Measurement of the angular distribution in pbar-p --> Psi(2S) --> e+ e-

We present the first measurement of the angular distribution for the exclusive process p ¯ p → ψ ( 2 S ) → e + e − based on a sample of 6844 events collected by the Fermilab E835 experiment. We find that the angular distribution is well described by the expected functional form d N d cos θ ∗ ∝ 1 + λ cos 2 θ ∗ , where θ ∗ is the angle between the antiproton and the electron in the center of mass frame, with λ = 0.67 ± 0.15 ( stat ) ± 0.04 ( sys ) . The measured value for λ implies a small but non-zero ψ ( 2 S ) helicity 0 formation amplitude in p ¯ p , comparable to what is observed in J / ψ decays to baryon pairs.