arXiv subjects
Publications and source records attributed to D. Schildknecht.
We investigate the mass range of the quark-antiquark fluctuations of the photon that are active in producing the total photoabsorption cross section in the color dipole picture, emphasizing the notion of color transparency and saturation. We consider the implications of measurements at future extensions of the available electron-proton-scattering energy.
We demonstrate that the transition photon radiation and pair creation can be interpreted as a diffractive phenomenon in terms of the light-cone wave functions in a way similar to the Good-Walker approach [6] to the diffraction dissociation. Our formulas for spectra agree with those obtained by Baier and Katkov [5] within the quasiclassical operator method. However, there is some disagreement with earlier results by Garibyan [4].
Deep inelastic scattering at low x can be described by essentially only two fitted parameters. The interpretation of J/Psi photoproduction in terms of the gluon structure function is elaborated upon.
The energy dependence of the virtual photoabsorption cross section in deep inelastic scattering (DIS) at low $x = Q^2/W^2 < 0.1$ may be described in terms of the saturation scale that in our approach depends on the energy, $Λ^2_{sat} = Λ^2_{sat} (W^2)$. We briefly summarize our recent findings that allow us to predict the exponent $C_2$ of $Λ^2_{sat} (W^2) \sim (W^2)^{C_2}$ in agreement with the previous result obtained from fitting the experimental data. The exponent $C_2$ depending on the relative magnitude of the longitudinal and transverse contribution to the structure function, direct measurements of the longitudinal part are urgently needed.
Saturation at low x appears as an almost unavoidable consequence of the two-gluon exchange generic structure. Consistency of the ansatz for the vector part of the color dipole cross section with conventional evolution determines the energy dependence of the saturation scale.
Saturation at low x appears as an almost unaboidable consequence of the two-gluon excange generic structure.
We examine the total cross section of virtual photons on protons, $σ_{γ^* p}(W^2,Q^2)$, at low $x \cong Q^2/W^2 \ll 1$ and its connection with ``elastic'' diffractive production $γ^*_{T,L}p \to X^{J=1}_{T,L} p$ in the two-gluon exchange dynamics for the virtual forward Compton scattering amplitude. Solely based on the generic structure of two-gluon exchange, we establish that the cross section is described by the (imaginary part of the) amplitude for forward scattering of $q \bar q$ vector states, $(q \bar q)^{J=1}_{T,L} p \to (q \bar q)^ {J=1}_{T,L} p$. The generalized vector dominance/color dipole picture (GVD/CDP) is accordingly established to only rest on the two-gluon-exchange generic structure. This is explicitly seen by the sum rules that allow one to directly relate the total cross section to the cross section for elastic diffractive forward production, $γ^*_{T,L} p\to (q \bar q)^{J=1}_{T,L} p$, of vector states.
The empirical scaling law, wherein the total photoabsorption cross section depends on the single variable $η=(Q^2 + m^2_0)/Λ^2 (W^2)$, provides empirical evidence for saturation in the sense of $σ_{γ^* p} (W^2, Q^2) / σ_{γp} (W^2) \to 1$ for $W^2 \to \infty$ at fixed $Q^2$. The total photoabsorption cross section is related to elastic diffraction in terms of a sum rule. The excess of diffractive production over the elastic component is due to inelastic diffraction that contains the production of hadronic states of higher spins. Motivated by the diffractive mass spectrum, the generalized vector dominance/color dipole picture (GVD/CDP) is extended to successfully describe the DIS data in the full region of $x \le 0.1$, all $Q^2 \ge 0$, where the diffractive two-gluon-exchange mechanism dominates.
We explore the consequences for diffractive production, gamma* p --> X p, in deep inelastic scattering at low values of x\sim Q^2/W^2 <<1 that follow from our recent representation of the total photoabsorption cross section, sigma_{gamma* p}, in the generalized vector dominance/ color dipole picture(GVD/CDP) that is based on the generic structure of the two-gluon-exchange from QCD. Sum rules are derived that relate the transverse and the longitudinal (virtual) photoabsorption cross section to diffractive forward production of q q-bar states that carry photon quantum numbers ("elastic diffraction"). Agreement with experiment in the W^2 and Q^2 dependence is found for M_X^2/Q^2<<1, where M_X is the mass of the produced system X. An additional component ("inelastic diffraction"), not actively contributing to the forward Compton amplitude, is needed for diffractive production at high values of M_X. Our previous theoretical representation of the total photoabsorption cross section sigma_{gamma* p}=sigma_{gamma* p}(eta), in terms of the scaling variable eta=(Q^2+m_0^2)/Lambda^2(W^2) is extended to include the entire kinematic domain, x=<0.1 and all Q^2 with Q^2>=0, where scaling in eta holds experimentally.
