Neutral Pion Electromagnetic Form Factor as a Bound System of 3 + 1 Dimensional QCD

We investigate a neutral pion electromagnetic form factor in momentum space and obtain Gaussian-like function for it. The characteristic form of our neutral pion electromagnetic form factor is consistent with the results published by Jefferson Lab Hall A Collaboration. .

Deuteron electromagnetic form factors and tensor polarization observables in the framework of the hard-wall AdS/QCD model

We study the electromagnetic form factors and tensor polarization observables of the deuteron in the framework of the hard-wall AdS/QCD model.We find a profile function for the bulk twistτ=6 vector field,which describes the deuteron on the boundary and fix the infrared boundary cut-off of AdS space in accordance with the ground state mass of the deuteron.We obtain the deuteron charge monopole,quadrupole,and magnetic dipole form factors and tensor polarization observables from the bulk Lagrangians for the deuteron and photon field interactions.We plot the momentum transfer dependence of the form factors and tensor polarization observables and compare our numerical results with those in the soft-wall model and experimental data.

Deuteron electromagnetic form factors in transverse plane with a phenomenological Lagrangian approach

A phenomenological Lagrangian approach is employed to study the electromagnetic properties of deuteron The deuteron is regarded as a loosely bound state of a proton and a neutron. The deuteron electromagnetic form factors are expressed in light-front representation in the transverse plane. The transverse charge density of the deuteron is discussed.

Electromagnetic form factors of neutron and neutral hyperons in the oscillating point of view

Based on the recent precise measurements by the BESIII collaboration for electron–positron annihilation into a neutron and antineutron pair,the effective form factors of the neutron were determined in the time-like region,and it was found that the effective form factors of the neutron are smaller than those of the proton.The effective form factors of the neutron show a periodic behaviour,similar to those of the proton.Here,a comparative analysis forΛ,∑^(0)and■hyperons is performed.Fits of the available data on the effective form factors ofΛ,∑^(0)and■with zero charge show an interesting phenomenon in the oscillating behavior of their effective form factors.However,this will need to be confirmed by future precise experiments.Both theoretical and experimental investigations of this phenomenon can shed light on the reaction mechanisms of the electron–positron annihilation processes.

Space- and time-like kaon electromagnetic form factors in perturbative QCD

We present a phenomenological analysis of the space- and time-like charged kaon electromagnetic form factors in factorized perturbative QCD （pQCD） by employing an analytic model for αs （Q2 ） and an infrared （IR） finite gluon propagator. In the space-like region, due to the lack of available experimental data above Q2 ～ 0.2 GeV2 , we only give our results for intermediate energies and make no comparison. In the time-like region, our results agree reasonably well with the available experimental data at moderate energies, including the CLEO data and the J/ψ result.

Excited nucleon electromagnetic form factors from broken spin-flavor symmetry

A group theoretical derivation of a relation between the N→△charge quadrupole transition and neutron charge form factors is presented.

A method for measuring D＊ electromagnetic form factors in e＋e- annihilation

We describe a method for measuring the electromagnetic form factors of the D＊ meson at time-like momentum transfer in e＋e- annihilation. This is to study the joint angular distribution of the e＋e- →γ＊→D＊＋D＊-, D＊＋ →D0π＋, and D＊-→＊D0π- processes. The magnitudes and relative phases of the charge, magnetic and quadrupole form factors can be determined. The method can also be applied to other vector particles.

Electromagnetic form factors of Λ_c in the space-like momentum region

We studied the electromagnetic form factors （EMFFs） of Ac and the contributions of the quark and diquark currents to the EMFFs of Ac in the space-like region in the Bethe-Salpeter equation approach with instantaneous approximation. In this picture, baryon Ac can be regarded as a two-body c（ud） system. We found that for different values of parameters the contribution of quark and diquark currents to the EMFFs of Ac is very different, while their total contribution to the EMFFs of Ac is similar. The EMFFs of ∧c are similar to those of other baryons （proton,≡ , and ∑＋） with a peak at ω= 1, where ω=v＇.v is the velocity transfer between the initial state （with velocity v） and the final state （with velocity vr） of Ac.

An estimate of two-photon exchange effect on deuteron electromagnetic form factors

The effect of the two-photon exchange on the deuteron electromagnetic form factors is estimated based on an effective Lagrangian approach. A numerical estimate calculation of the effect is discussed. In particular, the effect on the polarization observables is analyzed.

