ar X iv : h ep - p h / 01 06 18 8 v 1 1 7 Ju n 20 01 Scaling in γ ∗ p total cross sections and the generalized vector dominance / color dipole picture ∗

Scaling in γ * p total cross sections and the generalized vector dominance/color dipole picture * Abstract The scaling in σγ * p cross sections (for Q 2 /W 2 << 1) in terms of the scaling variable η = (Q 2 + m 2 0)/Λ 2 (W 2) is interpreted in the generalized vector dominance/color-dipole picture (GVD/CDP). The quantity Λ 2 (W 2) is identified as the average gluon transverse momentum absorbed by the q ¯ q state, < l 2 >= (1/6)Λ 2 (W 2). At any Q 2 , for W 2 → ∞, the cross sections for virtual and real photons became universal, σγ * p(W 2 , Q 2)/σγp(W 2) → 1. Two important observations[1] were made on deep inelastic scattering (DIS) at low values of the Bjorken scaling variable x bj ∼ = Q 2 /W 2 << 1, since HERA started running in 1993: i) The diffractive production of high-mass states (of masses M X < ∼ 30GeV) at an appreciable rate relative to the total virtual-photon-proton cross section, σ γ * p (W 2 , Q 2). The sphericity and thrust analysis[1] of the diffractively produced states revealed (approximate) agreement in shape with the final state found in e + e − annihilation at √ s = M X. This observation of high-mass diffractive production confirms the conceptual basis of generalized vector dominance (GVD)[2] that extends the role of the low-lying vector mesons in photoproduction[3] to DIS at arbitrary Q 2 , provided x bj << 1. ii)An increase of σ γ * p (W 2 , Q 2) with increasing energy considerably stronger[4] than the smooth " soft-pomeron " behavior known from photoproduction and hadron-hadron scattering. We have recently shown[5] that the data for total photon-proton cross sections,