5,000 research outputs found
The hard scale in the exclusive rho-meson production in diffractive DIS
We re-examine the issue of the pQCD factorization scale in the exclusive rho
production in diffractive DIS from the k_t-factorization point of view. We find
that this scale differs significantly from, and possesses much flatter Q^2
behavior than widely used value (Q^2 + m_\rho^2)/4. With these results in mind,
we discuss the Q^2 shape of the rho meson production cross section. We
introduce rescaled cross sections, which might provide further insight into the
dynamics of rho production. We also comment on the recent ZEUS observation of
energy-independent ratio sigma(gamma* p --> rho p) / sigma_{tot}(gamma*p).Comment: 14 pages, 7 eps figure
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.
Quenching of Leading Jets and Particles: the p_t Dependent Landau-Pomeranchuk-Migdal effect from Nonlinear k_t Factorization
We report the first derivation of the Landau-Pomeranchuk-Migdal effect for
leading jets at fixed values of the transverse momentum p_t in the beam
fragmentation region of hadron-nucleus collisions from RHIC (Relativistic Heavy
Ion Collider) to LHC (Large Hadron Collider). The major novelty of this work is
a derivation of the missing virtual radiative pQCD correction to these
processes - the real-emission radiative corrections are already available in
the literature. We manifestly implement the unitarity relation, which in the
simplest form requires that upon summing over the virtual and real-emission
corrections the total number of scattered quarks must exactly equal unity. For
the free-nucleon target, the leading jet spectrum is shown to satisfy the
familiar linear Balitsky-Fadin-Kuraev-Lipatov leading log(1/x) (LL-1/x)
evolution. For nuclear targets, the nonlinear k_t-factorization for the LL-1/x
evolution of the leading jet sepctrum is shown to exactly match the equally
nonlinear LL-1/x evolution of the collective nuclear glue - there emerges a
unique linear k_t-factorization relation between the two nonlinear evolving
nuclear observables. We argue that within the standard dilute uncorrelated
nucleonic gas treatment of heavy nuclei, in the finite energy range from RHIC
to LHC, the leading jet spectrum can be evolved in the LL-1/x
Balitsky-Kovchegov approximation. We comment on the extension of these results
to, and their possible reggeon field theory interpretation for, mid-rapidity
jets at LHC.Comment: 36 pages, 8 eps figs, revised, discussion on reggeon interpretation
and refs. adde
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.
Diffractive Hard Dijets and Nuclear Parton Distributions
Diffraction plays an exceptional role in DIS off heavy nuclei. First,
diffraction into hard dijets is an unique probe of the unintegrated glue in the
target. Second, because diffraction makes 50 per cent of total DIS off a heavy
target, understanding diffraction in a saturation regime is crucial for a
definition of saturated nuclear parton densities. After brief comments on the
Nikolaev-Zakharov (NZ) pomeron-splitting mechanism for diffractive hard dijet
production, I review an extension of the Nikolaev-Schafer-Schwiete (NSS)
analysis of diffractive dijet production off nuclei to the definition of
nuclear partons in the saturation regime. I emphasize the importance of
intranuclear final state interactions for the parton momentum distributions.Comment: 9 pages, 2 figures, to be published in Proceedings of the Workshop on
Exclusive Processes at High Momentum Transfer, Jefferson Lab, May 15-18,
2002. Typos corrected, discussion of the results extende
Nonlinear kT factorization for Forward Dijets in DIS off Nuclei in the Saturation Regime
We develop the QCD description of the breakup of photons into forward dijets
in small-x deep inelastic scattering off nuclei in the saturation regime. Based
on the color dipole approach, we derive a multiple scattering expansion for
intranuclear distortions of the jet-jet transverse momentum spectrum. A special
attention is paid to the non-Abelian aspects of the propagation of color
dipoles in a nuclear medium. We report a nonlinear -factorization
formula for the breakup of photons into dijets in terms of the collective
Weizs\"acker-Williams (WW) glue of nuclei as defined in ref.
\cite{Saturation,NSSdijet}. For hard dijets with the transverse momenta above
the saturation scale the azimuthal decorrelation (acoplanarity) momentum is of
the order of the nuclear saturation momentum QA. For minijets with the
transverse momentum below the saturation scale the nonlinear kT-factorization
predicts a complete disappearance of the jet-jet correlation. We comment on a
possible relevance of the nuclear decorrelation of jets to the experimental
data from the STAR-RHIC Collaboration.Comment: 40 pages, 7 figure
High Density QCD, Saturation and Diffractive DIS
We review a consistent description of the fusion and saturation of partons in
the Lorentz-contracted ultrarelativistic nuclei in terms of a nuclear
attenuation of color dipole states of the photon and collective
Weizs\"acker-Williams (WW) gluon structure function of a nucleus. Diffractive
DIS provides a basis for the definition of the WW nuclear glue. The point that
all observables for DIS off nuclei are uniquely calculable in terms of the
nuclear WW glue amounts to a new form of factorization in the saturation
regime.Comment: 13 pages, 4 figures, Invited talk at NATO Advanced Research Workshop
On Diffraction 2002, 31 Aug - 6 Sep 2002, Alushta, Ukrain
Nonlinear k_\perp-factorization for Gluon-Gluon Dijets Produced off Nuclear Targets
The origin of the breaking of conventional linear k_\perp-factorization for
hard processes in a nuclear environment is by now well established. The
realization of the nonlinear nuclear k_\perp-factorization which emerges
instead was found to change from one jet observable to another. Here we report
on an important technical progress, the evaluation of the four-gluon color
dipole cross section operator. It describes the coupled seven-channel
non-Abelian intranuclear evolution of the four-gluon color-singlet states. An
exact diagonalization of this seven-channel problem is possible for large
number of colors N_c and allows a formulation of nonlinear
k_\perp-factorization for production of gluon-gluon dijets. The momentum
spectra for dijets in all possible color representations are reported in the
form of explicit quadratures in terms of the collective nuclear unintegrated
glue. Our results fully corroborate the concept of universality classes.Comment: 53 pages, 9 figures, a few typos fixe
Non-linear BFKL dynamics: color screening vs. gluon fusion
A feasible mechanism of unitarization of amplitudes of deep inelastic
scattering at small values of Bjorken is the gluon fusion. However, its
efficiency depends crucially on the vacuum color screening effect which
accompanies the multiplication and the diffusion of BFKL gluons from small to
large distances. From the fits to lattice data on field strength correlators
the propagation length of perturbative gluons is fermi. The
probability to find a perturbative gluon with short propagation length at large
distances is suppressed exponentially. It changes the pattern of (dif)fusion
dramatically. The magnitude of the fusion effect appears to be controlled by
the new dimensionless parameter , with the diffraction cone
slope standing for the characteristic size of the interaction region. It
should slowly decrease at large . Smallness of the
ratio makes the non-linear effects rather weak even at lowest
Bjorken available at HERA. We report the results of our studies of the
non-linear BFKL equation which has been generalized to incorporate the running
coupling and the screening radius as the infrared regulator.Comment: 16 pages, 2 figures, version accepted for publication, references
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