arXiv subjects

Chung-Wen Kao

Publications and source records attributed to Chung-Wen Kao.

At least 19 recordsLinked to original sources

arXiv2026

Determination of charged pion unpolarised TMDPDFs from Drell-Yan measurements

We present an extraction of the unpolarised transverse momentum dependent (TMD) parton distribution functions (PDFs) for the valence quarks in the pion from experimental mea- surements of the Drell-Yan process. The procedure adopted in this project relies on the framework of TMD factorisation, and the inputs of nucleon TMDPDF, pion collinear PDF, as well as the Collins-Soper kernel. The non-perturbative behaviour of the TMDPDFs is parameterised using an ansatz function that contains three unknown parameters and implements an exponential decay at large transverse distances. This work employs the most recent results in perturbation theory, and utilises a precise estimation of the nucleon TMDPDF. We perform an investigation of data used in the fit. This investigation leads to important observations regarding the effects of the kinematics and binning methods in the experimental data. Our results agree qualitatively with the previous determinations from phenomenology and a lattice computation

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arXiv2020

Unpolarized Dihadron Fragmentation Functions in Nonlocal Chiral Quark Model

We have calculated the unpolarized dihadron fragmentation functions (uDiFFs) of pions and kaons using the nonlocal chiral-quark model (NLChQM) and evolved our results to the transferred momentum scale Q$^2$=4$\mathrm{GeV}^2$ by the QCD evolution equations. These uDiFFs have also been computed in the Nambu--Jona-Lasinio-jet (NJL-jet) model for the sake of comparison. We find that there is substantial difference between the results of these two models. Furthermore, the DiFFs of $u\to π^{+}π^{-}$ and $g\to π^{+}π^{-}$ at $Q^2=109\,\mathrm{GeV}^2$ in these two models are presented in comparison with the parametrizations fitted by the Monte Carlo event generator JETSET.

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arXiv2018

Charged Kaon multiplicities of Semi-inclusive DIS off the deuteron target

We investigate the results of the charged kaon multiplicities off the deuteron target from HERMES and COMPASS experiments. The discrepancy of the two data sets cannot be explained by different $Q^2$ values. Furthermore we examine the empirical parametrization of the fragmentation functions, DSS2017, and find that the agreement between the theoretical predictions and the HERMES data is less satisfactory as claimed.

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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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arXiv2013

Shear viscosity of quark matter at finite temperature in magnetic fields

We have applied the Green-Kubo formula to investigate the shear viscosity in the SU(2) light-flavor quark matter at finite temperature under the external strong magnetic field e|B| ~ m^2_pi. For this purpose, we employ the temperature-modified instanton model and the Schwinger method to induce the magnetic field. The quark spectral function with the finite width motivated by the instanton model is adopted to compute the shear viscosity. We find that shear viscosity increases as temperature increases even beyond the transition temperature T_0=170 MeV if temperature-dependent (TDP) model parameters is used. On the other hand, with temperature-independent ones the shear viscosity starts to drop when temperature goes beyond T_0. Although the magnetic field reduces the shear viscosity in terms of the magnetic catalysis, its effects are almost negligible in the chiral-restored phase even for very strong magnetic field, e|B| ~ 10^20 gauss. We also compute the ratio of the shear viscosity and entropy density eta/s. Our numerical results are well compatible with other theoretical results for a wide temperature regions. We obtain the parameterization of the temperature-dependent ratio from our numerical result as eta/s=0.27-0.87/t+1.19/t^2-0.28/t^3 with t = T/T_0 for T=(100 ~ 350) MeV and e|B|=0.

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arXiv2012

Fragmentation and quark distribution functions for the pion and kaon with explicit flavor-SU(3)-symmetry breaking

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 Q^2 by the DGLAP 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.

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arXiv2012

Fragmentation functions and parton distribution functions for the pion with the nonlocal interactions

We study the unpolarized fragmentation functions and 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. It turns out that the nonlocal interactions result in substantial differences in comparison to typical models with only local couplings. We also perform the high Q^2-evolution for our results calculated at a relatively low renormalization scale Q^2 ~ 0.36 GeV}^2 to be compared with the experimental data. Our results after evolution are in qualitatively good agreement with those data.

