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

Abstract

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