318 research outputs found

### A Gribov equation for the photon Green's function

We present a derivation of the Gribov equation for the gluon/photon Green's
function D(q). Our derivation is based on the second derivative of the
gauge-invariant quantity Tr ln D(q), which we interpret as the gauge-boson
`self-loop'. By considering the higher-order corrections to this quantity, we
are able to obtain a Gribov equation which sums the logarithmically enhanced
corrections. By solving this equation, we obtain the non-perturbative running
coupling in both QCD and QED. In the case of QCD, alpha_S has a singularity in
the space-like region corresponding to super-criticality, which is argued to be
resolved in Gribov's light-quark confinement scenario. For the QED coupling in
the UV limit, we obtain a \propto Q^2 behaviour for space-like Q^2=-q^2. This
implies the decoupling of the photon and an NJLVL-type effective theory in the
UV limit.Comment: 12 pages, 5 figures; version to be published in Eur. Phys. J.

### 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

### Nuclear effects in $g_{1A}(x,Q^2)$ at small $x$ in deep inelastic scattering on $^7$Li and $^3$He

We suggest to use polarized nuclear targets of $^7$Li and $^3$He to study
nuclear effects in the spin dependent structure functions $g_{1A}(x,Q^2)$.
These effects are expected to be enhanced by a factor of two as compared to the
unpolarized targets.
We predict a significant $x$ dependence at $10^{-4} \div 10^{-3} \leq x \leq
0.2$ of $g_{1A}(x,Q^2)/g_{1N}(x,Q^2)$ due to nuclear shadowing and nuclear
enhancement. The effect of nuclear shadowing at $x \approx 10^{-3}$ is of an
order of 16% for $g_{1A=7}^{n.s. 3/2}(x,Q^2)/g_{1N}^{n.s.}(x,Q^2)$ and 10% for
$g_{1A=3}^{n.s}(x,Q^2)/g_{1N}^{n.s.}(x,Q^2)$. By imposing the requirement that
the Bjorken sum rule is satisfied we model the effect of enhancement.
We find the effect of enhancement at $x \approx 0.125 (0.15)$ to be of an
order of $20 (55)%$ for $g_{1A=7}^{n.s. 3/2}(x,Q^2)/g_{1N}^{n.s.}(x,Q^2)$ and
$14 (40)%$ for $g_{1A=3}^{n.s}(x,Q^2)/g_{1N}^{n.s.}(x,Q^2)$, if enhancement
occupies the region $0.05 \leq x \leq 0.2$ ($0.1 \leq x \leq 0.2$). We predict
a 2% effect in the difference of the scattering cross sections of deep
inelastic scattering of an unpolarized projectile off $^7$Li with $M_{J}$=3/2
and $M_{J}$=1/2. We also show explicitly that the many-nucleon description of
deep inelastic scattering off $^7$Li becomes invalid in the enhancement region
$0.05 < x \leq 0.2$.Comment: 29 pages, 5 figures, RevTe

### Diffractive Processes at the LHC

We consider diffractive processes which can be measured at the LHC. Analysis
of diffractive events will give unique information about the high energy
asymptotics of hadron scattering. In semihard diffraction one may study the
partonic structure of the Pomeron. Central Exclusive Diffractive production
provides a possibility to investigate the new particles (Higgs bosons, SUSY
particles,...) in an exceptionally clean environment.Comment: 12 pages, To be published in the Proc. of the Gribov-75 Memorial
Workshop, Budapest, May 200

### Electroweak 2 -> 2 amplitudes for electron-positron annihilation at TeV energies

The non-radiative scattering amplitudes for electron-positron annihilation
into quark and lepton pairs in the TeV energy range are calculated in the
double-logarithmic approximation. The expressions for the amplitudes are
obtained using infrared evolution equations with different cut-offs for virtual
photons and for W and Z bosons, and compared with previous results obtained
with an universal cut-off.Comment: Revtex4, 17 pages, 7 figures. Some minor changes made, more refs
adde

