Parton evolution in the fixed flavor factorization scheme

It is argued that while the scale dependence of the parton distributions in the fixed flavor factorization scheme is governed by three active flavors, the scale dependence of the running coupling should nevertheless be better governed by a variable number of active flavors.Comment: 5 page

On the Determination of Spin-Dependent Parton Distributions in Semi-Inclusive Deep Inelastic Scattering

New polarized fragmentation functions are introduced and justified, in addition to those conventional ones assumed to be independent of the helicity of the parent parton. It is demonstrated that due to our present ignorance concerning these new parton-spin dependent leading-twist fragmentation functions, it is impossible to utilize current experiments on spin-dependent semi-inclusive deep inelastic lepton nucleon scattering to disentangle the separate polarized parton distributions.Comment: 8 pages, minor changes, 1 figure adde

On the Determination of the Polarized Sea Distributions of the Nucleon

The possibilities to determine the flavor structure of the polarized sea (antiquark) distributions of the nucleon via vector boson $(\gamma^*, W^{\pm}, Z^0)$ production at high energy polarized hadron--hadron colliders, such as the Relativistic Heavy--Ion Collider (RHIC), are studied in detail. In particular the perturbative stability of the expected asymmetries in two representative models for the (un)broken flavor structure are investigated by confronting perturbative QCD leading order predictions of the expected asymmetries with their next--to--leading order counterparts.Comment: 28 pages, LaTe

Spin-Dependent Structure Functions of Real and Virtual Photons

The implications of the positivity constraint, $|g_1^{\gamma(P^2)}(x,Q^2)| \leq F_1^{\gamma(P^2)}(x,Q^2)$, on the presently unknown spin--dependent structure function $g_1^{\gamma(P^2)}(x,Q^2)$ of real and virtual photons are studied at scales $Q^2\gg P^2$ where longitudinally polarized photons dominate physically relevant cross sections. In particular it is shown how to implement the physical constraints of positivity and continuity at $P^2=0$ in NLO calculations which afford a nontrivial choice of suitable (DIS) factorization schemes related to $g_1^{\gamma}$ and $F_1^{\gamma}$ and appropriate boundary conditions for the polarized parton distributions of real and virtual photons. The predictions of two extreme `maximal' and `minimal' saturation scenarios are presented and compared with results obtained within the framework of a simple quark `box' calculation expected to yield reasonable estimates in the not too small regions of $x$ and $P^2$.Comment: 33 pages, LaTeX, 12 figure

The Polarized and Unpolarized Photon Content of the Nucleon

The equivalent photon content of polarized and unpolarized nucleons (protons, neutrons), utilized in Weizs\"acker--Williams approximations, are presented. For this purpose a new expression for the elastic photon component of a polarized nucleon is derived. The inelastic photon components are obtained from the corresponding momentum evolution equations subject to the boundary conditions of their vanishing at some low momentum scale. The resulting photon asymmetries, important for estimating cross section asymmetries in photon induced subprocesses are also presented for some typical relevant momentum scales.Comment: LaTeX, 16 pages, 6 figure

Radiatively Generated Isospin Violations in the Nucleon and the NuTeV Anomaly

Predictions of isospin asymmetries of valence and sea distributions are presented which are generated by QED leading ${\cal{O}}(\alpha)$ photon bremsstrahlung effects. Together with isospin violations arising from nonperturbative hadronic sources (such as quark and target mass differences) as well as with even a conservative contribution from a strangeness asymmetry ($s\neq \bar{s}$), the discrepancy between the large NuTeV `anomaly' result for $\sin^2\theta_W$ and the world average of other measurements is removed.Comment: 10 pages, 2 figure