Partons and Their Transverse Momenta in Quantum Chromodynamics

It is argued that the parton distribution function ${\mathcal{P}}_{\frac{a}{A}}(x,{\overrightarrow{\mathrm{k}}}_T;\zeta )$ is well defined and useful in quantum chromodynamics. Here $\zeta =\frac{{\left(2\mathrm{}{P}_A·n\right)}^2}{(-n^2)}$ arises as a variable because of the use of the axial gauge $n·A=0\mathrm{}$. An approximation for ${\mathcal{P}}_{\frac{a}{A}}(x,k_T;\zeta )$ is given which is obtained from an approximate solution of the Bethe-Salpeter equation in the axial-gauge ladder model.