Breaking of k_\perp-factorization for Single Jet Production off Nuclei

The linear k_\perp-factorization is part and parcel of the pQCD description of high energy hard processes off free nucleons. In the case of heavy nuclear targets the very concept of nuclear parton density becomes ill-defined as exemplified by the recent derivation [2] of nonlinear nuclear k_\perp-factorization for forward dijet production in DIS off nuclei. Here we report a derivation of the related breaking of k_\perp-factorization for single-jet processes. We present a general formalism and apply it to several cases of practical interest: open charm and quark and gluon jet production in the central to beam fragmentation region of \gamma^*p,\gamma^*A, pp and pA collisions. We show how the pattern of k_\perp-factorization breaking and the nature and number of exchanged nuclear pomerons do change within the phase space of produced quark and gluon jets. As an application of the nonlinear k_\perp-factorization we discuss the Cronin effect. Our results are also applicable to the p_\perp-dependence of the Landau-Pomeranchuk-Migdal effect for, and nuclear quenching of, jets produced in the proton hemisphere of pA collisions.Comment: 55 pages, 9 eps figures, presentation shortened, a number of typos removed, to appear in Phys. Rev.

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.

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

Energy Dependence of the Pomeron Spin-Flip

There is no theoretical reason to think that the spin-flip component of the Pomeron is zero. One can measure the spin-flip part using Coulomb-nuclear interference (CNI). Perturbative QCD calculations show that the spin-flip component is sensitive to the smallest quark separation in the proton, while the non-flip part probes the largest separation. According to HERA results on the proton structure function at very low x the energy dependence of the cross-section correlates with the size of the color dipole. Analysing the data from HERA we predict that the ratio of the spin-flip to non-flip amplitude grows with energy as $r(s)\propto (1/x)^{0.1-0.2}$, violating Regge factorisation of the Pomeron.Comment: A few comments and references are added. Based on invited talks at the International Workshop on Diffraction Physics, Rio de Janeiro, February 16-20, 1998, and at DIS'98, Brussels, April 4-8, 199