990 research outputs found
The field strength correlator from QCD sum rules
The gauge invariant two-point correlator for the gluon field strength tensor
in analysed by means of the QCD sum rule method. To this end, we make use of a
relation of this correlator to a two-point function for a quark-gluon hybrid in
the limit of the quark mass going to infinity. From the sum rules a relation
between the gluon correlation length and the gluon condensate is obtained.Comment: 5 pages, 1 figure, QCD Euroconferenc
Pre-resonant Charmonium-Nucleon Cross Section in the Model of the Stochastic Vacuum
We calculate the nonperturbative charmonium - nucleon cross sections with the
model of the stochastic vacuum, which has been succesfully applied in many high
energy reactions. We also give a quantitative discussion of pre-resonance
formation and medium effects.Comment: Talk given at QCD Euroconference 99, Montpellier, 7-13 July 1999, 4
pages, Late
QCD sum rule analysis of the field strength correlator
The gauge invariant two-point correlator for the gluon field strength tensor
is analysed by means of the QCD sum rule method. To this end, we make use of a
relation of this correlator to a two-point function for a quark-gluon hybrid in
the limit of the quark mass going to infinity. From the sum rules a relation
between the gluon correlation length and the gluon condensate is obtained. We
briefly compare our results to recent determinations of the field strength
correlator on the lattice.Comment: 16 pages, 1 figur
Field strength correlators in QCD with dynamical fermions
We determine, by numerical simulations on a lattice, the gauge-invariant
two-point correlation functions of the gauge field strengths in the QCD vacuum
with four flavours of dynamical staggered fermions.Comment: Talk given at the ``XVth International Symposium on Lattice Field
Theory'', Edinburgh (UK), July 22nd-26th 1997 (LATTICE 97); 3 pages, LaTeX
file, uses ``espcrc2.sty''+ 2 PS figure
Odd C-P contributions to diffractive processes
We investigate contributions to diffractive scattering, which are odd under
C- and P-parity. Comparison of p- and p-p scattering indicates that
these odderon contributions are very small and we show how a diquark clustering
in the proton can explain this effect. A good probe for the odderon exchange is
the photo- and electroproduction of pseudo-scalar mesons. We concentrate on the
pi^0 and show that the quasi elastic pi^0-production is again strongly
suppressed for a diquark structure of the proton whereas the cross sections for
diffractive proton dissociation are larger by orders of magnitude and rather
independent of the proton structure.Comment: 18 pages, LaTex2e, graphicx package, 14 eps figures include
TVWS policies to enable efficient spectrum sharing
The transition from analogue to the Digital Terrestrial Television (DTV) in Europe is planned to be completed by the end of the year 2012. The DTV spectrum allocation is such that there are a number of TV channels which cannot be used for additional high power broadcast transmitters due to mutual interference and hence are left unused within a given geographical location, i.e. the TV channels are geographically interleaved. The use of geographically interleaved spectrum provides for the so-called TV white spaces (TVWS) an opportunity for deploying new wireless services. The main objective of this paper is to present the spectrum policies that are suitable for TVWS at European level, identified within the COGEU project. The COGEU project aims the efficient exploitation of the geographical interleaved spectrum (TVWS). COGEU is an ICT collaborative project supported by the European Commission within the 7th Framework Programme. Nine partners from seven EU countries representing academia, research institutes and industry are involved in the project. The COGEU project is a composite of technical, business, and regulatory/policy domains, with the objective of taking advantage of the TV digital switchover by developing cognitive radio systems that leverage the favorable propagation characteristics of the UHF broadcast spectrum through the introduction and promotion of real-time secondary spectrum trading and the creation of new spectrum commons regimes. COGEU will also define new methodologies for compliance testing and certification of TVWS equipment to ensure non-interference coexistence with the DVB-T European standard. The innovation brought by COGEU is the combination of cognitive access to TV white spaces with secondary spectrum trading mechanisms.telecommunications,spectrum management,secondary spectrum market,regulation,TV white spaces,cognitive radio
QCD sum rules in the effective heavy quark theory
We derive sum rules for the leptonic decay constant of a heavy-light meson in the effective heavy quark theory. We show that the summation of logarithms in the heavy quark mass by the renormalization group technique enhances considerably radiative corrections. Our result for the decay constant in the static limit agrees well with recent lattice calculations. Finite quark mass corrections are estimated
SrKZnMnAs: a ferromagnetic semiconductor with colossal magnetoresistance
A bulk diluted magnetic semiconductor (Sr,K)(Zn,Mn)As was
synthesized with decoupled charge and spin doping. It has a hexagonal
CaAlSi-type structure with the (Zn,Mn)As layer forming
a honeycomb-like network. Magnetization measurements show that the sample
undergoes a ferromagnetic transition with a Curie temperature of 12 K and
\revision{magnetic moment reaches about 1.5 /Mn under = 5 T
and = 2 K}. Surprisingly, a colossal negative magnetoresistance, defined as
, up to 38\% under a low field of = 0.1
T and to 99.8\% under = 5 T, was observed at = 2 K. The
colossal magnetoresistance can be explained based on the Anderson localization
theory.Comment: Accepted for publication in EP
Field strength correlators in full QCD
We study, by numerical simulations on a lattice, the behaviour of the
gauge-invariant two-point correlation functions of the gauge field strengths in
the QCD vacuum with dynamical fermions.Comment: 11 pages, LaTeX file, + 2 PS figure
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