1,124 research outputs found

    Pion Electromagnetic Form Factor in the KTK_T Factorization Formulae

    Full text link
    Based on the light-cone (LC) framework and the kTk_T factorization formalism, the transverse momentum effects and the different helicity components' contributions to the pion form factor Fπ(Q2)F_{\pi}(Q^2) are recalculated. In particular, the contribution to the pion form factor from the higher helicity components (λ1+λ2=±1\lambda_1+\lambda_2=\pm 1), which come from the spin-space Wigner rotation, are analyzed in the soft and hard energy regions respectively. Our results show that the right power behavior of the hard contribution from the higher helicity components can only be obtained by fully keeping the kTk_T dependence in the hard amplitude, and that the kTk_T dependence in LC wavefunction affects the hard and soft contributions substantially. A model for the twist-3 wavefunction ψp(x,k)\psi_p(x,\mathbf{k_\perp}) of the pion has been constructed based on the moment calculation by applying the QCD sum rules, whose distribution amplitude has a better end-point behavior than that of the asymptotic one. With this model wavefunction, the twist-3 contributions including both the usual helicity components (λ1+λ2=0\lambda_1+\lambda_2=0) and the higher helicity components (λ1+λ2=±1\lambda_1+\lambda_2=\pm 1) to the pion form factor have been studied within the modified pQCD approach. Our results show that the twist-3 contribution drops fast and it becomes less than the twist-2 contribution at Q210GeV2Q^2\sim 10GeV^2. The higher helicity components in the twist-3 wavefunction will give an extra suppression to the pion form factor. When all the power contributions, which include higher order in αs\alpha_s, higher helicities, higher twists in DA and etc., have been taken into account, it is expected that the hard contributions will fit the present experimental data well at the energy region where pQCD is applicable.Comment: 4 pages, 2 figures, Prepared for International Conference on QCD and Hadronic Physics, Beijing, China, 16-20 June 200

    Dominant Spin-Flip Effects for the Hadronic Produced J/ψJ/\psi Polarization at TEVATRON

    Full text link
    Dominant spin-flip effects for the direct and prompt J/ψJ/\psi polarizations at TEVATRON run II with collision energy 1.96 TeV and rapidity cut yJ/ψ<0.6|y^{J/\psi}|<0.6, have been systematically studied, especially, the spin-flip effect for the transition of (ccˉ)8[3S1](c\bar{c})_8[^3S_1] into J/ψJ/\psi has been carefully discussed. It is found that the spin-flip effect shall always dilute the J/ψJ/\psi polarization, and with a suitable choice of the parameters a0,1a_{0,1} and c0,1,2c_{0,1,2}, the J/ψJ/\psi polarization puzzle can be solved to a certain degree. At large transverse momentum ptp_t, α\alpha for the prompt J/ψJ/\psi is reduced by 50\sim50% for f0=v2f_0 = v^2 and by 80\sim80% for f0=1f_0=1. We also study the indirect J/ψJ/\psi polarization from the bb-decays, which however is slightly affected by the same spin-flip effect and then shall provide a better platform to determine the color-octet matrix elements.Comment: 19 pages, 5 figures. References added. Revised version to be published in Phys.Rev.