The Photon Structure Function at Small-x

It is shown that recent small-x measurements of the photon structure function F_2^{\gamma}(x,Q^2) by the LEP-OPAL collaboration are consistent with parameter-free QCD predictions at all presently accessible values of Q^2.Comment: 7 pages, LaTeX, 2 figure

Phenomenology of the Flavor-Asymmetry in the Light-Quark Sea of the Nucleon

A phenomenological ansatz for the flavor-asymmetry of the light sea distributions of the nucleon, based on the Pauli exclusion principle, is proposed. This ansatz is compatible with the measured flavor-asymmetry of the unpolarized sea distributions, $\bar{d}>\bar{u}$, of the nucleon. A prediction for the corresponding polarized flavor-asymmetry is presented and shown to agree with predictions of (chiral quark--soliton) models which successfully reproduced the flavor-asymmetry of the unpolarized sea.Comment: 5 pages, LaTeX, 2 figures, uses epsfi

Longitudinally Polarized Photoproduction of Inclusive Hadrons Beyond the Leading Order

We present a complete next-to-leading order QCD calculation for single-inclusive large-pT hadron production in longitudinally polarized lepton-nucleon collisions, consistently including ``direct'' and ``resolved'' photon contributions. This process could be studied experimentally at a future polarized lepton-proton collider like eRHIC at BNL. We examine the sensitivity of such measurements to the so far completely unknown parton content of circularly polarized photons.Comment: 15 pages, 7 eps figure

Leading logarithm calculation of the e^+ e^- -> e^+ \nu_e \bar{u} d cross section

We analytically evaluate in the leading logarithm approximation the differential cross section for e^+ e^- -> e^+ \nu_e \bar{u} d. We compare our order \alpha^4 \alpha_s^0 leading-log result to the order \alpha^4 \alpha_s^0 exact result obtained from the GRC4F Monte Carlo program. Finally we use the Gluck, Reya, Schienbien distribution of partons in a virtual photon, which incorporates both evolution and nonperturbative strong interaction contributions, to obtain better estimates of the differential cross section.Comment: 10 pages including 9 figure

Probing the parton densities of polarized photons at a linear e^+e^- collider

The present theoretical status of spin-dependent parton densities Delta f^{\gamma}(x,Q^2) of circularly polarized photons is briefly reviewed. It is then demonstrated that measurements of the deep-inelastic spin asymmetry A_1^{\gamma}\simeq g_1^{\gamma}/F_1^{\gamma} and of di-jet rapidity distributions at a future linear e^+e^- collider appear to be particularly suited for a determination the spin-dependent photonic quark and gluon densities, respectively. Special emphasis is devoted to a comparison of the different sources of polarized photons at a linear collider: the equivalent photon approximation and backscattered laser (Compton) photons. It is shown that backscattered laser photons are highly favorable, even indispensable, for decent measurements of the Delta f^{\gamma}(x,Q^2).Comment: 7 pages, LaTeX, 4 eps figures, uses amssymb, espcrc2 (included), and epsfig styles; Contribution to the Proceedings of the `International Conference on the Structure and Interactions of the Photon (Photon99)' [Nucl. Phys. B (Proc. Suppl.)], Freiburg, Germany, May 199

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

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

Dynamical QCD Predictions for Ultrahigh Energy Neutrino Cross Sections

Neutrino-nucleon total cross sections for neutrino energies up to ultrahigh energies (UHE), E_\nu=10^12 GeV, are evaluated within the framework of the dynamical (radiative) parton model. The expected uncertainties of these predictions do not exceed the level of about 20 % at the highest energies where contributions of parton distributions in the yet unmeasured region around x\simeq 10^-8 to 10^-9 are non-negligible. This is far more accurate than estimated uncertainties of about 2^+-1 due to ad hoc extrapolations of parton distributions to x<10^-5 required for calculating UHE cosmic neutrino event rates.Comment: 16 pages, LaTeX, 4 figures, uses epsfig and amssymb styl