(n\_0)\_l -(n\_0)\_l \[eq:panda\] and hence if the Reggeons are the same, $i=k$, the function $f_{0^-}$ should be symmetric under the interchange $3\leftrightarrow 4$.

-   $J^P(h) = 1^+$

    For the production of an axial vector state, both $\lambda_h = 0$ and $\lambda_h = \pm1$ components are present, and the vertex factor can be written as g\_[ik]{}\^h = f\_[1\^+]{}\^0 \_[ikl]{} (p\_[3]{})\_i(p\_[4]{})\_k e\_l + ( f\_[1\^+]{}\^1(p\_[3]{})\_i + f\_[1\^+]{}\^1(p\_[4]{})\_i) \_[ikl]{}(n\_0)\_ke\_l, \[eq:quail\] where the $f^i$ are functions of the scalar variables $p_{3\perp}^2, p_{4\perp}^2$ and $\vec p_{3\perp} \cdot \vec
    p_{4\perp}$, and where $\vec e$ is the polarization vector of the $1^+$ meson. If both of the exchanged Reggeons are the same ($i=k$), then the function $g_{ik}^h$ is symmetric under the interchange $3\leftrightarrow 4$. As a consequence, for small $\vec  p_{i\perp}$, f\_[1\^+]{}\^0 \~(p\_[3]{}\^2 - p\_[4]{}\^2), f\_[1\^+]{}\^1 = -f\_[1\^+]{}\^1. \[eq:rat\]

The form of the vertex factors $g_{ik}^h$ for the central production of states $h$ of higher spin can readily be constructed using equations [(\[eq:jackal\])]{}–[(\[eq:lion\])]{}.

It is interesting to note that the structure of the vertex factors given in [(\[eq:nightingale\])]{}–[(\[eq:rat\])]{} coincides, for small $\vec p_{i\perp}$, with that found using a non-conserved vector current model [@a7], which gives a good description of the experimental data of the WA102 Collaboration [@a2]. 