Polarization of theχc2inpp¯annihilation: Massless QCD versus diquarks

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