UNIFIED QCD EVOLUTION-EQUATIONS AND THE DOMINANT BEHAVIOR OF STRUCTURE FUNCTIONS AT LOW-X

We consider a system of evolution equations for quark and gluon struct ure functions satisfying the leading-logarithmic behaviour due to both QCD collinear (LLQ(2)) and infrared (LL1/x) singularities. We show th at these equations leave undetermined an arbitrary regular function of j in the Mellin-transformed weights. We consider the constraints resu lting from energy-momentum conservation and from the decoupling of qua rk loops in the leading j-plane singularity. These constraints can be fulfilled without influencing the leading-log terms. As a particular c onsequence of the second constraint, the location of the leading singu larity is determined in terms of the (LL1/x) and (LLQ(2)) kernels. It leads to a value significantly lower than the LL1/x evaluation, while remaining at j > 1, and compatible with the behaviour of structure fun ctions observed at HERA.