Measurement of the angular distribution in p̄p→ψ(2S) →e+e

We present the first measurement of the angular distribution for the exclusive process p̄p→ψ(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 dNd/cosθ* ∝1+λcos2θ*, 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(2S) helicity 0 formation amplitude in p̄p, comparable to what is observed in J/ψ decays to baryon pairs. © 2005 Elsevier B.V. All rights reserved.