7,060 research outputs found
Exclusive annihilation p pbar -> gamma gamma in a generalized parton picture
Exclusive proton-antiproton annihilation into two photons at large s (~10
GeV^2) and |t|,|u| ~ s can be described by a generalized parton picture
analogous to the 'soft mechanism' in wide-angle real Compton scattering. The
two photons are emitted in the annihilation of a single fast quark and
antiquark. The matrix element describing the transition of the p-pbar system to
a q-qbar pair can be related to the timelike proton elastic form factors as
well as to the quark/antiquark distributions measured in inclusive
deep-inelastic scattering. The reaction could be studied with the proposed
1.5-15 GeV high-luminosity antiproton storage ring (HESR) at GSI.Comment: 4 pages, revtex4, 3 eps figure
Strong coupling analysis of diquark condensation
The phenomenon of diquark condensation at non-zero baryon density and zero
temperature is analyzed in the strong coupling limit of lattice QCD. The
results indicate that there is attraction in the quark-quark channel also at
strong coupling, and that the attraction is more effective at high baryon
density, but for infinite coupling it is not enough to produce diquark
condensation. It is argued that the absence of diquark condensation is not a
peculiarity of the strong coupling limit, but persists at sufficiently large
finite couplings.Comment: 3 pages, 2 figures, Lattice2004(non-zero
Pion distribution amplitudes within the instanton model of QCD vacuum
Pion transition form factor for the process at
space-like values of photon momenta is calculated within the effective
quark-meson model with the interaction induced by instanton exchange. The
leading and next-to-leading order power asymptotics of the form factor and the
relation between the light-cone pion distribution amplitudes of twists 2 and 4
and the dynamically generated quark mass are found.Comment: 5 pages, 4 eps figures. Revised version: references and figures adde
One-loop self-energy correction to the 1s and 2s hyperfine splitting in H-like systems
The one-loop self-energy correction to the hyperfine splitting of the 1s and
2s levels in H-like low-Z atoms is evaluated to all orders in Z\alpha. The
results are compared to perturbative calculations. The residual higher-order
contribution is evaluated. Implications to the specific difference of the
hyperfine structure intervals 8\Delta \nu_2 - \Delta \nu_1 in He^+ are
investigated.Comment: 17 pages, RevTeX, 3 figure
Determination of quark-antiquark component of the photon wave function for u, d, s quarks
Based on the data for the transitions pi0, eta, eta' -> gamma gamma^*(Q^2)
and reactions of the e^+ e^- -annihilations, e^+ e^- -> rho0, omega, phi and
e^+ e^--> hadrons at 1<E_{e^+e^-}<3.7 GeV, we determine the light-quark
components of the photon wave function gamma^*(Q^2) -> q anti-q (q= u, d, s)
for the region 0< Q^2 <1 (GeV/c)^2.Comment: 17 pages, some typos correcte
Exclusive Photoproduction of Large Momentum-Transfer K and K* Mesons
The reactions gamma p -> K+ Lambda and gamma p -> K* Lambda are analyzed
within perturbative QCD, allowing for diquarks as quasi-elementary constituents
of baryons. The diquark-model parameters and the quark-diquark distribution
amplitudes of proton and Lambda are taken from previous investigations of
electromagnetic baryon form factors and Compton-scattering off protons.
Unpolarized differential cross sections and polarization observables are
computed for different choices of the K and K* distribution amplitudes. The
asymptotic form of the K distribution amplitude (proportional to x1 x2) is
found to provide a satisfactory description of the K photoproduction data.Comment: 32 pages, 7 figures available as tared, compressed and uuencoded
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