Azimuthal asymmetry in unpolarized π N Drell-Yan process

Taking into account the effect of final state interaction, we calculate the non-zero (na\"{i}ve) $T$-odd transverse momentum dependent distribution $h_1^{\perp}(x,\kp^2)$ of the pion in a quark-spectator-antiquark model with effective pion-quark-antiquark coupling as a dipole form factor. Using the model result we estimate the $\cos 2\phi$ asymmetries in the unpolarized $\pi^- N$ Drell-Yan process which can be expressed as $h_1^{\perp}\times\bar{h}_1^{\perp}$. We find that the resulting $h_{1\pi}^\p(x,\kp^2)$ has the advantage to reproduce the asymmetry that agrees with the experimental data measured by NA10 Collaboration. We estimate the $\cos2\phi$ asymmetries averaged over the kinematics of NA10 experiments for 140, 194 and 286 GeV $\pi^-$ beam and compare them with relevant experimental data.