Measurement of the angular distribution in p¯p→ψ(2S)→e+e−

We present the first measurement of the angular distribution for the exclusive process p¯p→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 dNdcosθ∗∝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 p¯p, comparable to what is observed in J/ψ decays to baryon pairs.