22 research outputs found
Renormalization group equations in resonance chiral theory
The use of the equations of motion and meson field redefinitions allows the
development of a simplified resonance chiral theory lagrangian: terms including
resonance fields and a large number of derivatives can be reduced into
corresponding O(p2) resonance operators, containing the lowest possible number
of derivatives. This is shown by means of the explicit computation of the pion
vector form-factor up to next-to-leading order in 1/Nc. The study of the
renormalization group equations for the corresponding couplings demonstrates
the existence of an infrared fixed point in the resonance theory. The
possibility of developing a perturbative 1/Nc expansion in the slow running
region around the fixed point is shown here.Comment: 6 pages, 3 figures. Final version as published. References added.
Extended explanations. The interrelation between the IR fixed point and the
UV constraints has been further studie
Vector Meson Dominance as the first order of a sequence of Pade Approximants
The use of Pade Approximants for the analysis of the pion vector form-factor
is discussed and justified in this talk. The method is tested first in a
theoretical model and applied then on real experimental data. It is shown how
the Pade Approximants provide a convenient and reliable framework to
incorporate both low and high energy information in the euclidean region,
leading to improved determinations of the low energy parameters such as, e.g.,
the quadratic radius ^pi_V.Comment: 4 pages, 3 figures, espcrc2 style. To appear in the proceedings of
the 14th International QCD Conference, QCD 08, 7-12 July 2008, Montpellier
(France
extension of the large-- partial wave dispersion relations
Continuing our previous work(JHEP 0706:030,2007), large-- techniques and
partial wave dispersion relations are used to discuss scattering
amplitudes. We get a set of predictions for low-energy chiral
perturbation theory couplings. They are provided in terms of the masses and
decay widths of scalar and vector mesons.Comment: 7 page
Renormalizable Sectors in Resonance Chiral Theory: S -> pi pi Decay Amplitude
We develop a resonance chiral theory without any a priori limitation on the
number of derivatives in the hadronic operators. Through an exhaustive analysis
of the resonance lagrangian and by means of field redefinitions, we find that
the number of independent operator contributing to the S -> pi pi decay
amplitude is finite: there is only one single-trace operator (the cd term) and
three multi-trace terms. The deep implication of this fact is that the
ultraviolet divergences that appear in this amplitude at the loop level can
only appear through these chiral invariant structures. Hence, a renormalization
of these couplings renders the amplitude finite.Comment: 4 page
Effective Theory Description of Weak Annihilation in B-> Xu l nu Decays
The semileptonic B-> Xu l nu decays allow a pretty clean determination of the
CKM matrix element |Vub|. Nevertheless, the presence of weak-annihilation
effects near the end-point of the q2 spectrum introduces uncertainties in the
inclusive calculation, requiring the use of non-perturbative techniques like
heavy meson chiral perturbation theory and large NC limit.Comment: 3 pages, 2 figures. To appear in the proceedings of the 7th
International Conference on Hyperons, Charm And Beauty Hadrons (BEACH 2006),
2nd-8th July 2006, Lancaster, Englan
Some Remarks on the Pade Unitarization of Low-Energy Amplitudes
We present a critical analysis of Pade-based methods for the unitarization of
low energy amplitudes. We show that the use of certain Pade Approximants to
describe the resonance region may lead to inaccurate determinations. In
particular, we find that in the Linear Sigma Model the unitarization of the low
energy amplitude through the inverse amplitude method produces essentially
incorrect results for the mass and width of the sigma. Alternative sequences of
Pades are studied and we find that the diagonal sequences (i.e., [N/N]) have
much better convergence properties.Comment: 12 pages, 4 fig
Interpolating between low and high energy QCD via a 5D Yang-Mills model
We describe the Goldstone bosons of massless QCD together with an infinite
number of spin-1 mesons. The field content of the model is SU(Nf)xSU(Nf)
Yang-Mills in a compact extra-dimension. Electroweak interactions reside on one
brane. Breaking of chiral symmetry occurs due to the boundary conditions on the
other brane, away from our world, and is therefore spontaneous. Our
implementation of the holographic recipe maintains chiral symmetry explicit
throughout. For intermediate energies, we extract resonance couplings. These
satisfy sum rules due to the 5D nature of the model. These sum rules imply,
when taking the high energy limit, that perturbative QCD constraints are
satisfied. We also illustrate how the 5D model implies a definite prescription
for handling infinite sums over 4D resonances. Taking the low energy limit, we
recover the chiral expansion and the corresponding non-local order parameters.
All local order parameters are introduced separately.Comment: Corresponds to published version, with some typos correcte
Spin-1 Correlators at Large NC: Matching OPE and Resonance Theory up to O(alpha_s)
The relation between the quark-gluon description of QCD and the hadronic
picture is studied up to order alpha_s. The analysis of the spin-1 correlators
is developed within the large NC framework. Both representations are shown to
be equivalent in the euclidean domain, where the Operator Product Expansion is
valid. By considering different models for the hadronic spectrum at high
energies, one is able to recover the alpha_s running in the correlators, to fix
the rho(770) and a1(1260) couplings, and to produce a prediction for the values
of the condensates. The Operator Product Expansion is improved by the large NC
resonance theory, extending its range of validity. Dispersion relations are
employed in order to study the minkowskian region and some convenient sum
rules, specially sensitive to the resonance structure of QCD, are worked out. A
first experimental estimate of these sum rules allows a cross-check of former
determinations of the QCD parameters and helps to discern and to discard some
of the considered hadronical models. Finally, the truncated resonance theory
and the Minimal Hadronical Approximation arise as a natural approach to the
full resonance theory, not as a model.Comment: 36 pages, 19 figures. Minor changes (added reference,...). Paper as
finally appeared in pres
Quantum Loops in the Resonance Chiral Theory: The Vector Form Factor
We present a calculation of the Vector Form Factor at the next-to-leading
order in the 1/N_C expansion, within the framework of Resonance Chiral Theory.
The calculation is performed in the chiral limit, and with two dynamical quark
flavours. The ultraviolet behaviour of quantum loops involving virtual
resonance propagators is analyzed, together with the kind of counterterms
needed in the renormalization procedure. Using the lowest-order equations of
motion, we show that only a few combinations of local couplings appear in the
final result. The low-energy limit of our calculation reproduces the standard
Chiral Perturbation Theory formula, allowing us to determine the resonance
contribution to the chiral low-energy couplings, at the next-to-leading order
in 1/N_C, keeping a full control of their renormalization scale dependence.Comment: 27+1 pages, 9 figure
K pi vector form factor, dispersive constraints and tau -> nu_tau K pi decays
Recent experimental data for the differential decay distribution of the decay
by the Belle collaboration are described by a
theoretical model which is composed of the contributing vector and scalar form
factors and . Both form factors are constructed
such that they fulfil constraints posed by analyticity and unitarity. A good
description of the experimental measurement is achieved by incorporating two
vector resonances and working with a three-times subtracted dispersion relation
in order to suppress higher-energy contributions. The resonance parameters of
the charged meson, defined as the pole of in the
complex -plane, can be extracted, with the result MeV and MeV. Finally, employing the
three-subtracted dispersion relation allows to determine the slope and
curvature parameters and
of the vector form factor
directly from the data.Comment: 16 pages, 1 figure, version to appear in Eur. Phys. J.