decay channels or angular correlations of the accompanying particles in the production process, especially the correlations between the outgoing protons in the central exclusive production process, $pp\to p + h + p$, shown in Fig. \[fig:1\](a).

![(a) The central production of a state $h$ by double-Reggeon exchange.  (b) The double-Pomeron exchange contribution to $pp\to p + h + p$, which dominates at high energies, where the $+$ signs are used to indicate the presence of Pomeron-induced rapidity gaps.\[fig:1\]](fig1.eps){height="5cm"}

The advantage of the latter approach is that it offers the possibility, not only to separate different states by accurately measuring the missing mass, but also to distinguish between scalar and pseudoscalar new heavy objects, which is difficult from studying their decay products. In this paper we begin by studying the general implications of applying Reggeon techniques to describe such exclusive processes. At very high energies, and in the central region ($x_F\simeq0$), the double-Pomeron process, Fig 1(b), should give the dominant contribution.

The theory of double-Reggeon (and multi-Reggeon) exchanges was developed long ago [@a1]. However the revival of interest in these processes is related to the new effects observed in the central production of resonances by the WA102 experiment [@a2] in the 