The Spin Structure of the Nucleon

We present an overview of recent experimental and theoretical advances in our understanding of the spin structure of protons and neutrons.Comment: 84 pages, 29 figure

Naturalness and ultraviolet structure of gauge theories with massive fermions

According to the principle of naturalness a small, with respect to the cutoff, mass parameter entering a quantum field system is natural only when it is compatible with some symmetry in the limit where it vanishes. In this paper, advantage is taken of the liberty afforded by the renormalization procedure in order to harmonize the cutoff with the physical mass in a non-Abelian gauge field theory with spin-1/2 matter fields. The ultraviolet structure of the theory, from such a vantage point, is explored at the level of the full fermionic propagator, as well as the vertex function, using the world line approach. An interplay between this ultraviolet structure and the infrared behavior of the same system, but from the customary viewpoint ‘’cutoff much greater than mass,” is pointed out. Direct implications for open fermionic lines in the world line path integral casting of field theories are also made

Worldline approach to eikonals for QED and linearized quantum gravity and their off mass shell extensions

We construct the worldline expression pertaining to a four-point process involving the scattering of two spin-1/2 particles via photon exchange. Restricting our attention to the case of forward scattering at extremely high energies, we show how to formulate the corresponding eikonal version of the four-point Green’s function. We proceed to distinguish between the on and off mass shell cases within the framework of our description. For the on mass shell situation we recover the well-known result for the QED eikonal which corresponds to the infinite Coulomb phase. The (slightly) off mass shell case is confronted next. We produce a relevant expression for the eikonal phase in analytic form. Finally, we extend our considerations to a linearized quantum gravitational model and recover, via a series of elementary steps, the 0th order eikonal result for Planckian scattering (both for on and off mass shell)

Green's function approach to infrared factorization and finite eikonal corrections

The infrared sector of a generic gauge theory with spin-1/2 matter fields and, for simplicity, only one mass scale, is factored out via a procedure which relies on a path integral (worldline) casting of the field system. The basic idea is to employ a velocity expansion which imposes the spin-1/2 particle’s mass as a cutoff for the factorized sector, Anomalous dimensions characterizing the infrared regime are derived in connection with two- and three-point Green’s Functions. Finally an off mass shell expansion of the propagator is achieved which contains genuine corrections to the eikonal approximation. (C) 1997 Academic Press

Worldline approach to Sudakov-type form factors in non-Abelian gauge theories

We calculate Sudakov-type form factors for isolated spin-1/2 particles (fermions) entering non-Abelian gauge-field systems. We consider both the on-and the off-mass-shell case using a methodology which rests on a worldline casting of field theories. The simplicity and utility of our approach derives from the fact that we are in a position to make, a priori, a more transparent separation (factorization), with respect to a given scale, between short-and long-distance physics than diagramatic methods. (C) 1997 Elsevier Science B.V

Chiral symmetry and the delta-nucleon transition form factors

The three complex form factors entering the Delta–>N gamma* vertex are calculated to O(epsilon(3)) in the framework of a chiral effective theory with explicit Delta(1232) degrees of freedom included. It is shown that the low q(2) behavior of the form factors is governed by pi N, pi Delta loop effects. Predictions are given for the q(2) dependence of the three transition multipoles M1(q(2)), E2(q(2)), C2(q(2)). Furthermore, the role of the presently unknown low energy constants that affect the values of the multipole ratios EMR(q(2)) and CMR(q(2)) is elucidated. [S0556-2821(99)04215-0]

Next-to-soft corrections to high energy scattering in QCD and gravity

SCOAP