5,099 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.

### Production of two $c \bar c$ pairs in gluon-gluon scattering in high energy proton-proton collisions

We calculate cross sections for $g g \to Q \bar Q Q \bar Q$ in the
high-energy approximation in the mixed (longitudinal momentum fraction, impact
parameter) and momentum space representations. Besides the total cross section
as a function of subsystem energy also differential distributions (in quark
rapidity, transverse momentum, $Q Q$, $Q \bar Q$ invariant mass) are presented.
The elementary cross section is used to calculate production of $(c \bar c) (c
\bar c)$ in single-parton scattering (SPS) in proton-proton collisions. We
present integrated cross section as a function of proton-proton center of mass
energy as well as differential distribution in $M_{(c \bar c)(c \bar c)}$. The
results are compared with corresponding results for double-parton scattering
(DPS) discussed recently in the literature. We find that the considered SPS
contribution to $(c \bar c)(c \bar c)$ production is at high energy ($\sqrt{s}
>$ 5 TeV) much smaller than that for DPS contribution.Comment: 17 pages, 11 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

### 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 ${\cal
F}(x,Q^{2})$ 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
${\cal F}(x,Q^{2})$ from the experimental data on the small-x proton structure
function $F_{2p}(x,Q^{2})$. We give convenient parameterizations for ${\cal
F}(x,Q^{2})$ 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 ${\cal F}(x,Q^{2})$ 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

### Glue in the pomeron from nonlinear k_\perp-factorization

We derive the nonlinear k_\perp-factorization for the spectrum of jets in
high-mass diffractive deep inelastic scattering as a function of three hard
scales - the virtuality of the photon Q^2, the transverse momentum of the jet
and the saturation scale Q_A. In contrast to all other hard reactions studied
so far, we encounter a clash between the two definitions of the glue in the
pomeron -- from the inclusive spectrum of leading quarks and the small-\beta
evolution of the diffractive cross section. This clash casts a further shadow
on customary applications of the familiar collinear factorization to a pQCD
analysis of diffractive deep inelastic scattering.Comment: 9 page

### The BFKL-Regge factorization and $F_2^b$, $F_2^c$, $F_L$ at HERA: physics implications of nodal properties of the BFKL eigenfunctions

The asymptotic freedom is known to split the leading-$\log$ BFKL pomeron into
a series of isolated poles in the complex angular momentum plane. One of our
earlier findings was that the subleading hard BFKL exchanges decouple from such
experimentally important observables as small-$x$ charm, $F_2^c$, and the
longitudinal, $F_L$, structure functions of the proton at moderately large
$Q^2$. For instance, we predicted precocious BFKL asymptotics of $F_2^c(x,Q^2)$
with intercept of the rightmost BFKL pole
\alpha_{\Pom}(0)-1=\Delta_{\Pom}\approx 0.4. On the other hand, the small-$x$
open beauty photo- and electro-production probes the vacuum exchange for much
smaller color dipoles which entails significant subleading vacuum pole
corrections to the small-$x$ behavior. In view of the accumulation of the
experimental data on small-$x$ $F_{2}^{c}$, $F_{2}^{b}$ and $F_{L}$ we extend
our early predictions to the kinematical domain covered by new HERA
measurements. Our parameter-free results agree well with the determination of
$F_2^c$, $F_L$ and published H1 results on $F_2^b$ but slightly overshoot the
very recent (2008, preliminary) H1 results on $F_2^b$.Comment: 13 pages, 4 figures, JETP Letters, Fig.5 remove

### The Wave Function of 2S Radially Excited Vector Mesons from Data for Diffraction Slope

In the color dipole gBFKL dynamics we predict a strikingly different Q^2 and
energy dependence of the diffraction slope for the elastic production of ground
state V(1S) and radially excited V'(2S) light vector mesons. The color dipole
model predictions for the diffraction slope for \rho^0 and \phi^0 production
are in a good agreement with the data from the fixed target and collider HERA
experiments. We present how a different form of anomalous energy and Q^2
dependence of the diffraction slope for V'(2S) production leads to a different
position of the node in radial wave function and discuss a possibility how to
determine this position from the fixed target and HERA data.Comment: 20 pages and 6 figures. Title change

### Asymptotic behaviour of the total cross section of p-p scattering and the Akeno cosmic ray data

I present a new determination of the total cross section for proton-proton
collisions from the recent Akeno results on absorption of the cosmic ray
protons in the p-Air collisions. Extrapolation to the SSC energy suggests
$\sigma_{tot}(p-p) \approx (160-170) mb$. I also comment on a possible
sensitivity of the p-Air cross section determinations to assumptions on the
inelasticity of nuclear collisions at high energy.Comment: . 6 pages, 0 figure

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