Electroproduction of Single Pions with Large Transverse Momenta

We report measurements of the electroproduction reaction $\mathrm{ep}\to e{\pi }^{+}n$ for pions produced between 60 and 90° in the virtual-photon-proton center-of-mass system. The cross section is studied as a function of the center-of-mass energy, $W$, and the square of the virtual-photon mass, $-Q^2$, in the region $1.2 \mathrm{GeV}<~W<~3.0 \mathrm{GeV}$, $1.2 \mathrm{Ge}{\mathrm{V}}^2<~Q^2<~4.4 \mathrm{Ge}{\mathrm{V}}^2$. The data show a strong $W$ dependence in accord with simple quark-model predictions but exhibit a $Q^2$ dependence which is difficult to explain in the context of such a model.