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N. G. Stefanis

Publications and source records attributed to N. G. Stefanis.

120 records · Page 7Linked to original sources
arXiv1996

Nonperturbative calculation of Green and vertex functions in terms of particle contours

The infrared regime of fermionic Green and vertex functions is studied analytically within a geometric approach which simulates soft interactions by an {\it effective} theory of contours. Expanding the particle path integral in terms of dominant contours at large distances, all-order results in the coupling constant are obtained for the renormalized fermion propagator and a universal vertex function with physical characteristics close to those associated with the Isgur-Wise function in the weak decays of heavy mesons. The extension to the ultraviolet regime is scetched.

hep-th↗
arXiv1996

End-Point Behavior of Exclusive Processes: The Twilight Regime of Perturbative QCD

A selected set of topics along the borderline between perturbative and nonperturbative QCD in exclusive reactions are studied. Specific problems, related to different mechanisms of momentum transfer to an intact hadron, are discussed. Calculations of the space-like form factors of the pion and the nucleon are reviewed within a convolution scheme of short-distance (hard) and large-distance (soft) contributions which takes into account soft gluon emission and the intrinsic transverse hadron size. The failure of this scheme to reproduce the existing experimental data signals sizeable higher-order perturbative corrections (a K-factor of order two) and/or higher-twist contributions.

hep-ph↗
arXiv1996

Multi-Instanton Effects in QCD Sum Rules for the Pion

Multi-instanton contributions to QCD sum rules for the pion are investigated within a framework which models the QCD vacuum as an instanton liquid. It is shown that in singular gauge the sum of planar diagrams in leading order of the $1/N_{c}$ expansion provides similar results as the effective single-instanton contribution. These effects are also analysed in regular gauge. Our findings confirm that at large distances the correlator functions are more adequately described in the singular gauge rather than in the regular one.

hep-th↗
arXiv1994

Intrinsic Transverse Size Effect

Two recently proposed concepts to improve the perturbative calculation of exclusive amplitudes, gluonic radiative corrections (Sudakov factor) and confinement size effects (intrinsic transverse momentum) are combined to study the neutron magnetic form factor in the space-like region. We find that nucleon distribution amplitudes modelled on the basis of current QCD sum rules indicate overlap with the existing data at the highest measured values of momentum transfer. However, sizeable higher-order perturbative corrections (K-factor) and/or higher-twist contributions cannot be excluded, although they may be weaker than in the proton case.

hep-ph↗
arXiv1994

A Critical Analysis of the Proton Form Factor with Sudakov Suppression and Intrinsic Transverse Momentum

The behavior of the proton magnetic form factor is studied within the modified hard scattering picture, which takes into account gluonic radiative corrections in terms of transverse separations. We parallel the analysis given previously by Li and make apparent a number of serious objections. The appropriate cut-off needed to render the form-factor calculation finite is both detailed and analyzed by considering different cut-off prescriptions. The use of the maximum interquark separation as a common infrared cut-off in the Sudakov suppression factor is proposed, since it avoids difficulties with the $α_{s}$-singularities and yields a proton form factor insensitive to the inclusion of the soft region which therefore can be confidently attributed to perturbative QCD. Results are presented for a variety of proton wave functions including also their intrinsic transverse momentum. It turns out that the perturbative contribution, although theoretically self-consistent for $Q^{2}$ larger than about $6$~GeV${}^{2}$ to $10$~GeV${}^{2}$, is too small compared to the data.

hep-ph↗
arXiv1994

Evolution Effects on the Nucleon Distribution Amplitude

We study the Brodsky-Lepage evolution equation for the nucleon and construct an eigenfunction basis by including contributions of up to polynomial order $9$. By exployting the permutation symmetry $P_{13}$ of these eigenfunctions, a basis of symmetrized Appell polynomials can be constructed in which the diagonalization of the evolution kernel is considerably simplified. The anomalous dimensions are calculated and found to follow a power-law behavior. As an application, we consider the Brodsky-Huang-Lepage ansatz. An algorithm is developed to properly incorporate such higher order contributions in a systematic way.

hep-ph↗
arXiv1994

Heterotic Approach to the Nucleon Distribution Amplitude

We give an in-depth analysis of the determination procedure of the recently proposed heterotic nucleon distribution amplitude which hybridizes the best features of the Chernyak-Ogloblin-Zhitnitsky and the Gari-Stefanis models. With respect to the QCD sum-rule constraints, optimized versions of these amplitudes are derived in terms of which a "hybridity" angle can be introduced to systematically classify all models.

hep-ph↗
arXiv1994

On Proton and Delta Wave Functions

Physical wave functions for the nucleon and the $Δ^{+}$ isobar are presented, which unify the best features of previous models. With these wave functions we can calculate elastic form factors and the decays of the charmonium levels ${}^{3}S_{1}$, ${}^{3}P_{1}$, ${}^{3}P_{2}$ into $p\bar p$ in agreement with the data. A striking scaling behavior between $R=|G_{\text{M}}^{\text{n}}|/G_{\text{M}}^{\text{p}}$ and the coefficient $B_{4}$ of the Appell polynomial decomposition of the nucleon distribution amplitude is found; the implications for elastic nucleon cross sections are discussed.

hep-ph↗
arXiv1992

Analysis of $Δ^{+}(1232)$ Isobar Observables, with Improved Quark Distribution Amplitudes

A model distribution amplitude for the $Δ^{+}(1232)$ isobar is proposed, derived on the basis of the QCD sum-rule calculations of Farrar et al. combined with those of Carlson and Poor. The transition form factor $γpΔ^{+}$ is calculated modeling the proton by selected distribution amplitudes. Furthermore, predictions are made for some exclusive charmonium decays into $Δ\barΔ$. The obtained results are compared with other theoretical models and with the available data.

hep-ph↗
arXiv1992

On the Nucleon Distribution Amplitude: The Heterotic Solution

We present a new nucleon distribution amplitude which amalgamates features of the Chernyak-Ogloblin-Zhitnitsky model with those of the Gari-Stefanis model. This "heterotic" solution provides the possibility to have asymptotically a small ratio \hbox{$\vert G_{M}^{n}\vert/G_{M}^{p}\le 0.1$}, while fulfilling most of the sum-rule requirements up to the third order. Using this nucleon distribution amplitude we calculate the electromagnetic and weak nucleon form factors, the transition form factor $γp Δ^{+}$ and the decay widths of the charmonium levels $^3S_{1}$, $^3P_{1}$, and $^3P_{2}$ into $p\bar p$. The agreement with the available data is remarkable in all cases.

hep-ph↗