Pion electromagnetic form factor in Coulomb gauge QCD

The electromagnetic pion form factor and radius is calculated in a model which can be derived from Coulomb gauge QCD in an instantaneous approximation discussed recently. The results agree well with experimental data without further adjustment of parameters. They are also compared to results obtained in the Nambu-Jona-Lasinio and in the Vector Meson Dominance model.     

Pion form factor in the NLC QCD SR approach

We present results of a calculation of the electromagnetic pion form factor within the framework of QCD sum rules with nonlocal condensates and using a perturbative spectral density which includes O(α _s ) contributions.     

QCD sum rules and electromagnetic polarizabilities of a nucleon

We investigate the electromagnetic polarizabilities of a nucleon using the method of QCD sum rules. The diagrams in the operator product expansion are taken into account up to dimension 6. We find that the polarizabilities of a nucleon can be expressed by means of three new kinds of susceptibilities of quark condensates which represent the response under weak external electromagnetic fields. Estimating the susceptibilities with the QCD sum rules itself based on the formalism by Balitsky et al., we see that good agreement is obtained for the neutron electric polarizability.     

Calculation of electromagnetic ρπ formfactor from QCD sum rules

Electromagnetic ρπ formfactor at intermediate momentum transfer, 0.7 GeV²≦Q ²≦3 GeV², is calculated using QCD sum rules for the vertex function of two vector and one axial vector currents. In this region the results obtained are consistent within 25% accuracy with the vector meson dominance model predictions and can be regarded as its theoretical justification.     

Electromagnetic form factors of the nucleon from perturbative QCD and QCD sum rules

Perturbative quantum chromodynamics (QCD) and QCD sum rules are combined to calculate nucleon wave functions in the light-cone framework. The use of these wave functions leads to predictions for the Dirac form factors of the proton and neutron which are in remarkable agreement with experiment.     

QCD sum rules versus experiment

We show the compatibility of Shifman, Vainshtein and Zakharov (SVZ) sum rules with the sum rules involving the electro-magnetic mass difference of pions and the axial form factor in radiative π^± decay. In spite of good agreement between them, QCD sum rules are unable to explain clearly the experimental data of τ-decay in the axial channel.     

QCD sum rules: charmonium

The dimensiond=4 gluon condensateφ ₁ is determined from an analysis of charmonium, taking model dependent estimates ofd=6.8 condensates into account. ForS-waves, there is a compatibility region, where both these and model dependent higher state corrections to ground state dominance are small, enablingφ ₁ to be determined from a fit to the data. ForP-waves the estimated higher dimension contributions are large in the region of ground state dominance, and noφ ₁ determination is possible. Theφ ₁ value obtained from the fit to theS-wave ratios is 3 5 times bigger than that obtained by RRY using the plateau method. The methods are compared for the vector current. We show that the higher state correction used by RRY does not fit the data; and that the plateau method is much more sensitive to thed=6.8 condensates than the ratio used here. When reasonable estimates of both these and the higher state corrections are taken into account, there is compatibility between the two methods.     

Sum rules and the pion form factor in QCD

We propose a new approach to the investigation of the pion electromagnetic form factor in QCD based on the systematic use of the QCD sum rule technique. The theoretical curve obtained for F_π(Q²) is in good agreement with existing experimental data.     

QCD sum rules at finite temperature

Finite energy and Laplace transform QCD sum rules atT0 are analyzed, and predictions for vacuum condensates are compared with the low temperature expansion of the energy density and pressure. Results show a serious disagreement which indicates a breakdown of the FESR programme already at dimension four, and which invalidates Laplace transform sum rules, at least in their straightforward extension to finite temperature.     

Symmetry energy from QCD sum rules

We review the symmetry energy in the context of AdS/CFT correspondence. After constructing D -brane configurations corresponding to dense system in boundary theory, we calculate the symmetry energy by solving DBI action of D branes in confining and deconfining phase. We conclude that the density dependence of the symmetry energy has scaling law, whose power depends only on the dimensionality of the branes and space-time.     

QCD sum rules with finite masses

The concept of QCD sum rules is extended to bound states composed of particles with finite mass such as scalar quarks or strange quarks. It turns out that mass corrections become important in this context. The number of relevant corrections is analyzed in a systematic discussion of the IR- and UV-divergencies, leading in general to a finite number of corrections. The results are demonstrated for a system of two massless quarks and two heavy scalar quarks.We wish to thank Dr. Lech Mankiewicz for very helpful discussions. This work was supported by DFG (G. Hess program).     

Elastic and transition form factors of light pseudoscalar mesons from QCD sum rules

We revisit F _π(Q ²) and F _Pγ(Q ²), P = π, η, η′, making use of the local-duality (LD) version of QCD sum rules. We give arguments that the LD sum rule provides reliable predictions for these form factors at Q ² ≥ 5–6 GeV², the accuracy of the method increasing with Q ² in this region. For the pion elastic form factor, the well-measured data at small Q ² give a hint that the LD limit may be reached already at relatively low values of momentum transfers, Q ² ≈ 4–8 GeV²; we therefore conclude that large deviations from LD in the region Q ² = 20–50 GeV² seem very unlikely. The data on the (η, η′) → γγ* form factors meet the expectations from the LD model. However, the BaBar results for the π ⁰ → γγ* form factor imply a violation of LD growing with Q ² even at Q ² ≈ 40 GeV², at odds with the η, η′ case and with the general properties expected for the LD sum rule.