In this work we study the double-differential cross sections ͑DDCSs͒ of ejected electrons in single-ionization collisions of protons with helium atoms. The final state of the emitted electron is modeled by the correlated wave function ⌽ 2 , a confluent hypergeometric function of two variables. We introduce a series representation of the ⌽ 2 wave function in terms of two-body Coulomb-like states, corresponding to the electron-projectile and electron-target relative motions. We consider undistorted and eikonal initial states that give rise to the Born ⌽ 2 and eikonal initial-state ⌽ 2 approximations, respectively. In both cases we obtain analytic series representations of the transition matrices, which exhibit a strong numerical convergence. We obtain DDCSs in ionhelium collisions in the intermediate-to high-energy regime with the Born ⌽ 2 approximation and show that it qualitatively agrees with the available experimental data. ͓S1050-2947͑98͒06610-4͔