Electromagnetic form factors of Σ^(+) and Σ^(-) in the vector-meson dominance model

Based on the recent measurements of the e^(+)e^(-)→∑^(+)∑^(-)^(-)and e^(+)e^(-)→∑^(-)∑^(-)^(+)processes by the Beijing SpectrometerⅢ(BESⅢ)collaboration,the electromagnetic form factors of the hyperon Σ^(+) and Σ^(-) in the timelike region are investigated using the vector-meson dominance model,where the contributions of theρ,ω,and φ mesons are taken into account.The model parameters are determined from the BESⅢexperimental data of the timelike effective form factors|G_(eff)|of the Σ^(+) and Σ^(-) baryons for center-of-mass energies from 2.3864 to 3.02 GeV.It is found that we can provide quantitative descriptions of the available data using as few as one adjustable model parameter.We then progress to an analysis of the electromagnetic form factors in the spacelike region and evaluate the spacelike form factors of the hyperons Σ^(+) and Σ^(-).The electromagnetic form factors obtained for the Σ^(+) and Σ^(-) baryons are comparable with those of other model calculations.

Nucleon Form Factors and N-△ Transitions in a Hypercentral Constituent Quark Model with Meson Cloud

We study the nucleon form factors and the nucleon-△（1232） transitions in a framework of hypercentral constituent quark model. The pion meson cloud effect is taken into account explicitly. Our results show that the pion cloud contributes substantially to the nucleon form factors as well as to the helicity amplitudes of △（1232）, and it gives an improved agreement compared to the experimental.

Contributions to the nucleon form factors from bubble and tadpole diagrams

The nonlocal chiral effective theory is applied to investigate the electromagnetic and strange form factors of nucleons.The bubble and tadpole diagrams are included in the calculation.With the contributions from bubble and tadpole diagrams,the obtained electromagnetic form factors are close to the results without these contributions as long as the low energy constants c_(1) and c_(2) are properly chosen,while the magnitudes of strange form factors become larger.The electromagnetic form factors are in good agreement with the experimental results,while the magnitudes of strange form factors are larger than the lattice data.

Form factor measurements with ANDA at FAIR

The high intensity and high energy antiproton beams which will be produced at FAIR open the possibility to determine time-like electromagnetic form factors in a wide kinematical range, through the annihilation reaction： p ＋ p → e^＋ ＋ e^-. The status of the proposed experiment as well as the expected results are presented on the basis of realistic simulations. The impact of these measurements on the understanding of the nucleon structure, of the asymptotic properties of form factors and of the reaction mechanism are discussed using model independent statements based on symmetry properties of the strong and electromagnetic interactions in connection with space-like data.

Time-like proton form factor measurement with ANDA

The electromagnetic probe is an excellent tool to investigate the structure of the nucleon. The nearly 4π detector PANDA, will allow to make a precise determination of the electromagnetic form factors of the proton in the time-like region with unprecedented precision. In the one-photon exchange approximation, the center of mass unpolarized differential cross section of the reaction pp → e＋e- is a linear combination of the squared moduli of the electric GE and magnetic GM proton form factors. The precise measurement of the angular distribution over almost full angular range then directly gives these quantities. At present only two experiments have provided the ratio R=｜ GE｜/｜GM｜ but with large statistical uncertainties. It is shown that with strict PID cuts and a kinematic fit, the dominant background, pp→π＋π-, can be supressed to much less than 1% of the signal, without affecting the extraction of the ratio R. PANDA will therefore offer a unique opportunity to measure the ratio with a precision ranging from 〈1% at low q2 up to 30 % for q2 = 14 （GeV/c）2.

Deuteron form factors in a phenomenological approach

The electromagnetic form factors of the deuteron, particularly its quadrupole form factor, are studied with the help of a phenomenological Lagrangian approach where the vertex of the deuteron proton-neutron with D-state contribution is explicitly taken into account. The results show the importance of this contribution to the deuteron quadrupole form factor in the approach.

Magnetic and quadrupole moments of the Z_(c)(4020)^(+),Z_(c)(4050)^(+),and Z_(c)(4600)^(+)states in the diquark-antidiquark picture

