BI-TP 2001/29 The γ ∗ p total cross section at low x ∗
نویسنده
چکیده
The scaling in σ γ * p (W 2 , Q 2) 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 become universal, σ γ * p (W 2 , Q 2)/σ γp (W 2) → 1. The gluon density corresponding to the color-dipole cross section in the appropriate limit is found to be consistent with the results from QCD fits. 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, 2] 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) [3, 4] that extends the role of the low-lying vector mesons in photoproduction [5] 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 [6] than the smooth " soft-pomeron " behavior known from photoproduction and hadron-hadron scattering. We have recently shown [7] that the data for total photon-proton cross sections, including virtual as well as real photons, show a scaling behavior. In good approximation, σ γ * p (W 2 , Q 2) = σ γ * p (η), (1)
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Scaling and asymptotic behavior of the γ ∗ p total cross section at low x ⋆
The scaling in σ γ * 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 re...
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