Evolution of high-mass diffraction from the light quark valence component of the pomeron

We analyze the contribution from excitation of the $(q\bar q)(f\bar f),(q\bar q)g_1...g_n(f\bar f)$ Fock states of the photon to high mass diffraction in DIS. We show that the large $Q^2$ behavior of this contribution can be described by the DLLA evolution from the non-perturbative $f\bar f$ valence state of the pomeron. Although of higher order in pQCD, the new contribution to high-mass diffraction is comparable to that from the excitation of the $q\bar q g$ Fock state of the photon.Comment: 12 pages, 2 figures, the oublished version. The slight numerical errors corrected, all conclusions are retaine

The Onset of Color Transparency in (e,e′p)(e,e'p) Reactions on Nuclei

Quantum filtering of the ejectile wave packet from hard $ep$ scattering on bound nucleons puts stringent constraints on the onset of color transparency in $(e,e'p)$ reactions in nuclei at moderate energies. Based on multiple-scattering theory, we derive a novel formula for nuclear transparency and discuss its energy dependence in terms of a color transparency sum rule.Comment: 14 pages (LATEX), 3 figures (not included, available from the authors), KFA-IKP(TH)-1992-1

The Running BFKL: Resolution of Caldwell's Puzzle

The HERA data on the proton structure function, $F_2(x,Q^2)$, at very small $x$ and $Q^2$ show the dramatic departure of the logarithmic slope, $\partial F_2/\partial\log Q^2$, from theoretical predictions based on the DGLAP evolution. We show that the running BFKL approach provides the quantitative explanation for the observed $x$ and/or $Q^2$ -dependence of $\partial F_2/\partial\log Q^2$.Comment: 7 pages, Latex, 4 Figures, P

Unitarity cutting rules for the nucleus excitation and topological cross sections in hard production off nuclei from nonlinear k_t-factorization

At the partonic level, a typical final state in small-x deep inelastic scattering off nuclei and hard proton-nucleus collisions can be characterized by the multiplicity of color-excited nucleons. Within reggeon field theory, each color-excited nucleon is associated with the unitarity cut of the pomeron exchanged between the projectile and nucleus. In this communication we derive the unitarity rules for the multiplicity of excited nucleons, alias cut pomerons, alias topological cross sections, for typical hard dijet production processes. We demonstrate how the coupled-channel non-Abelian intranuclear evolution of color dipoles, inherent to pQCD, gives rise to the reggeon field theory diagrams for final states in terms of the uncut, and two kinds of cut, pomerons. Upon the proper identification of the uncut and cut pomeron exchanges, the topological cross sections for dijet production follow in a straightforward way from the earlier derived nonlinear k_t - factorization quadratures for the inclusive dijet cross sections. The concept of a coherent (collective) nuclear glue proves extremely useful for the formulation of reggeon field theory vertices of multipomeron - cut and uncut - couplings to particles and between themselves. A departure of our unitarity cutting rules from the ones suggested by the pre-QCD Abramovsky-Kancheli-Gribov rules, stems from the coupled-channel features of intranuclear pQCD. We propose a multiplicity re-summation as a tool for the isolation of topological cross sections for single-jet production.Comment: 53 pages, 16 eps-figures, to appear in Phys. Rev.

Double-exchange theory of ferroelectric polarization in orthorhombic manganites with twofold periodic magnetic texture

We argue that many aspects of improper ferroelectric activity in orthorhombic manganites can be rationalized by considering the limit of infinite intra-atomic splitting between the majority- and minority-spin states (or the double exchange limit), which reduces the problem to the analysis of a spinless double exchange (DE) Hamiltonian. We apply this strategy to the low-energy model, derived from the first-principles calculations, and combine it with the Berry-phase theory of electric polarization. We start with the simplest two-orbital model, describing the behavior of the eg bands, and apply it to the E-type antiferromagnetic (AFM) phase, which in the DE limit effectively breaks up into one-dimensional zigzag chains. We derive an analytical expression for the electronic polarization (Pel) and explain how it depends on the orbital ordering and the energy splitting Delta between eg states. Then, we evaluate parameters of this model, starting from a more general five-orbital model for all Mn 3d bands and constructing a new downfolded model for the eg bands. From the analysis of these parameters, we conclude that the behavior of Pel in realistic manganites corresponds to the limit of large Delta. We further utilize this property in order to derive an analytical expression for Pel in a general two-fold periodic magnetic texture, based on the five-orbital model and the perturbation-theory expansion for the Wannier functions in the first order of 1/Delta. This expression explains the functional dependence of Pel on the relative directions of spins. Furthermore, it suggests that Pel is related to the asymmetry of the transfer integrals, which should simultaneously have symmetric and antisymmetric components. Finally, we explain how the polarization can be switched between orthorhombic directions a and c by inverting the zigzag AFM texture in every second ab plane.Comment: 41 page, 10 figure