1,527 research outputs found
Nuclear shadowing in inclusive and tagged deuteron structure functions and extraction of F_2^p-F_2^n at small x from electron-deuteron collider data
We review predictions of the theory of leading twist nuclear shadowing for
inclusive unpolarized and polarized deuteron structure functions F_2^D, g_1^D
and b_1^D and for the tagged deuteron structure function F_2^D(x,Q^2,\vec{p}).
We analyze the possibility to extract the neutron structure function F_2^n from
electron-deuteron data and demonstrate that an account of leading twist nuclear
shadowing leads to large corrections for the extraction of F_2^n from the
future deuteron collider data both in the inclusive and in the tagged structure
function modes. We suggest several strategies to address the extraction of
F_2^n and to measure at the same time the effect of nuclear shadowing via the
measurement of the distortion of the proton spectator spectrum in the
semi-inclusive e D \to e^{\prime}NX process. We address the issue of the final
state interactions in the e D \to e^{\prime}NX process and examine how they
affect the extraction of F_2^n.Comment: 19 pages, 7 figures. Published in Mod. Phys. Lett.
Photon parton distributions in nuclei and the EMC effect
Photons as well as quarks and gluons are constituents of the infinite
momentum frame (IMF) wave function of an energetic particle. They are mostly
equivalent photons whose amplitude follows from the Lorentz transformation of
the particle rest frame Coulomb field into the IMF and from the conservation of
the electromagnetic current. We evaluate in a model independent way the
dominant photon contribution \propto \alpha_{em}(Z^2/A^{4/3})\ln(1/R_{A}m_{N}x)
to the nuclear structure functions as well as the term \propto \alpha_{em}Z/A.
In addition we show that the definition of x consistent with the exact
kinematics of eA scattering (with exact sum rules) works in the same direction
as the nucleus field of equivalent photons. Combined, these effects account for
the bulk of the EMC effect for x\le 0.5 where Fermi motion effects are small.
In particular for these x the hadronic mechanism contribution to the EMC effect
does not exceed \sim 3% for all nuclei. Also the A-dependence of the hadronic
mechanism of the EMC effect for x > 0.5 is significantly modified.Comment: 14 pages, 3 figures, final version published in Phys.Rev.
Active-to-absorbing state phase transition in the presence of fluctuating environments: Weak and strong dynamic scaling
We investigate the scaling properties of phase transitions between survival
and extinction (active-to-absorbing state phase transition, AAPT) in a model,
that by itself belongs to the directed percolation (DP) universality class,
interacting with a spatio-temporally fluctuating environment having its own
non-trivial dynamics. We model the environment by (i) a randomly stirred fluid,
governed by the Navier-Stokes (NS) equation, and (ii) a fluctuating surface,
described either by the Kardar-Parisi-Zhang (KPZ) or the Edward-Wilkinson (EW)
equations. We show, by using a one-loop perturbative field theoretic set up,
that depending upon the spatial scaling of the variance of the external forces
that drive the environment (i.e., the NS, KPZ or EW equations), the system may
show {\em weak} or {\em strong dynamic scaling} at the critical point of active
to absorbing state phase transitions. In the former case AAPT displays scaling
belonging to the DP universality class, whereas in the latter case the
universal behavior is different.Comment: 17 pages, 2 figures, accepted in PR
Large distance behaviour of light cone operator product in perturbative and nonperturbative QCD regimes
We evaluate the coordinate space dependence of the matrix elements of the
commutator of the electromagnetic and gluon currents in the vicinity of the
light-cone but at large distances within the parton model, DGLAP, the
resummation approaches to the small x behaviour of DIS processes, and for the
Unitarity Bound. We find that an increase of the commutator with relative
distance as is the generic property of QCD
at small but fixed space-time interval in perturbative and
nonperturbative QCD regimes. We explain that the factor follows within the
dipole model (QCD factorization theorem) from the properties of Lorents
transformation. The increase of disappeares at central impact parameters
if cross section of DIS may achieve the Unitarity Limit. We argue that such
long range forces are hardly consistent with thermodynamic equilibrium while a
Unitarity Limit may signal equilibration. Possible implications of this new
long range interaction are briefly discussed.Comment: 23 page
The casuality and/or energy-momentum conservation constraints on QCD amplitudes in small x regime
The causality and/or the energy-momentum constraints on the amplitudes of
high energy processes are generalized to QCD. The constraints imply that the
energetic parton may experience at most one inelastic collision only and that
the number of the constituents in the light cone wave function of the
projectile is increasing with the collision energy and the atomic number.Comment: 24 pages,8 figures. The paper is streamlined, some references are
changed and misprints are eliminate
On the behaviour of single scale hard small processes in QCD near the black disc limit
We argue that at sufficiently small Bjorken where pQCD amplitude rapidly
increases with energy and violates probability conservation the shadowing
effects in the single-scale small hard QCD processes can be described by an
effective quantum field theory of interacting quasiparticles. The
quasiparticles are the perturbative QCD ladders. We find, within the WKB
approximation, that the smallness of the QCD coupling constant ensures the
hierarchy among many-quasiparticle interactions evaluated within physical
vacuum and in particular, the dominance in the Lagrangian of the triple
quasiparticle interaction. It is explained that the effective field theory
considered near the perturbative QCD vacuum contains a tachyon relevant for the
divergency of the perturbative QCD series at sufficiently small . We solve
the equations of motion of the effective field theory within the WKB
approximation and find the physical vacuum and the transitions between the
false (perturbative) and physical vacua. Classical solutions which dominate
transitions between the false and physical vacua are kinks that cannot be
decomposed into perturbative series over the powers of . These kinks
lead to color inflation and the Bose-Einstein condensation of quasiparticles.
