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

TL;DR

This paper reports the first measurement of the angular distribution in antiproton-proton to psi(2S) decays to electron-positron pairs, providing insights into the helicity amplitudes involved.

Contribution

It presents the first experimental determination of the angular distribution parameter for this process, confirming theoretical expectations and comparing it to J/psi decay observations.

Findings

Measured angular distribution parameter λ = 0.67 ± 0.15 (stat.) ± 0.04 (sys.)

Angular distribution follows expected form with small non-zero helicity 0 amplitude

Results suggest similar helicity dynamics in psi(2S) and J/psi decays

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.