Introduction

In section 4.8 we demonstrated that the occurrence of Gribov–Pomeranchuk fixed poles at wrong-signature nonsense points, generated by the third double spectral function, ρsu, requires that there be cuts in the t-channel angular-momentum plane. Otherwise it is impossible to satisfy t-channel unitarity. We have also found in section 6.8 that, despite the many successes of Regge pole phenomenology, there are some features of the data that poles alone cannot explain. These are mainly failures of factorization, and it seems natural to try an invoke Regge cuts, which correspond to the exchange of two or more Reggeons and so are not expected to factorize, to make good these defects.

Unfortunately we still have a much less complete understanding of the properties of Regge cuts than of the properties of poles. On the phenomenological side, this is mainly because it is difficult to be sure whether cuts or poles are responsible for what is observed, since the main tests, logs behaviour (see (8.5.12) below) and lack of factorization, are hard to apply. Though cuts do not have to factorize some models suggest that they do, at least approximately. We shall review some of these problems in section 8.7.

Also the various theoretical models which have been used to gain insight into the behaviour of Regge poles (discussed in chapter 3) are harder to apply to cuts. For example in potential scattering, which has only elastic unitarity and no third double spectral function, there are no Regge cuts if the potentials are well behaved.