794 research outputs found
Exclusive electroproduction of J/psi mesons
A nonperturbative calculation of elastic electroproduction of the J/psi meson
is presented and compared to the experimental data.
Our model describes well the observed dependences of the cross sections on
the photon virtuality Q2 and on the energy, and the measured ratio R of
longitudinal to transverse cross sections.Comment: Five *.eps figure
Nonperturbative QCD treatment of photoproduction
We present a nonperturbative QCD calculation of elastic meson
production in photon-proton scattering at high energies. Using light cone wave
functions of the photon and vector mesons, and the framework of the model of
the stochastic QCD vacuum, we calculate the differential and integrated elastic
cross sections for \gamma p \goto J/\psi p . With an energy dependence
following the two-pomeron model we are able to give a consistent description of
the integrated cross sections and the differential cross sections at low
in the range from 20 GeV up to the highest HERA energies. We discuss different
approaches to introduce saturation and find no specific effects up to energies
presently available. We also calculate and compare to experiments the cross
section for photoproduction.Comment: 24 pages, 10 figures, 4 table
Direct extraction of the chiral quark condensate and bounds on the light quark masses
We select sum rules from which one can extract directly reliable limits on
the size of the chiral symmetry breaking light quark condensate
. Combined results from the nucleon and mass-splitting
sum rules give a result compatible with the standard value: , through the determinations of the quark-gluon mixed
condensate. The vector form factor of the D\rar K^*l\nu semi-leptonic decay
leads to the range . The upper
limit combined with the Gell-Mann-Oakes-Renner (GMOR) relation implies the
interesting lower bound on the sum of light quark masses: 9.4 MeV, which combined with the ratio of light quark masses from chiral
perturbation theory leads to MeV. The lower
limit combined with the positivity of the contribution to the GMOR
relation leads to the upper bound : 15.7 MeV, which is
independent on the nature of chiral symmetry breaking.Comment: Minor modifications and corrections. Version to appear in Phys. Lett.
Universality in the Electroproduction of Vector Mesons
We study universality in the electroproduction of vector mesons using a
unified nonperturbative approach which has already proved to reproduce
extremely well the available experimental data. In this framework, after the
extraction of factors that are specific of each vector meson, we arrive at a
reduced integrated elastic cross section which is universal. Our calculations
suggest a finite infrared behavior for the strong coupling constant.Comment: 22 pages, 10 figure
Exclusive production of vector mesons and by real and virtual photons
We present a non-perturbative QCD calculation of high-energy diffractive
photo- and leptoproduction of vector mesons , and on a
nucleon. The initial photon splits up in a -dipole and transforms
into a vector meson by scattering on the quark-diquark nucleon. The
dipole-dipole scattering amplitude is provided by the non-perturbative model of
the stochastic QCD vacuum, the wave functions result from considerations on the
light-cone. We assume the physical - and -states to be mixed
states of an active 2S-excitation and a rest whose coupling to the photon is
suppressed. We obtain good agreement with the experimental data and get an
understanding of the markedly different spectrum in the -invariant
mass for photoproduction and -annihilation.Comment: Talk given in Montpellier, QCD Euroconference 98, to be published in
the proceedings Nucl.Phys. B (Proc.Suppl.). 5 pages, LaTeX2e, 13 ps/eps
figures included, needs style file espcrc2MOD.sty (appended
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
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