Single photo and electroproduction of pions at EBAC@JLAB

Within the Excited Baryon Analysis Center we have performed a dynamical coupled-channels analysis of the available p(e,e'π)N data in the region of W ≤ 1.6 GeV and Q~2≤ 1.45 (GeV/c)~2. The channels included are γN, πN, ηN, and ππN which has π△, ρN, and σN components. With the hadronic parameters of the model determined in our previous investigations of πN→πN reaction, we have found that the available data in the considered W≤1.6 GeV region can be fitted well by only adjusting the bare γ~*N→N~* helicity amplitudes for the lowest N~* states in P_(33), P_(11), S_(11) and D_(13) partial waves. The meson cloud effect, as required by the unitarity conditions, on the γ~*N→ N~* form factors are examined.     

Hard exclusive electroproduction of pions

We investigate the exclusive electroproduction of mesons from nucleons. To leading-twist, leading-order accuracy, the corresponding production amplitude can be decomposed into pseudovector and pseudoscalar parts. Both can be expressed in terms of quark double distribution functions of the nucleon. While the pseudovector contribution is connected to ordinary polarized quark distributions, the pseudoscalar part can be related to one-pion t-channel exchange. We observe that the pseudovector part of the production amplitude is important at . On the other hand, for , contributions from one-pion exchange dominate. Received: 4 January 1999 / Published online: 15 July 1999     

Nuclear electroproduction of negative pions below threshold

We calculate the electroproduction of negative pions into bound atomic states. The effect of the strong π-nucleus interaction is included in the calculation.     

Pomeron-Odderon interference effects in electroproduction of two pions

We study Pomeron-Odderon interference effects giving rise to charge and single-spin asymmetries in diffractive electroproduction of a pair. We calculate these asymmetries, originating from both longitudinal and transverse polarizations of the virtual photon, in the framework of QCD and in the Born approximation, in a kinematical domain accessible to HERA experiments. We predict a sizable charge asymmetry with a characteristic dependence on the invariant mass of the pair, which makes this observable very important for establishing the magnitude of the Odderon exchange in hard processes. The single-spin asymmetry turns out to be rather small. We briefly discuss future improvements of our calculations and their possible effects on the results. Received: 18 July 2002 / Published online: 25 October 2002     

Evaluation of multipoles for photo- and electroproduction of pions

With this paper we start a systematic study of the multipole amplitudes for electro- and photoproduction of pions in the low energy region. It is based on the integral equations following from fixedt dispersion relations and the Watson theorem. We start with a discussion of general mathematical problems one encounters when solving the singular integral equations. Especially the non-uniqueness of their solutions poses difficult problems which cannot be solved within the framework of the dispersion theory. As boundary conditions “from outside” we use at threshold the predictions of the static theory. But thereby only the magnetic dipole excitation of the first isobar is determined. For the electric quadrupole amplitude one needs additional conditions.     

Evaluation of multipoles for photo- and electroproduction of pions

In this paper we present the detailed results of an evaluation of the multipole amplitudes for photoproduction of pions on nucleons. We have solved the dispersion relations for the multipolesE ₀₊,M _1?,M ₁₊,E ₁₊,E _2?2 andM _2? by the Muskhelishvili-Omnès method using an appropriate ansatz for the inelastic multipole amplitudes. Four phenomenological parameters occuring in our theory have been determined by a least squares fit of theπ ⁰ experimental different cross section data. Except at pion CM-angles between 50°0 and 80°0 and at 180°0 inπ ⁰ production there is satisfactory agreement between theory and experiment for all measured differential cross sections, polarizations and asymmetries ofπ ⁰- andπ ⁺-photoproduction.     

核子共振态的螺旋度振幅的计算

相对论夸克模型用来计算面和N^*的电产生振幅.介子云的贡献是一个重要的课题.计算表明考虑了介子云的作用对Δ和N^*(1440)螺旋度振幅的解释将会有明显的改进.这有助于进一步认识Roper共振态的性质.     

Inclusive photo- and electroproduction of pions in the triple Regge region

We develop a Regge pole expansion including spin for inclusive photo- and electroproduction of pions in the photon fragmentation region and see to what extent the available data at intermediate energies can be explained in terms of Regge poles alone. It is shown that a pure Regge pole model is not adequate, although some qualitative features are reproduced.     

