Central pseudoscalar production in pp scattering and the gluon contribution to the proton spin

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 here, but we argue that an extension of the experiment could probe the gluon contribution to the proton spin