$\phi \phi$ production in high-energy hadron-hadron collisions and enhancement near threshold

Excessive $\phi \phi$ production has been seen in $\pi p\to \phi \phi X$ and $\mathrm{pp}\to \phi \phi X$ with an enhancement near $\phi \phi$ threshold. A lowest-order phenomenological model to interpret this as an evidence of gluon-gluon collisions appears to be consistent with data, and suggests that similar other reactions parallel to photon-photon collisions may also be looked for.