58 research outputs found
An Analytical Expression for the Non-Singlet Structure Functions at Small in the Double Logarithmic Approximation
A simple analytic expression for the non-singlet structure function
is given. The expression is derived from the result of Ref. [1] obtained by low
resummation of the quark ladder diagrams in the double logarithmic
approximation of perturbative QCD.Comment: 5 pages, A few comments and refs are adde
On the forward-backward charge asymmetry in e+e- -annihilation into hadrons at high energies
The forward-backward asymmetry in e+ e- annihilation into a quark-antiquark
pair is considered in the double-logarithmic approximation at energies much
higher than the masses of the weak bosons. It is shown that after accounting to
all orders for the exchange of virtual photons and W, Z -bosons one is lead to
the following effect (asymmetry): quarks with positive electric charge (e.g. u,
\bar{d}) tend to move in the e+ - direction whereas quarks with negative
charges (e.g. d, \bar{u}) tend to move in the e- - direction. The value of the
asymmetry grows with increasing energy when the produced quarks are within a
cone with opening angle, in the cmf, \theta_0\sim 2M_Z/\sqrt{s} around the e+e-
-beam. Outside this cone, at \theta_0 << \theta << 1, the asymmetry is
inversely proportional to \theta .Comment: 17 Pages, 2 Tables, 4 Figures. Hadronization effects to the asymmetry
are considered with more detail
Production of electroweak bosons in e+e- annihilation at high energies
Production of electroweak bosons in e+e- annihilation into quarks and into
leptons at energies much greater than 100 Gev is considered. We account for
double-logarithmic contributions to all orders in electroweak couplings. It is
assumed that the bosons are emitted in the multi-Regge kinematics. The explicit
expressions for the scattering amplitudes of the process are obtained. It is
shown that the cross sections of the photon and Z production have the identical
energy dependence and asymptotically their ratio depends only on the Weinberg
angle whereas the energy dependence of the cross section of the W production is
suppressed by factor s^{-0.4} compared to them.Comment: Revtex4, 16 pages, 7 figures, 2 table
QCD running coupling effects for the non-singlet structure function at small x
A generalization of the leading-order DGLAP evolution at small x is performed
for the non-singlet structure function by resumming the leading-order DGLAP
anomalous dimension to all orders in the QCD coupling. Explicit expressions are
obtained for the non-singlet structure function of the deep inelastic
scattering, taking into account both the double-logarithmic and the
single-logarithmic contributions, including the running alpha_s effects. It is
shown that when these contributions are included, the asymptotic small-x
behaviour is power-like, with an exponent of about 0.4.Comment: Latex, 20 pages, 7 figure
Factorization and infrared properties of non-perturbative contributions to DIS structure functions
In this paper we present a new derivation of the QCD factorization. We deduce
the k_T- and collinear factorizations for the DIS structure functions by
consecutive reductions of a more general theoretical construction. We begin by
studying the amplitude of the forward Compton scattering off a hadron target,
representing this amplitude as a set of convolutions of two blobs connected by
the simplest, two-parton intermediate states. Each blob in the convolutions can
contain both the perturbative and non-perturbative contributions. We formulate
conditions for separating the perturbative and non-perturbative contributions
and attributing them to the different blobs. After that the convolutions
correspond to the QCD factorization. Then we reduce this totally unintegrated
(basic) factorization first to the k_T- factorization and finally to the
collinear factorization. In order to yield a finite expression for the Compton
amplitude, the integration over the loop momentum in the basic factorization
must be free of both ultraviolet and infrared singularities. This obvious
mathematical requirement leads to theoretical restrictions on the
non-perturbative contributions (parton distributions) to the Compton amplitude
and the DIS structure functions related to the Compton amplitude through the
Optical theorem. In particular, our analysis excludes the use of the singular
factors x^{-a} (with a > 0) in the fits for the quark and gluon distributions
because such factors contradict to the integrability of the basic convolutions
for the Compton amplitude. This restriction is valid for all DIS structure
functions in the framework of both the k_T- factorization and the collinear
factorization if we attribute the perturbative contributions only to the upper
blob.Comment: 19 pages, 6 figure
Parton interactions in the Bjorken limit of QCD
We consider the Bjorken limit in the framework of the effective action
approach and discuss its similarities to the Regge limit. The proposed
effective action allows for a rather simple calculation of the known evolution
kernels. We represent the result in terms of two-parton interaction operators
involving gluon and quark operators depending on light-ray position and
helicity and analyze their symmetry properties.Comment: 32 pages LaTex, 4 eps-figures, comments added, minor correction
Intercepts of the non-singlet structure functions
Infrared evolution equations for small- behaviour of the non-singlet
structure functions and are obtained and solved in the
next-to-leading approximation, to all orders in , and including
running effects. The intercepts of these structure functions, i.e.
the exponents of the power-like small- behaviour, are calculated. A detailed
comparison with the leading logarithmic approximation (LLA) and DGLAP is made.
We explain why the LLA predictions for the small- dependence of the
structure functions may be more reliable than the prediction for the
dependence in the range of explored at HERA.Comment: 26 pages, LaTeX, 5 Postscript figure
The Small-x Behaviour of the Singlet Polarized Structure Function g_2 in the Double Logarithmic Approximation
The small-x behavior of the singlet contributions to the polarized structure
function g_2(x,Q^2) is calculated in the double-logarithmic approximation of
perturbative QCD. The dominant contribution is due to the gluons which, in
contrast to the unpolarized case, mix with the fermions also in the small-x
domain. We find a power-like growth in 1/x in the odd-signature parts of the
amplitude with the same power as in the singlet function g_1(x,Q^2) at x<< 1.Comment: 20 pages, latex, 3 ps figure
On the mechanisms governing gas penetration into a tokamak plasma during a massive gas injection
A new 1D radial fluid code, IMAGINE, is used to simulate the penetration of gas into a tokamak plasma during a massive gas injection (MGI). The main result is that the gas is in general strongly braked as it reaches the plasma, due to mechanisms related to charge exchange and (to a smaller extent) recombination. As a result, only a fraction of the gas penetrates into the plasma. Also, a shock wave is created in the gas which propagates away from the plasma, braking and compressing the incoming gas. Simulation results are quantitatively consistent, at least in terms of orders of magnitude, with experimental data for a D 2 MGI into a JET Ohmic plasma. Simulations of MGI into the background plasma surrounding a runaway electron beam show that if the background electron density is too high, the gas may not penetrate, suggesting a possible explanation for the recent results of Reux et al in JET (2015 Nucl. Fusion 55 093013)
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