ar X iv : h ep - p h / 00 07 25 7 v 1 2 4 Ju l 2 00 0 COLOUR DIPOLES and SATURATION

We employ values of the colour dipole cross section extracted from electropro-duction and photoproduction data to show that gluon saturation effects are not required by the current HERA data but will become important in the THERA energy region. 1 The colour dipole model Singly dissociative diffractive γ p processes are conveniently described in the rest frame of the hadron using a picture in which the incoming photon dissociates into a hadronic state which subsequently interacts with the proton. In the colour dipole model, 1,2 the dominant states are assumed to be q ¯ q states of given transverse size. Specifically |γ = dzd 2 r ψ where r is the transverse size of the pair, z is the fraction of light cone energy carried by the quark and ψ(z, r) is the light cone wave function of the photon. Assuming that these states are scattering eigenstates (i.e. that z, r remain unchanged in diffractive scattering) one obtains σ γ * p T,L = dzd 2 r |ψ for the γ * p total cross-section in deep inelastic scattering, where σ(s, r, z) is the total cross-section for scattering dipoles of specified (z, r) from a proton at fixed s = W 2. This " dipole cross-section " is a universal quantity for singly-dissociative diffractive processes on a proton target, playing a similarly fundamental role in, for example, open diffraction, exclusive vector meson production and deeply virtual Compton scattering. 2 The dipole cross-section We have extracted the dipole cross-section from DIS and real photoabsorption data assuming a form with two terms with a Regge type s dependence and no dependence on z: σ(s, r) = a sof t (r)s λS + a hard (r)s λH (3)