Transverse momentum distribution in hadrons. What can we learn from QCD?

Abstract

We discuss some QCD constraints on light-cone $\pi$ meson wave function $\psi(\k, x)$ . The analysis is based on such general methods as dispersion relations, duality and PCAC. We calculate the asymptotical behavior of the wave function ($wf$) at the end-point region ($x\to 1$ and $\k\to\infty$) by analysing the corresponding large $n-$th moments in transverse $\la\vec{k}_{\perp}^{2n}\ra \sim n!$ and longitudinal $\la (2x-1)^n\ra \sim 1/n^2$ directions. This information fixes the asymptotic behavior of $wf$ at large $\k$ (which is turned out to be Gaussian commonly used in the phenomenological analyses). We discuss one particular application of the obtained results. We calculate the nonleading "soft" contribution to the pion form factor at intermediate momentum transfer. We argue, that due to the specific properties of $\psi(\k, x)$, the corresponding contribution can temporarily {\bf simulate} the leading twist behavior in the extent region of $ Q^2:~~3 GeV^2\leq Q^2\leq 40 GeV^2 $, where $Q^2 F(Q^2)\sim const.$ Such a mechanism, if it is correct, would be an explanation of the phenomenological success of the dimensional counting rules at available, very modest energies for many different processes. We discuss some inclusive amplitudes (like Drell Yan and Deep Inelastic) where intrinsic $\pi$ meson structure might be essential. The relation to the valence quark model is also discussed.