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 $\frac{dN}{d\cos {\theta }^{\ast }}$ $\propto$ 1 + $\lambda {\cos }^2{\theta }^{\ast }$, where ${\theta }^{\ast }$ 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.

Accepted by Phys. Lett. B

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