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arXiv2011

Unpolarized fragmentation function for the pion and kaon via the nonlocal chiral-quark model

In this talk we present our recent studies for the unpolarized fragmentation functions for the pion and kaon, employing the nonlocal chiral quark model, which manifests the nonlocal interaction between the quarks and pseudoscalar mesons, in the light-cone frame. It turns out that the nonlocal interaction produces considerable differences in comparison to typical local-interaction models.

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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.

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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.

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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.

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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.

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arXiv2010

Lambda(1520) photoproduciton off the proton target with Regge contributions

We investigate the Lambda(1520,3/2^-)=Lambda* photoproduction off the proton target with the Regge contributions to go beyond the resonance region, in addition to the tree-level Born terms consisting of the s, t, and u channels, and contact term. For this purpose, the Reggeized propagators for the K and K* exchanges in the t channel are employed in a gauge-invariant manner. We also take into account the contribution from a nucleon resonance D_{13}(2030). 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. It turns out that the Regge contributions are necessary to explain the high-energy data beyond E_{gamma}~4 GeV especially for the angular dependences in the forward-scattering region. On the contrary, the polarization observables are insensitive to the Regge contributions due to the contact-term dominance and gauge invariance. We also compute the K^{-}-angle distribution function in the Gottfried-Jackson frame, using the polarization-transfer coefficients in the z direction. It shows nontrivial theta and E_{gamma} dependences in the forward-scattering region.

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arXiv2008

U-boson and the HyperCP exotic events

We show that the very light spin-1 gauge U-boson of the extra $U(1)'$ gauge model in the framework of the supersymmetric standard model extension can be a good candidate of the new light particle suggested by the HyperCP experiment. We demonstrate that the flavor changing neutral currents (FCNCs) for the HyperCP events in the decay of $Σ^{+}\to p μ^{+} μ^{-}$ can be generated at both tree and loop levels. In particular, we find that the loop induced $s\to d U$ transition due to the tensor-type interaction with the dimension-5 electric dipole operator plays a very important role on the FCNCs. Our explanation of the HyperCP data with the spin-1 U-boson is different from that based on a light pseudoscalar Higgs boson or sgoldstino in the literature. In particular, the U-boson involves a rich phenomenology in particle physics as well as cosmology.

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arXiv2007

Is there model-independent evidence of the two-photon-exchange effect in the electron-proton elastic scattering cross section?

We re-analyze the data of the elastic electron proton scattering to look for model-independent evidence of the two-photon-exchange (TPE) effect. In contrast to previous analyses, TPE effect is parametrized in forms which are free of kinematical-singularity, in addition to being consistent with the constraint derived from crossing symmetry and the charge conjugation. Moreover, we fix the value of $R=G_E/G_M$ as determined from the data of the polarization transfer experiment. We find that, at high $Q^2 \geq 2 GeV^2$ values, the contribution of the TPE effect to the slope of $σ_R$ vs. $ε$ is large and comparable with that arising from $G_{E}$. It also behaves quasi-linearly in the region of current data, namely, in the range of $0.2 < ε< 0.95$. Hence the fact that the current elastic $ep$ cross section data shows little nonlinearity with respect to $ε$ can not be used to exclude the presence of the TPE effect. More precise data at extreme angles will be crucial for a model-independent extraction of the TPE effect.

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arXiv2006

Leading Chiral Corrections to the Nucleon Generalized Parton Distributions

Using heavy baryon chiral perturbation theory we study the leading chiral corrections to the complete set of nucleon generalized parton distributions (GPDs). We compute the leading quark mass and momentum transfer dependence of the moments of nucleon GPDs through the nucleon off-forward twist-2 matrix elements. These results are then applied to get insight on the GPDs and their impact parameter space distributions.

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arXiv2005

Many Body Methods and Effective Field Theory

In the framework of pionless nucleon-nucleon effective field theory we study different approximation schemes for the nuclear many body problem. We consider, in particular, ladder diagrams constructed from particle-particle, hole-hole, and particle-hole pairs. We focus on the problem of finding a suitable starting point for perturbative calculations near the unitary limit (k_Fa)->infinity and (k_Fr)-> 0, where k_F is the Fermi momentum, a is the scattering length and r is the effective range. We try to clarify the relationship between different classes of diagrams and the large g and large D approximations, where g is the fermion degeneracy and D is the number of space time dimensions. In the large D limit we find that the energy per particle in the strongly interacting system is 1/2 the result for free fermions.

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