Understanding Geometric Scaling at Small x

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Geometric scaling is a novel scaling phenomenon observed in deep inelastic scattering at small x: the total virtual photon-proton cross section depends upon the two kinematical variables Q² and x only via their combination Q² R₀²(x), with R₀²(x) ∝ xλ. At sufficiently low Q², below the saturation scale Qₛ²(x) (a few GeV²), this phenomenon finds a natural explanation as a property of the Color Glass Condensate, the high-density matter made of saturated gluons. To explain the experimental observation of geometric scaling up to much higher values of Q², of the order of 100 GeV², we study the solution to the BFKL equation subjected to a saturation boundary condition at Q² Qₛ²(x). We find that the scaling extends indeed above the saturation scale, within a window 1 < (Q²/Qₛ²) << (Qₛ²/Λ² QCD), which is consistent with phenomenology.