561 research outputs found
Multi-Instanton Effects in QCD Sum Rules for the Pion
Multi-instanton contributions to QCD sum rules for the pion are investigated
within a framework which models the QCD vacuum as an instanton liquid. It is
shown that in singular gauge the sum of planar diagrams in leading order of the
expansion provides similar results as the effective single-instanton
contribution. These effects are also analysed in regular gauge. Our findings
confirm that at large distances the correlator functions are more adequately
described in the singular gauge rather than in the regular one.Comment: 11 pages RevTeX is use
The leading-twist pion and kaon distribution amplitudes from the QCD instanton vacuum
We investigate the leading-twist light-cone distribution amplitudes for the
pion and kaon, based on the nonlocal chiral quark model from the instanton
vacuum. Effects of explicit flavor SU(3)-symmetry breaking are taken into
account. The Gegenbauer moments are computed, analyzed and compared with those
of other models. The one-loop QCD evolution of the moments is briefly
discussed. The transverse momentum distributions are also discussed for the
pion and kaon light-cone wave functions.Comment: 17 pages, 12 figures. Accepted for publication in Physical Review
Rare decay pi0 -> e+e-: theory confronts KTeV data
Within the dispersive approach to the amplitude of the rare decay pi0 -> e+e-
the nontrivial dynamics is contained only in the subtraction constant. We
express this constant, in the leading order in (m_e/\Lambda)^2 perturbative
series, in terms of the inverse moment of the pion transition form factor given
in symmetric kinematics. By using the CELLO and CLEO data on the pion
transition form factor given in asymmetric kinematics the lower bound on the
decay branching ratio is found. The restrictions following from QCD allow us to
make a quantitative prediction for the branching B(pi0 -> e+e-) =(6.2\pm
0.1)*10^{-8} which is 3\sigma below the recent KTeV measurement. We confirm our
prediction by using the quark models and phenomenological approaches based on
the vector meson dominance. The decays \eta -> l^+l^- are also discussed.Comment: 7 pages, 1 figur
How the recent BABAR data for P to \gamma\gamma* affect the Standard Model predictions for the rare decays P to l+l-
Measuring the lepton anomalous magnetic moments and the rare decays
of light pseudoscalar mesons into lepton pairs , serve as
important tests of the Standard Model. To reduce the theoretical uncertainty in
the standard model predictions, the data on the charge and transition form
factors of the light pseudoscalar mesons play a significant role. Recently, new
data on the behavior of the transition form factors at
large momentum transfer were supplied by the BABAR collaboration. There are
several problems with the theoretical interpretation of these data: 1) An
unexpectedly slow decrease of the pion transition form factor at high momenta,
2) the qualitative difference in the behavior of the pion form factor and the
and form factors at high momenta, 3) the inconsistency of
the measured ratio of the and form factors with the
predicted one. We comment on the influence of the new BABAR data on the rare
decay branchings.Comment: 11 pages, 3 figure
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
The Generalized Star Product and the Factorization of Scattering Matrices on Graphs
In this article we continue our analysis of Schr\"odinger operators on
arbitrary graphs given as certain Laplace operators. In the present paper we
give the proof of the composition rule for the scattering matrices. This
composition rule gives the scattering matrix of a graph as a generalized star
product of the scattering matrices corresponding to its subgraphs. We perform a
detailed analysis of the generalized star product for arbitrary unitary
matrices. The relation to the theory of transfer matrices is also discussed
Leading-twist pion and kaon distribution amplitudes in the gauge-invariant nonlocal chiral quark model from the instanton vacuum
We investigate the leading-twist light-cone distribution amplitudes for the
pion and kaon based on the gauge-invariant nonlocal chiral quark model from the
instanton vacuum in the presence of external axial-vector currents. We find
that the nonlocal contribution from the gauge invariance has much effects on
the pion distribution amplitudes, while it changes mildly the kaon ones. We
also study the Gegenbauer moments of the distribution amplitudes and compare
them with the empirical analysis of the CLEO data.Comment: 14 pages, 10 figure
Ballistic transport in disordered graphene
An analytic theory of electron transport in disordered graphene in a
ballistic geometry is developed. We consider a sample of a large width W and
analyze the evolution of the conductance, the shot noise, and the full
statistics of the charge transfer with increasing length L, both at the Dirac
point and at a finite gate voltage. The transfer matrix approach combined with
the disorder perturbation theory and the renormalization group is used. We also
discuss the crossover to the diffusive regime and construct a ``phase diagram''
of various transport regimes in graphene.Comment: 23 pages, 10 figure
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