### Nuclear shadowing in deep inelastic scattering on nuclei: leading twist versus eikonal approaches

We use several diverse parameterizations of diffractive parton distributions,
extracted in leading twist QCD analyses of the HERA diffractive deep inelastic
scattering (DIS) data, to make predictions for leading twist nuclear shadowing
of nuclear quark and gluon distributions in DIS on nuclei. We find that the
HERA diffractive data are sufficiently precise to allow us to predict large
nuclear shadowing for gluons and quarks, unambiguously. We performed detailed
studies of nuclear shadowing for up and charm sea quarks and gluons within
several scenarios of shadowing and diffractive slopes, as well as at central
impact parameters. We compare these leading twist results with those obtained
from the eikonal approach to nuclear shadowing (which is based on a very
different space-time picture) and observe sharply contrasting predictions for
the size and Q^2-dependence of nuclear shadowing. The most striking differences
arise for the interaction of small dipoles with nuclei, in particular for the
longitudinal structure function F_{L}^{A}.Comment: 43 pages, 16 figures, requires JHEP style fil

### A global reanalysis of nuclear parton distribution functions

We determine the nuclear modifications of parton distribution functions of
bound protons at scales $Q^2\ge 1.69$ GeV$^2$ and momentum fractions
$10^{-5}\le x\le 1$ in a global analysis which utilizes nuclear hard process
data, sum rules and leading-order DGLAP scale evolution. The main improvements
over our earlier work {\em EKS98} are the automated $\chi^2$ minimization,
simplified and better controllable fit functions, and most importantly, the
possibility for error estimates. The resulting 16-parameter fit to the N=514
datapoints is good, $\chi^2/{\rm d.o.f}=0.82$. Within the error estimates
obtained, the old {\em EKS98} parametrization is found to be fully consistent
with the present analysis, with no essential difference in terms of $\chi^2$
either. We also determine separate uncertainty bands for the nuclear gluon and
sea quark modifications in the large-$x$ region where they are not stringently
constrained by the available data. Comparison with other global analyses is
shown and uncertainties demonstrated. Finally, we show that RHIC-BRAHMS data
for inclusive hadron production in d+Au collisions lend support for a stronger
gluon shadowing at $x<0.01$ and also that fairly large changes in the gluon
modifications do not rapidly deteriorate the goodness of the overall fits, as
long as the initial gluon modifications in the region $x\sim 0.02-0.04$ remain
small.Comment: 33 pages, 14 figure

### Quarkonium from the Fifth Dimension

Adding fundamental matter of mass m_Q to N=4 Yang Mills theory, we study
quarkonium, and "generalized quarkonium" containing light adjoint particles. At
large 't Hooft coupling the states of spin<=1 are anomalously light (Kruczenski
et al., hep-th/0304032). We examine their form factors, and show these hadrons
are unlike any known in QCD. By a traditional yardstick they appear infinite in
size (as with strings in flat space) but we show that this is a failure of the
yardstick. All of the hadrons are actually of finite size ~ \sqrt{g^2N}/m_Q,
regardless of their radial excitation level and of how many valence adjoint
particles they contain. Certain form factors for spin-1 quarkonia vanish in the
large-g^2N limit; thus these hadrons resemble neither the observed J/Psi
quarkonium states nor rho mesons.Comment: 57 pages, LaTeX, 5 figure

### Soft-Gluon Resummation for Bottom Fragmentation in Top Quark Decay

We study soft-gluon radiation in top quark decay within the framework of
perturbative fragmentation functions. We present results for the b-quark energy
distribution, accounting for soft-gluon resummation in both the MSbar
coefficient function and in the initial condition of the perturbative
fragmentation function. The results show remarkable improvement and the b-quark
energy spectrum in top quark decay exhibits very little dependence on
factorization and renormalization scales. We present some hadron-level results
in both x_B and moment space by including non-perturbative information
determined from e+e- data.Comment: 19 pages, 6 figures, JHEP style. Few changes after referee report,
one reference added, numerical results unchange

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