Polarization Asymmetry Zero in Heavy Quark Photoproduction and Leptoproduction Cross Sections

We demonstrate two novel features of the sea-quark contributions to the polarized structure functions and photoproduction cross sections, a zero sum rule and a zero crossing point of the polarization asymmetry, which can be traced directly to the dynamics of the perturbative tree-graph gluon-splitting contributions. In particular, we show that the Born contribution of massive quarks arising from photon-gluon fusion gives zero contribution to the logarithmic integral over the polarization asymmetry $\int {d\nu \over \nu}\Delta \sigma(\nu,Q^2)$ for any photon virtuality. The vanishing of this integral in the Bjorken scaling limit then implies a zero gluon-splitting Born contribution to the Gourdin-Ellis-Jaffe sum rule for polarized structure functions from massive sea quarks. The vanishing of the polarization asymmetry at or near the canonical position predicted by perturbative QCD provides an important tool for verifying the dominance of the photon-gluon fusion contribution to charm photoproduction and for validating the effectiveness of this process as a measure of the gluon polarization $\Delta G(x,Q^2)$ in the nucleon. The displacement of the asymmetry zero from its canonical position is sensitive to the virtuality of the gluon in the photon-gluon fusion subprocess, and it can provide a measure of intrinsic and higher-order sea quark contributions.Comment: LaTex, 13 page

Final-State Interactions and Single-Spin Asymmetries in Semi-Inclusive Deep Inelastic Scattering

Recent measurements from the HERMES and SMC collaborations show a remarkably large azimuthal single-spin asymmetries A_{UL} and A_{UT} of the proton in semi-inclusive pion leptoproduction. We show that final-state interactions from gluon exchange between the outgoing quark and the target spectator system lead to single-spin asymmetries in deep inelastic lepton-proton scattering at leading twist in perturbative QCD; i.e., the rescattering corrections are not power-law suppressed at large photon virtuality Q^2 at fixed x_{bj}. The existence of such single-spin asymmetries requires a phase difference between two amplitudes coupling the proton target with J^z_p = + 1/2 and -1/2 to the same final state, the same amplitudes which are necessary to produce a nonzero proton anomalous magnetic moment. We show that the exchange of gauge particles between the outgoing quark and the proton spectators produces a Coulomb-like complex phase which depends on the angular momentum L_z of the proton's constituents and is thus distinct for different proton spin amplitudes. The single-spin asymmetry which arises from such final-state interactions does not factorize into a product of distribution function and fragmentation function, and it is not related to the transversity distribution delta q(x,Q) which correlates transversely polarized quarks with the spin of the transversely polarized target nucleon.Comment: Version to appear in Physics Letters B. Typographical errors corrected in Eqs. (13) and (14

Cluster density functional theory for lattice models based on the theory of Mobius functions

Rosenfeld's fundamental measure theory for lattice models is given a rigorous formulation in terms of the theory of Mobius functions of partially ordered sets. The free-energy density functional is expressed as an expansion in a finite set of lattice clusters. This set is endowed a partial order, so that the coefficients of the cluster expansion are connected to its Mobius function. Because of this, it is rigorously proven that a unique such expansion exists for any lattice model. The low-density analysis of the free-energy functional motivates a redefinition of the basic clusters (zero-dimensional cavities) which guarantees a correct zero-density limit of the pair and triplet direct correlation functions. This new definition extends Rosenfeld's theory to lattice model with any kind of short-range interaction (repulsive or attractive, hard or soft, one- or multi-component...). Finally, a proof is given that these functionals have a consistent dimensional reduction, i.e. the functional for dimension d' can be obtained from that for dimension d (d'<d) if the latter is evaluated at a density profile confined to a d'-dimensional subset.Comment: 21 pages, 2 figures, uses iopart.cls, as well as diagrams.sty (included