Measurement of the angular distribution in $\bar p p\to \psi(2S)\to e^+e^-$

Abstract

We present the first measurement of the angular distribution for the exclusive process $\bar p p \to \psi(2S)\to e^+e^-$ 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 $\frac{dN}{d\cos\theta^{*}}$ $\propto$ 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 $\pm$ 0.15 (stat.) $\pm$ 0.04 (sys.) The measured value for $\lambda$ implies a small but non zero $\psi(2S)$ helicity 0 formation amplitude in $\bar p p$, comparable to what is observed in $J/\psi$ decays to baryon pairs.