However, from the discussion above, it is clear that these results simply follow from the general rules of Reggeon theory. They are not connected with a particular vector current model of the Pomeron, but rather follow from the fact that the product of the parity and signature of the Pomeron is $+1$. Moreover the Pomeron has positive signature and corresponds to the analytic continuation from angular momenta $J^P = 2^+,4^+,\dots$ in the $t$-channel. The same results would be obtained if a tensor current model were used.

On the other hand, the detailed structure of the amplitudes $f_m^k(p_{3\perp}^2, p_{4\perp}^2, \vec
p_{3\perp}\!\cdot\!\vec p_{4\perp})$ (with $m=0^+,0^-,1^+,2^+,\dots$) depends on dynamics and cannot be predicted from the general principles of Regge theory. For example, if the heavy state $h$ is strongly coupled to gluons and is produced perturbatively via the diagram shown in Fig. \[fig:3\](a), then in the forward direction ($p_{3\perp}, p_{4\perp}\ll Q_\perp$) the vertex factor $f_{0^+}$ does not depend on $p_3$ or $p_4$. Moreover, as was shown in [@Liverpool; @KMRmm; @INC], there exists a $J_z=0$, parity-even, selection rule, for production by gluon–gluon fusion where each of these active gluons comes from colour-singlet digluon $t$-channel exchange, see Fig. \[fig:3\]. As a consequence the production of 