Scheme Dependence at Small x

We discuss the evolution of F_2^p at small x, emphasizing the uncertainties related to expansion, fitting, renormalization and factorization scheme dependence. We find that perturbative extrapolation from the measured region down to smaller x and lower Q^2 may become strongly scheme dependent.Comment: 8 pages, LaTeX with epsfig, 2 uuencoded figure

Anomaly-Induced Magnetic Screening in 2+1 dimensional QED at Finite Density

We show that in 2+1 dimensional Quantum Electrodynamics an external magnetic field applied to a finite density of massless fermions is screened, due to a $2+1$-dimensional realization of the underlying $2$-dimensional axial anomaly of the space components of the electric current. This is shown to imply screening of the magnetic field, i.e., the Meissner effect. We discuss the physical implications of this result.Comment: 8 pages, DFTT-93-10 [ Eq.(15) and (16) were scrambled in previous version

BFKL at NNLO

We present a recent determination of an approximate expression for the O(alpha_s^3) contribution chi_2 to the kernel of the BFKL equation. This includes all collinear and anticollinear singular contributions and is derived using duality relations between the GLAP and BFKL kernels.Comment: 8 pages. Talk presented at 12th International Conference on Elastic and Diffractive Scattering: Forward Physics and QCD, Hamburg, DESY, Germany, 21-25 May 200

On the Sudakov Form Factor, and a Factor of Two

I answer a question that Roman Jackiw asked me, and I draw some lessons from the answer. The question is: why is the Sudakov form factor larger by a factor of two, if computed for off-shell fermions, in comparison to the onshell case? The answer sheds some light on the interplay between infrared and collinear singularities \u2014 and the importance of factors of two

A determination of alpha_s from scaling violations with truncated moments

We describe a determination of the strong coupling alpha_s(M_Z) from scaling violations of the nonsinglet DIS structure function, which is based on two novel techniques aimed at controlling and minimizing the theoretical error: a neural network parametrization of BCDMS and NMC data, and QCD evolution by means of truncated Mellin moments.Comment: 5 pages, no figures. Talk given by L. Magnea at QCD02, Montpellier, July 200

The sigma term and the quark number operator in QCD

We discuss the relationship of the forward matrix element of the operator $\bar\psi\psi$, related to the so-called sigma term, to the quark number. We show that in the naive quark model in the canonical formalism these quantities coincide in the limit of small average quark momenta. In the QCD parton model defined through light-front quantization this result is preserved at leading perturbative order but it receives radiative corrections. We analyze the theoretical and phenomenological consequences of this result, which provides a bridge between a current algebra quantity, the sigma term, and a deep-inelastic quantity, the parton number.Comment: 30 pages, 1 figure, DFTT-92-6 (April 1993