  Central pseudoscalar production in $pp$ scattering is suppressed at small
values of $Q_\perp$, where $Q$ is defined as the difference between the momenta
transferred from the two protons. Such a behaviour is expected if the
production occurs through the fusion of two vectors. Photon exchange could
provide the dominant contribution at low transferred momenta, but we argue that
an extension of the experiment could probe the gluon contribution to the proton
spin.
