[constant]{}\
g\_[ik]{}\^[h(0\^-)]{} & \~& (p\_[3]{} p\_[4]{})n\_0  \~|t\_1|\^|t\_2|\^\[eq:rat2\]

at small $t$.

Amplitudes for $h(0^{\pm})$ production
--------------------------------------

To see how the dynamics modify this behaviour we have first to describe how the cross sections for the exclusive production of $h(0^\pm)$ Higgs bosons are calculated. We use the formalism of Ref. [@KMR; @KMRmm; @INC]. The amplitudes are described by the diagram shown in Fig. \[fig:3\](a), where the hard subprocesses $gg\to h(0^\pm)$ are initiated by gluon–gluon fusion and where the second $t$-channel gluon is needed to screen the colour flow across the rapidity gap intervals.

![(a) The QCD diagram for double-diffractive exclusive production of a Higgs boson $h$, $pp\to p + h + p$, where the gluons of the hard subprocess $gg\to h$ are colour screened by the second $t$-channel gluon.  (b) The rescattering or absorptive corrections to $pp\to p + h + p$, where the shaded region represents the multi-Pomeron exchanges of Fig. \[fig:2\].\[fig:3\]](fig3.eps){height="5cm"}

The Born amplitudes are of the form T\_h = N f\_g(x\_1, x\_1’, Q\_3\^2, \^2; t\_1)f\_g(x\_2,x\_2’,Q\_4\^2,\^2; t\_2) \[eq:rat3\] The overall normalization constant $N$ can be written in terms of the $h\to gg$ decay width [@INC], and the $gg\to h$ vertex factors are

[l l l]{} V\_[h(0\^+)]{} & = & (Q\_- 