emitted at rapidity $y$.

Saturation and The Balitsky-Kovchegov Equation 
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Consider the scattering of two onium states against each other. Using the dipole formulation described above, this process can be viewed as the scattering of a collection of color dipoles against each other. The single pomeron exchange then corresponds to the approximation in which two dipoles (from the two different states) interact via the exchange of a color neutral gluon pair, independently of the rest of the dipoles. This pomeron is known as the BFKL pomeron. The total cross section would then be given by the number of dipoles in one onium state times the number of dipoles in the other onium state times the cross section for dipole-dipole scattering due to the two gluon exchange. However, as the center of mass energy increases the number of dipoles will increase, and when the number becomes sufficiently large the total cross section will violate the unitarity bounds. The reason is that at higher energies, second order processes must be taken into account, and these will slow down the growth of the total cross section. The higher order scatterings are called multiple pomeron exchanges.

Consider the scattering of a photon off a 