Polarization of theχc2inpp¯annihilation: Massless QCD versus diquarks

The measurement of the diagonal spin-density matrix elements ${\ensuremath{\rho}}_{\mathrm{MM}}$ of the ${\ensuremath{\chi}}_{c2}$ produced in exclusive $p\overline{p}$ annihilation, via its radiative decay into $\frac{J}{\ensuremath{\psi}\ensuremath{\gamma}}$, can discriminate between different nucleon models. Whereas perturbative massless QCD predicts the ${\ensuremath{\chi}}_{c2}$ to be entirely in $M=\ifmmode\pm\else\textpm\fi{}1$ states (i.e., only ${\ensuremath{\rho}}_{11}={\ensuremath{\rho}}_{\ensuremath{-}1\ensuremath{-}1}\ensuremath{\ne}0$, quark-diquark models of the nucleon allow also $M=0$ states ${\ensuremath{\rho}}_{00}\ensuremath{\ne}0$. Predictions of a particular model are given.