The account of the quantum fluctuations around the WKB solution reveals the
appearance of the "massless" particles-- "phonons". It is explained that
"phonons" are relevant for the black disc behaviour of small processes,
leading to a Froissart rise of the cross-section. The condensation of the
ladders produces a color network occupying a "macroscopic" longitudinal volume.
We discuss briefly the possible detection of new QCD effects.Comment: 24 pages, 1 Figure. References added, and several misprints
eliminate
On the universality of cross sections of hadron-hadron(nuclei) collisions at superhigh energies
We analyze the pattern of the onset of complete absorption (the black limit)
in the high energy hadron-hadron collisions. The black limit arises due to the
hard and soft interaction dynamics as a function of the impact parameters b.
Both hard and soft mechanisms provide universal dependence of the partial
amplitude of the high energy elastic hadron- hadron scattering on the impact
parameter and result in the radius of interaction proportional to
. We find that with increase of the collision energies hard
interactions lead to a faster increase of the impact parameter range where the
partial wave amplitudes are approaching the unitarity limit. Consequently, we
argue that at super high energies whenthe radius of hadronic interactions
significantly exceeds static radii of the interacting hadrons(nuclei) the ratio
of total cross sections of nucleon-nucleon, meson-nucleon, hadron-nucleus,
nucleus-nucleus collisions becomes equal to one. The same universality is also
expected for the structure functions of nuclei: , at very small , and for the ratio at superhigh energies. We analyze how accounting
for the energy dependence of the interaction radii changes the geometry of
hadron-nucleus and nucleus-nucleus collisions, the energy dependence of total,
absorption and inelastic cross sections, the distribution over the number of
wounded nucleons in proton-nucleus collisions and find that these effects are
noticeable already for the LHC energies and even more so close to the
Greisen-Zatsepin-Kuzmin limit.Comment: 24 pages, 9 figure
Analytic Expression for the Joint x and Q^2 Dependences of the Structure Functions of Deep Inelastic Scattering
We obtain a good analytic fit to the joint Bjorken-x and Q^2 dependences of
ZEUS data on the deep inelastic structure function F_2(x, Q^2). At fixed
virtuality Q^2, as we showed previously, our expression is an expansion in
powers of log (1/x) that satisfies the Froissart bound. Here we show that for
each x, the Q^2 dependence of the data is well described by an expansion in
powers of log Q^2. The resulting analytic expression allows us to predict the
logarithmic derivatives {({\partial}^n F_2^p/{{(\partial\ln Q^2}})^n)}_x for n
= 1,2 and to compare the results successfully with other data. We extrapolate
the proton structure function F_2^p(x,Q^2) to the very large Q^2 and the very
small x regions that are inaccessible to present day experiments and contrast
our expectations with those of conventional global fits of parton distribution
functions.Comment: 4 pages, 3 figures, a few changes in the text. Version to be
published in Physical Review Letter
Probing the QCD pomeron in doubly tagged e^+ e^- collisions
We calculate the total cross-section for collisions and
for the process with two tagged leptons
assuming dominance of the QCD pomeron exchange. We solve the BFKL equation
including the dominant subleading effects generated by the consistency
constraint which restricts the available phase-space of the emitted gluons to
the region inwhich the virtuality of the exchanged gluons is dominated by their
transverse momentum squared. Estimate of the possible soft pomeron contribution
to the cross-section is also presented. At very high CM
energies the calculated total cross-section exhibits
effective power-law behaviour with . We
confront our results with the recent measurements at LEP and give predictions
for the energies which can be accessible at TESLA and at other future linear
colliders.Comment: 12 pages, LaTeX, 3 figure
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