J/ψ衰变中f0(975)的实验研究

本文基于北京正负电子对撞机(BEPC)上北京谱仪(BES)所收集到的国家自然科学基金资助.7.8×10⁶J/ψ事例,系统研究了J/ψ→φπ⁺π^－和J/ψ→ωπ⁺π^－两个衰变道,给出了过程的分支比以及f₀的位置和宽度参数,并对J/ψ→φf₀,f₀→π⁺π^－过程的角分布进行了拟合,首次确定出该过程的螺旋度振幅比.     

Analytical investigation of γN→N*,Δ* transition helicity amplitudes

We study the structure of nonstrange baryons by analytically calculating the electromagnetic transition helicity amplitudes of the nucleon and Δ resonances. We employ an improved hypercentral constituent quark model and obtain the corresponding eigenenergies and eigenfunctions in closed forms. Then, we calculate the transverse and longitudinal helicity amplitudes for nucleon and Δ resonances. The comparison of evaluated observables and experimental data indicates good agreement between the proposed model and available data.     

P-wave contributions to B_((s))→ψKπ decays in perturbative QCD approach

We study the quasi-two-body decays B_((s)) ψ[K~*(892) K~*(1410) K~*(1680)] ψKπ by employing the perturbative QCD(PQCD) factorization approach, where the charmonia ψ represents J/ψ and ψ(2S). The corresponding decay channels are studied by constructing the kaon-pion distribution amplitude(DA) Φ_(Kπ)~P, which comprises important final state interactions between the kaon and pion in the resonant region. Relativistic Breit-Wigner formulas are adopted to parameterize the time-like form factor F_(Kπ) appearing in the kaon-pion DAs. The SU(3) flavor symmetry breaking effect resulting from the mass difference between the kaon and pion is taken into account,which makes significant contributions to the longitudinal polarizations. The observed branching ratios and the polarization fractions of B_((s)) ψK~*(892) ψKπ are accommodated by tuning hadronic parameters for the kaon-pion DAs. The PQCD predictions for B_((s)) ψ[K~*(1410) K~*(1680)] ψKπ modes from the same set of parameters can be tested by precise data obtained in the future from LHCb and Belle Ⅱ experiments.     

Is there a hadron spin-flip contribution to the Coulomb-hadron interference at small momentum transfer and high energies?

The analysing power A_N is examined in the range of the Coulomb-hadron interference on the basis of the experimental data from p_L = 6 GeV/c up to 200 GeV/c taking account of a phenomenological analysis at p_L = 6 GeV/c and a dynamic high-energy spin model. The results are compared with the new RHIC data at p_L = 100 GeV/c. The new experimental data obtained at RHIC indicate small contributions of the hadron spin-flip amplitude.     

Violation of the Okubo-Zweig-Iizuka rule in the decay mode B(1235) →φ(1020) +π

The Okubo-Zweig-Iizuka rule forbids the transitionB(1235)→φπ. Unitarity, however, induces a non-vanishing amplitude forB→φπ. We estimate the amplitude using dispersion relation. The absorptive part is shown to be small contributing less than 100 keV to the partial width Γ (B→φπ). For the dispersive part we evaluate the contributions of the and intermediate states and point out that the state does not contribute significantly. The dispersive part contributes about 300 keV to the partial width. Thus unitarity correction to the Okubo-Zweig-Iizuka rule gives the partial width Γ(B→φπ) to be around 300–400 keV compared with the present experimental upper limit of 1·9 MeV.     

Meson-production experiments at COSY-Jülich

Selected results from experiments at COSY-Jülich are presented: an attempt to measure the mass of the η meson with high precision (ANKE facility),first steps towards the detection of rare η decays (WASA),and several measurements of KK-pair production (ANKE,COSY-11,MOMO).     

Photoproduction of pions at low energy

Presented in this paper is a field theory model for photoproduction of pions at low energy. In particular, it is shown that certain interaction terms, which were previously considered to be essential in order to explain the experimental situation corresponding to the spin-3/2 resonances, are really unnecessary and that the presence of these terms are not justifiable from the theoretical point of view. Further, the free parameters in the theory representing the off-mass-shell effects of the spin-3/2 particles are fixed on the basis of theoretical considerations.