\,\mathrm{fm}^{-3}$, $κ$ shows its maximum $6.22\,\mathrm{GeV}^2$ at $T\approx10$ MeV, then decreases exponentially down to $κ\approx0.2\,\mathrm{GeV}^2$. We also compute the ratio of $κ$ and the entropy density, i.e. $κ/s$ as a function of $(μ,T)$ which is a monotonically decreasing function for a wide range of $T$, then approaches a lower bound at very high $T$: $κ/s_\mathrm{min}\gtrsim0.3\,\mathrm{GeV}^{-1}$ in the vicinity of $μ=0$.

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arXiv2015

Photoproduction of $Λ(1405)$ with the two-pole structure

We investigate the $Λ(1405,1/2^-)\equivΛ^*$ photoproduction off the proton target, i.e. $γp\to K^+Λ^*$, considering explicitly its two-pole structure, the higher ($Λ^*_H:\,1430\,\mathrm{MeV}$) and lower ($Λ^*_L:\,1390\,\mathrm{MeV}$) mass-pole contributions, suggested by the chiral-unitary model (ChUM) approaches. For this purpose, we construct a two-body process model, which mimics the Dalitz process, $γp\to K^+πΣ$, assuming that the mass of $Λ^*$ as the invariant mass of $π$ and $Σ$, i.e. $M_{Λ^*}\sim M_{πΣ}$. We employ the effective Lagrangian method with the tree-level Born approximation, using the gauge-invariant prescription for the phenomenological form factors. We provide the numerical results for the energy and angular dependences, $π$-$Σ$ invariant-mass distribution, and so on. It turns out that the model parameters determined from ChUM reproduce the experimental data qualitatively well, supporting the two-pole structure. Moreover, the nucleon resonance contribution near the threshold plays an important role to describe the data.

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arXiv2014

Jet-quenching parameter $\hat{q}$ in the nonperturbative region

We investigate the jet-quenching parameter $\hat{q}$ for the quark-gluon plasma (QGP) for $N_c=3$, defined nonperturbatively with the Wilson loop in the light-cone (LC) coordinate, at finite temperature $(T)$. Considering the effective static (heavy) quark-antiquark potential $V_{\bar{Q}Q}=σL+C-A/L$, where $L$ indicates the LC transverse separation between the quarks, we obtain $\hat{q}\approx8V_{\bar{Q}Q}/L^2$. The $T$ dependences for $L$ and other relevant parameters are extracted from the $T$-dependent instanton (trivial-holonomy caloron) length parameters and the lattice QCD data. By choosing $L\approx{a}\approx\barρ_T$, in which $a$ and $\barρ_T$ denote the lattice spacing and the $T$-dependent average (anti)instanton size, respectively, we acquire numerically that $\hat{q}=(5\sim25)\,\mathrm{GeV/fm}$ for $T=(0\sim0.6)$ GeV, and these values are well consistent with other estimations from AdS/CFT and experimental analyses. It turns out that $\hat{q}$ is produced almost by the Coulomb and constant potentials of $V_{\bar{Q}Q}$. We also observe that the ratio $T^3/\hat{q}$ turns into being saturated to $\sim4.45\times10^{-2}$ for $T\gtrsim0.4$ GeV, indicating the strongly-coupled QGP, and $\hat{q}$ behaves proportionally to $T^3$ for high $T$.

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arXiv2013

Study of Baryon Resonances in the Photoproduction gamma p ---> K* Sigma(1190)

In this talk, we report the theoretical studies of baryon resonances for the gamma p ---> K*0 Sigma+ and gamma p ---> K*+ Sigma0 reaction processes by making use of the effective Lagrangian method in the tree-level Born approximation. The numerical results for the total and differential cross sections are shown with the presently available data. It turns out that the contributions from the N* and Delta* resonances are almost negligible near the threshold energy. On the other hand, the effects of the K exchange in the t channel and Delta(1232) in the s channel are considerably larger than other ones in the scattering process, being in good agreement with the experimental data.

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arXiv2013

K*Sigma photoproduction off the proton target with baryon resonances

We investigate the photoproduction of K*0Sigma+ and K*+Sigma0 off the proton target, employing the effective Lagrangian approach at the tree-level Born approximation. In addition to the (s, t, u)-channel Born diagrams, we take into account various baryon-resonance contributions such as F15(2000), D13(2080), G17(2190), D15(2200), F35(2000), G37(2200), F37(2390), and Sigma*(1385, 3/2+) in a fully covariant manner. We present the numerical results for the energy and angular dependences for the cross sections in comparison to available experimental data. The single-polarization observables, i.e. the photon-beam, recoil and target baryon polarization asymmetries are computed as well for future experiments. We observe from the numerical results that the resonance contributions play a minor role in producing the strength of the cross sections, being different from the K*Lambda photoproduction. In contrast, it turns out that the Delta(1232)-pole contribution and strange-meson exchanges in the t-channel dominate the scattering process. On the other hand, the higher resonances influence the polarization observables such as the recoil and target asymmetries.

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arXiv2013

Electroproduction of $Λ(1520)$ off the nucleon target with the nucleon resonances

We investigate the unpolarized electroproduction of $Λ(1520)\equivΛ^*$ off the nucleon target, using the effective Lagrangian method at the tree-level Born approximation with the nucleon-resonance contributions from $S_{11}(2090)$, $D_{13}(2080)$, and $D_{15}(2200)$. First, we compute the 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 $D_{13}$ plays an important role to reproduce the electroproduction data properly. The numerical results for the $ϕ$ distribution shows obvious different structures from that 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 becomes about a half of 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 saturated almost by the resonance contributions. Finally, the contact-term dominance, which is the key ingredient for the $Λ^*$ electromagnetic productions, is briefly discussed.

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arXiv2013

Quark-gluon mixed condensate for the SU(2) light-flavor sector at finite temperature

We investigate the quark-gluon mixed condensate m^2_0 = for the SU(2) light-flavor sector at finite temperature (T). Relevant model parameters, such as the average (anti)instanton size, inter-(anti)instanton distance, and constituent-quark mass at zero virtuality, are modified as functions of T, employing the trivial-holonomy caloron solution. By doing that, we observe correct chiral restoration patterns depending on the current-quark mass m. We also perform the two-loop renormalization-group (RG) evolution for the both condensates by increasing the renormalization scale mu=(0.6~2.0) GeV. It turns out that the mixed condensate is insensitive to the RG evolution, whereas the quark condensate become larger considerably by the evolution. Numerically, we obtain - ^1/5 = (0.45 ~ 0.46) GeV at T=0 within the present theoretical framework, and the mixed condensate plays the role of the chiral order parameter for finite T. The ratio of the two condensates m^2_0 is almost flat below the chiral transition T (T_0), and increases rapidly beyond it. From a simple linear parametrization, we obtain m^2_0(T)/m^2_0(0) ~ (0.07, 0.47) T/T_0 + (1,0.6) for (T T_0) at mu=0.6 GeV. The present results are compared with other theoretical ones including the lattice QCD simulations, and show qualitatively good agreement with them.

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