40 research outputs found
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
Photon distribution amplitudes and light-cone wave functions in chiral quark models
The leading- and higher-twist distribution amplitudes and light-cone wave
functions of real and virtual photons are analyzed in chiral quark models. The
calculations are performed in the nonlocal quark model based on the instanton
picture of QCD vacuum, as well as in the spectral quark model and the
Nambu--Jona-Lasinio model with the Pauli-Villars regulator, which both treat
interaction of quarks with external fields locally. We find that in all
considered models the leading-twist distribution amplitudes of the real photon
defined at the quark-model momentum scale are constant or remarkably close to
the constant in the variable, thus are far from the asymptotic limit form.
The QCD evolution to higher momentum scales is necessary and we carry it out at
the leading order of the perturbative theory for the leading-twist amplitudes.
We provide estimates for the magnetic susceptibility of the quark condensate
and the coupling , which in the nonlocal model turn out
to be close to the estimates from QCD sum rules. We find the higher-twist
distribution amplitudes at the quark model scale and compare them to the
Wandzura-Wilczek estimates. In addition, in the spectral model we evaluate the
distribution amplitudes and light-cone wave functions of the -meson.Comment: 24 pages, 15 figure
Pion pole light-by-light contribution to g-2 of the muon in a nonlocal chiral quark model
We calculate the pion pole term of the light-by-light contribution to the
of the muon in the framework of an effective chiral quark model with
instanton-like nonlocal quark--quark interaction. The full kinematic dependence
of the pion-photon transition form factors is taken into account. The
dependence of form factors on the pion virtuality decreases the result by about
15% in comparison to the calculation where this dependence is neglected.
Further, it is demonstrated that the QCD constraints suggested by Melnikov and
Vainshtein are satisfied within the model. The corresponding contributions
originate from the box diagram as well from the pion-pole term. Our chiral
nonlocal model result for the pion-pole light-by-light contribution to
of the muon is , which is in the ball park
of other effective-model calculations.Comment: 15 pages, 7 figure
Instanton Contribution to the Quark Form Factor
The nonperturbative effects in the quark form factor are considered in the
Wilson loop formalism. The properties of the Wilson loops with cusp
singularities are studied taking into account the perturbative and
nonperturbative contributions, where the latter are considered within the
framework of the instanton liquid model. For the integration path corresponding
to this form factor -- the angle with infinite sides -- the explicit expression
for the vacuum expectation value of the Wilson operator is found to leading
order. The calculations are performed in the weak-field limit for the instanton
vacuum contribution and compared with the one- and two-loop order results for
the perturbative part. It is shown that the instantons produce the powerlike
corrections to the perturbative result, which are comparable in magnitude with
the perturbative part at the scale of order of the inverse average instanton
size. It is demonstrated that the instanton contributions to the quark form
factor are exponentiated to high orders in the small instanton density
parameter.Comment: Version coincident with the journal publication. LaTeX, 15 pages, 1
figur
Instanton Corrections to Quark Form Factor at Large Momentum Transfer
Within the Wilson integral formalism, we discuss the structure of
nonperturbative corrections to the quark form factor at large momentum transfer
analyzing the infrared renormalon and instanton effects. We show that the
nonperturbative effects determine the initial value for the perturbative
evolution of the quark form factor and attribute their general structure to the
renormalon ambiguities of the perturbative series. It is demonstrated that the
instanton contributions result in the finite renormalization of the
next-to-leading perturbative result and numerically are characterized by a
small factor reflecting the diluteness of the QCD vacuum within the instanton
liquid model.Comment: Version coincident with the journal publication, 9 pages; REVTe
Generalized Quark Transversity Distribution of the Pion in Chiral Quark Models
The transversity generalized parton distributions (tGPDs) of the the pion,
involving matrix elements of the tensor bilocal quark current, are analyzed in
chiral quark models. We apply the nonlocal chiral models involving a
momentum-dependent quark mass, as well as the local Nambu--Jona-Lasinio with
the Pauli-Villars regularization to calculate the pion tGPDs, as well as
related quantities following from restrained kinematics, evaluation of moments,
or taking the Fourier-Bessel transforms to the impact-parameter space. The
obtained distributions satisfy the formal requirements, such as proper support
and polynomiality, following from Lorentz covariance. We carry out the
leading-order QCD evolution from the low quark-model scale to higher lattice
scales, applying the method of Kivel and Mankiewicz. We evaluate several
lowest-order generalized transversity form factors, accessible from the recent
lattice QCD calculations. These form factors, after evolution, agree properly
with the lattice data, in support of the fact that the spontaneously broken
chiral symmetry is the key element also in the evaluation of the transversity
observables.Comment: 17 pages, 17 figures, regular pape
Transversity Form Factors and Generalized Parton Distributions of the pion in chiral quark models
The transversity Generalized Parton Distributions (tGPDs) and related
transversity form factors of the pion are evaluated in chiral quark models,
both local (Nambu--Jona-Lasinio) and nonlocal, involving a momentum-dependent
quark mass. The obtained tGPDs satisfy all a priori formal requirements, such
as the proper support, normalization, and polynomiality. We evaluate
generalized transversity form factors accessible from the recent lattice QCD
calculations. These form factors, after the necessary QCD evolution, agree very
well with the lattice data, confirming the fact that the spontaneously broken
chiral symmetry governs the structure of the pion also in the case of the
transversity observables.Comment: 6 pages, 3 figures, presented by WB at LIGHTCONE 2011, 23 - 27 May,
2011, Dalla
Unbiased analysis of CLEO data at NLO and pion distribution amplitude
We discuss different QCD approaches to calculate the form factor
F^{\gamma^*\gamma\pi}(Q^2) of the \gamma^*\gamma\to\pi^{0} transition giving
preference to the light-cone QCD sum rules (LCSR) approach as being the most
adequate. In this context we revise the previous analysis of the CLEO
experimental data on F^{\gamma^*\gamma\pi}(Q^{2}) by Schmedding and Yakovlev.
Special attention is paid to the sensitivity of the results to the (strong
radiative) \alpha_s-corrections and to the value of the twist-four coupling
\delta^2. We present a full analysis of the CLEO data at the NLO level of
LCSRs, focusing particular attention to the extraction of the relevant
parameters to determine the pion distribution amplitude, i.e., the Gegenbauer
coefficients a_2 and a_4. Our analysis confirms our previous results and also
the main findings of Schmedding and Yakovlev: both the asymptotic, as well as
the Chernyak--Zhitnitsky pion distribution amplitudes are completely excluded
by the CLEO data. A novelty of our approach is to use the CLEO data as a means
of determining the value of the QCD vacuum non-locality parameter \lambda^2_q =
/ =0.4 GeV^2, which specifies the average virtuality of
the vacuum quarks.Comment: 25 pages, 5 figures, 4 tables; format and margins corrected to fit
page size; small changes in the text and correction of misprint