further information on the structure of this function, extra dynamical input is needed (see Section \[sec:3\]).

-   $J^P(h) = 0^-$

    For the central production of a pseudoscalar particle, the vertex factor takes the form g\_[ik]{}\^h = f\_[0\^-]{}(p\_[3]{}\^2, p\_[4]{}\^2, p\_[3]{}p\_[4]{}) \_[ikl]{} (p\_[3]{})\_i(p\_[4]{})\_k (n\_0)\_l, \[eq:ocelot\] where $\vec n_0$ is the unit vector in the direction of the colliding hadrons (in the c.m.s.). In this case, according to [(\[eq:koala\])]{}, all amplitudes with $m_1,m_2=0$ are zero, and so $|m_1| = |m_2| = 1$ are the leading terms. According to [(\[eq:mouse\])]{} this corresponds to helicity amplitudes which are proportional to $(-t_1)^{\frac{1}{2}}(-t_2)^{\frac{1}{2}}$ for small $t$. Thus the cross section behaves as $\sim|t_1||t_2|$. Also the angular distribution contains a factor $\sin^2\phi$, which is evident from either [(\[eq:ocelot\])]{} or [(\[eq:jackal\])]{} and [(\[eq:koala\])]{}. Again the function $f_{0^-}$ is not predicted from general principles.

    Note that the characteristic $\sin^2\phi$ dependence of the angular distribution, and the $t$-behaviour at small $t$, which are observed by the WA102 Collaboration for $\eta$, and $\eta'$ production [@a2], are direct consequences of the general properties of the double-Regge-exchange amplitudes. This behaviour is valid not only for double-Pomeron exchange, but also for $P\!f, f\!f, \rho\rho, A_2A_2, \omega\omega,\dots$ exchanges. Under the interchange of the Reggeons, $i\leftrightarrow k$, 