1,124 research outputs found

### Pion Electromagnetic Form Factor in the $K_T$ Factorization Formulae

Based on the light-cone (LC) framework and the $k_T$ factorization formalism,
the transverse momentum effects and the different helicity components'
contributions to the pion form factor $F_{\pi}(Q^2)$ are recalculated. In
particular, the contribution to the pion form factor from the higher helicity
components ($\lambda_1+\lambda_2=\pm 1$), 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 $k_T$
dependence in the hard amplitude, and that the $k_T$ dependence in LC
wavefunction affects the hard and soft contributions substantially. A model for
the twist-3 wavefunction $\psi_p(x,\mathbf{k_\perp})$ 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 ($\lambda_1+\lambda_2=0$) and the
higher helicity components ($\lambda_1+\lambda_2=\pm 1$) 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 $Q^2\sim 10GeV^2$. 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 $\alpha_s$,
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 $J/\psi$ Polarization at TEVATRON

Dominant spin-flip effects for the direct and prompt $J/\psi$ polarizations
at TEVATRON run II with collision energy 1.96 TeV and rapidity cut
$|y^{J/\psi}|<0.6$, have been systematically studied, especially, the spin-flip
effect for the transition of $(c\bar{c})_8[^3S_1]$ into $J/\psi$ has been
carefully discussed. It is found that the spin-flip effect shall always dilute
the $J/\psi$ polarization, and with a suitable choice of the parameters
$a_{0,1}$ and $c_{0,1,2}$, the $J/\psi$ polarization puzzle can be solved to a
certain degree. At large transverse momentum $p_t$, $\alpha$ for the prompt
$J/\psi$ is reduced by $\sim50%$ for $f_0 = v^2$ and by $\sim80%$ for $f_0=1$.
We also study the indirect $J/\psi$ polarization from the $b$-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.

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