1,398 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.
Probing coherent charmonium photoproduction off light nuclei at medium energies
We demonstrate how the elementary amplitudes , the
amplitude of the nondiagonal transition, and
the total and cross sections can be determined from
measurements of the coherent and photoproduction off light
nuclei at moderate energies. For this purpose we provide a detailed numerical
analysis of the coherent charmonium photoproduction off silicon within the
generalized vector dominance model (GVDM) adjusted to account for the physics
of charmonium models and color transparency phenomenon.Comment: 8 pages, 5 figures (color
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.
Cold Nuclear Matter Effects on J/psi and Upsilon Production at the LHC
The charmonium yields are expected to be considerably suppressed if a
deconfined medium is formed in high-energy heavy-ion collisions. In addition,
the bottomonium states, with the possible exception of the Upsilon(1S) state,
are also expected to be suppressed in heavy-ion collisions. However, in
proton-nucleus collisions the quarkonium production cross sections, even those
of the Upsilon(1S), are also suppressed. These "cold nuclear matter" effects
need to be accounted for before signals of the high density QCD medium can be
identified in the measurements made in nucleus-nucleus collisions. We identify
two cold nuclear matter effects important for midrapidity quarkonium
production: "nuclear absorption", typically characterized as a final-state
effect on the produced quarkonium state and shadowing, the modification of the
parton densities in nuclei relative to the nucleon, an initial-state effect. We
characterize these effects and study the energy, rapidity, and impact-parameter
dependence of initial-state shadowing in this paper.Comment: to be 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
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
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
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
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
Prompt neutrino fluxes from atmospheric charm
We calculate the prompt neutrino flux from atmospheric charm production by
cosmic rays, using the dipole picture in a perturbative QCD framework, which
incorporates the parton saturation effects present at high energies. We compare
our results with the next-to-leading order perturbative QCD result and find
that saturation effects are large for neutrino energies above 10^6 GeV, leading
to a substantial suppression of the prompt neutrino flux. We comment on the
range of prompt neutrino fluxes due to theoretical uncertainties.Comment: 13 pages with 11 figures; expanded discussion, added references,
version to be published in Phys. Rev.
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