The magnetic and quadrupole moments of the Z_(c)(4020)^(+),Z_(c)(4050)^(+),and Z_(c)(4600)^(+)states are calculated within the QCD light-cone sum rules.The compact diquark-antidiquark interpolating currents and the distribution amplitudes of the on-shell photon are used to extract the magnetic and quadrupole moments of these states.The magnetic moments are acquired asμZ_(c)=0.50+0.22−0.22μN,μZ_(c)^(1)=1.22+0.34−0.32μN,andμZ_(c)^(2)=2.40+0.53−0.48μN for the Z_(c)(4020)^(+),Z_(c)(4050)^(+),and Z_(c)(4600)^(+)states,respectively.The magnetic moments evaluated for the Z_(c)4020)+,Z_(c)(4050)+,and Z_(c)(4600)+states are sufficiently large to be experimentally measurable.The magnetic moment is an excellent platform for studying the internal structure of hadrons governed by the quark-gluon dynamics of QCD because it is the leading-order response of a bound system to a weak external magnetic field.The quadrupole moment results are DZ_(c)=(0.20+0.05−0.04)×10^(−3)fm^(2),DZ_(c)^(1)=(0.57+0.07−0.08)×10^(−3)fm^(2),and DZ_(c)^(2)=(0.30+0.05−0.04)×10^(−3)fm^(2)for the Z_(c)(4020)^(+),Z_(c)(4050)^(+),and Z_(c)(4600)^(+)states,respectively.We obtain a non-zero,but small,value for the quadrupole moments of the Z_(c)states,which indicates a non-spherical charge distribution.The nature and internal structure of these states can be elucidated by comparing future experimental data on the magnetic and quadrupole moments of the Z_(c)(4020)^(+),Z_(c)(4050)^(+),and Z_(c)(4600)^(+)states with the results of the present study.

Solution of the Nucleon Structure Problem from a Field Theory of Fermions and Bosons and the Origin of the Proton Stability

A bound state formalism derived from a fermion-boson symmetric Lagrangian has been used to calculate the nucleon masses, the charge neutrality of the neutron, the magnetic moments and the electromagnetic form factor ratios μpGEp/GMpand μnGEn/GMn. A quantitative description is obtained, assuming a mixing of a scalar bound state of 3(f f¯)fstructure with its corresponding vector (f f¯)fstate (f indicating massless elementary fermions). Only a few parameters are needed, mainly fixed by energy and momentum conservation. The nucleon stability is explained by an extra binding in the confinement potential, negative for electric and positive for magnetic binding of the proton, and opposite for the neutron. The stronger electric extra binding of the proton allows a decay of the neutron to proton and electron.

核子电磁形状因子振荡行为的新视角

本文研究了质子和中子的类时电磁形状因子.作者基于手征有效场论,通过求解李普曼-施温格方程得到了核子-反核子对散射振幅.以此为输入,利用扭曲波恩近似方法描述了核子末态相互作用,计算了正负电子湮灭到正反核子对的过程.通过拟合相移、截面以及微分截面等实验数据,定下了有效场论的未知耦合常数.进一步研究发现,可以利用两个分数振荡振子来描述核子电磁类时形状因子的振荡行为:一个表现为过阻尼振荡,在阈值附近占主导地位;另一个类似于欠阻尼振荡,在高能区起到重要作用.这种振荡行为对理解硬光子诱导的核子内部极化电荷的分布起着至关重要的作用.

Exploring the light-quark interaction

Two basic motivations for an upgraded JLab facility are the needs： to determine the essential nature of light-quark confinement and dynamical chiral symmetry breaking （DCSB）; and to understand nucleon structure and spectroscopy in terms of QCD＇s elementary degrees of freedom. During the next ten years a programme of experiment and theory will be conducted that can address these questions. We present a Dyson- Schwinger equation perspective on this effort with numerous illustrations, amongst them： an interpretation of string^breaking; a symmetry-preserving truncation for mesons; the nucleon＇s strangeness σ-term; and the neutron＇s charge distribution.

A proposed U.S./China theoretical/experimental collaborative effort on baryon resonance extraction

In this paper we discuss the reasons for our work towards establishing a new collaboration between Jefferson Lab （JLab） and the Institute of High Energy Physics （IHEP） in Beijing. We seek to combine experimentalists and theorists into a dedicated group focused on better understanding the current and future data from JLab and from the Beijing Electron Positron Collider （BEPC）. Recent JLab results on the extraction of single- and double-polarization observables in both the lπ- and 2π-channel show their high sensitivity to small production amplitudes and therefore their importance for the extraction of resonance parameters. The Beijing Electron Spectrometer （BES） at the BEPC has collected high statistics data on J/ψ production. Its decay into baryon-antibaryon channels offers a unique and complementary way of probing nucleon resonances. The CEBAF Large Acceptance Spectrometer, CLAS, has access to N＊ form factors at high Q2 which is advantageous for the study of dynamical properties of nucleon resonances, while the low-background BES results will be able to provide guidance for the search for less-dominant excited states at JLab. Moreover, with the recently approved experimental proposal Nucleon Resonance Studies with CLAS12 and the high-quality data streaming from BES-Ⅲand CLAS, the time has come for forging a new Trans-Pacific collaboration of theorists and experimentalists on NSTAR physics.