5,323 research outputs found
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.
The Running BFKL: Resolution of Caldwell's Puzzle
The HERA data on the proton structure function, , at very small
and show the dramatic departure of the logarithmic slope, , from theoretical predictions based on the DGLAP
evolution. We show that the running BFKL approach provides the quantitative
explanation for the observed and/or -dependence of .Comment: 7 pages, Latex, 4 Figures, P
Evolution of high-mass diffraction from the light quark valence component of the pomeron
We analyze the contribution from excitation of the Fock states of the photon to high mass diffraction in
DIS. We show that the large behavior of this contribution can be
described by the DLLA evolution from the non-perturbative 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 Fock state of the photon.Comment: 12 pages, 2 figures, the oublished version. The slight numerical
errors corrected, all conclusions are retaine
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
adde
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
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
Exchange Field Induced Magnetoresistance in Colossal Magnetoresistance Manganites
The effect of an exchange field on electrical transport in thin films of
metallic ferromagnetic manganites has been investigated. The exchange field was
induced both by direct exchange coupling in a ferromagnet/antiferromagnet
multilayer and by indirect exchange interaction in a ferromagnet/paramagnet
superlattice. The electrical resistance of the manganite layers was found to be
determined by the absolute value of the vector sum of the effective exchange
field and the external magnetic field.Comment: 5 pages, 4 figure
Anatomy of the differential gluon structure function of the proton from the experimental data on F_2p
The use of the differential gluon structure function of the proton introduced by Fadin, Kuraev and Lipatov in 1975 is called upon in
many applications of small-x QCD. We report here the first determination of
from the experimental data on the small-x proton structure
function . We give convenient parameterizations for based partly on the available DGLAP evolution fits (GRV, CTEQ &
MRS) to parton distribution functions and on realistic extrapolations into soft
region. We discuss an impact of soft gluons on various observables. The
x-dependence of the so-determined varies strongly with Q^2
and does not exhibit simple Regge properties. None the less the hard-to-soft
diffusion is found to give rise to a viable approximation of the proton
structure function F_{2p}(x,Q^2) by the soft and hard Regge components with
intercepts \Delta_{soft}=0 and \Delta_{hard}\sim 0.4.Comment: 37 pages, 25 figure
Saturation Effects in Deep Inelastic Scattering at low and its Implications on Diffraction
We present a model based on the concept of saturation for small and
small . With only three parameters we achieve a good description of all Deep
Inelastic Scattering data below . This includes a consistent treatment
of charm and a successful extrapolation into the photoproduction regime. The
same model leads to a roughly constant ratio of diffractive and inclusive cross
section.Comment: 24 pages, 12 figures, Latex-fil
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