The empirical scaling law, wherein the total photoabsorption cross section depends on the single variable eta=(Q^2+m_0^2)/Lambda^2(W^2), provides empirical evidence for saturation in the sense of sigma_{gamma* p}(W^2,Q^2)/sigma_{gamma p}(W^2) --> 1 for W^2 --> infinity at fixed Q^2. The total photoabsorption cross section is related to elastic diffraction in terms of a sum rule. The excess of diffractive production over the elastic component is due to inelastic diffraction that contains the production of hadronic states of higher spins. Motivated by the diffractive mass spectrum, the generalized vector dominance/color dipole picture (GVD/CDP) is extended to successfully describe the DIS data in the full region of x=<0.1, all Q^2>=0, where the diffractive two-gluon-exchange mechanism dominates.
A comparison of the H1 data on the longitudinal structure function, $F_L$, at small $x$ with the predictions from the generalized vector dominance / color dipole picture (GVD/CDP) is presented. Using the set of parameters previously determined in the fits to the total cross section, $σ_{γ^* p}$, we find good agreement with the data for $F_L$. Scaling in $η= (Q^2 + m^2_0) / Λ^2 (W^2)$ is discussed in detail for the longitudinal and transverse photoabsorption cross sections.
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 $q \bar q$ state, $<\vec l_\bot^{~2}> = (1/6) Λ^2 (W^2)$. At any $Q^2$, for $W^2 \to \infty$, the cross sections for virtual and real photons become universal,$σ_{γ^*p}(W^2,Q^2)/ σ_{γp} (W^2) \to 1$. 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.
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 $q \bar q$ state, $<\vec l^{~2}> = (1/6) Λ^2 (W^2)$. At any $Q^2$, for $W^2 \to \infty$, the cross sections for virtual and real photons become universal, $σ_{γ^*p}(W^2,Q^2)/σ_{γp}(W^2) \to 1$. 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.
Including the new HERA data, the $γ^* p$ total cross section is analysed in the generalized vector dominance/colour-dipole picture (GVD/CDP) that contains scaling in $η= (Q^2 + m^2_0) / Λ^2 (W^2)$, where $Λ^2 (W^2)$ is an increasing function of $W^2$. At any $Q^2$, for $W^2 \to \infty$, the cross sections for virtual and real photons become identical, $σ_{γ^* p} (W^2, Q^2) / σ_{γp} (W^2) \to 1$. The gluon density deduced from the colour-dipole cross section fulfills the leading order DGLAP relationship. Evolution à la DGLAP breaks down for $η\lsim 0.1$.
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 $q \bar q$ state, $<\vec l^{~2}> = (1/6) Λ^2 (W^2)$. At any $Q^2$, for $W^2 \to \infty$,the cross sections for virtual and real photons became universal, $σ_{γ^*p}(W^2,Q^2)/σ_{γp}(W^2) \to 1$.
We give a detailed account of the recently formulated generalized vector dominance/colour-dipole picture (GVD/CDP) of deep-inelastic scattering at low $x\cong Q^2/W^2$, including photoproduction. The approach, based on $γ^*(q \bar q)$ transitions, $q \bar q$ propagation and diffractive $(q \bar q)p$ scattering via the generic structure of the two-gluon exchange, provides a unique and quantitatively successful theory for the $γ^* p$ total cross section, $σ_{γ^* p} (W^2,Q^2)$, at low $x$. The GVD/CDP is shown to imply the empirical low-$x$ scaling law, $σ_{γ^* p} (W^2,Q^2)=σ_{γ^* p} (η)$ with $η=(Q^2+m_0^2)/Λ^2(W^2)$, that was established by a model-independent analysis of the experimental data.
We summarize recent work on low-x deep inelastic scattering. The generalized vector dominance/color-dipole picture (GVD/CDP) implies a scaling behavior for $σ_{γ^* p} (W^2, Q^2)\cong σ_{γ^* p}(η)$, with $η= (Q^2 + m^2_0)/Λ^2 (W^2)$ and yields an excellent representation of the experimental results on $σ_{γ^* p}(η)$.
We briefly summarize the equivalence of off-diagonal generalized vector dominance and the colour-dipole approach to deep-inelastic scattering (DIS) in the diffraction region of values of $x \simeq Q^2/W^2 << 1$.