Measurement of the angular distribution in (p)over-barp→ψ(2S)→ e+e-

We present the first measurement of the angular distribution for the exclusive process (p) over barp -> psi(2S) -> 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 theta* proportional to 1 + lambda cos(2) theta*, where theta* is the angle between the antiproton and the electron in the center of mass frame, with lambda = 0.67 +/- 0.15(stat) +/- 0.04(sys). The measured value for lambda implies a small but non-zero psi(2S) helicity 0 formation amplitude in (p) over barp, comparable to what is observed in J/psi decays to baryon pairs. (c) 2005 Elsevier B.V. All rights reserved.