958 research outputs found
A Parametrization for
We discuss various models and Chiral Perturbation Theory results for the
form factors and . We check in how much a simple
parametrization with a few parameters can be used to extract information from
experiment.Comment: 19 pages, 14 figure
QCD Short-distance Constraints and Hadronic Approximations
This paper discusses a general class of ladder resummation inspired hadronic
approximations. It is found that this approach naturally reproduces many
successes of single meson per channel saturation models (e.g. VMD) and NJL
based models. In particular the existence of a constituent quark mass and a gap
equation follows naturally. We construct an approximation that satisfies a
large set of QCD short-distance and large constraints and reproduces many
hadronic observables.
We show how there exists in general a problem between QCD short-distance
constraints for Green Functions and those for form factors and cross-sections
following from the quark-counting rule. This problem while expected for Green
functions that do not vanish in purely perturbative QCD also persists for many
Green functions that are order parameters.Comment: 27 page
On the two-loop contributions to the pion mass
We derive a simplified representation for the pion mass to two loops in
three-flavour chiral perturbation theory. For this purpose, we first determine
the reduced expressions for the tensorial two-loop 2-point sunset integrals
arising in chiral perturbation theory calculations. Making use of those
relations, we obtain the expression for the pion mass in terms of the minimal
set of master integrals. On the basis of known results for these, we arrive at
an explicit analytic representation, up to the contribution from K-K-eta
intermediate states where a closed-form expression for the corresponding sunset
integral is missing. However, the expansion of this function for a small pion
mass leads to a simple representation which yields a very accurate
approximation of this contribution. Finally, we also give a discussion of the
numerical implications of our results.Comment: Typos corrected and minor changes in Table 2. Published version. 19
pages, 1 figure, 2 table
2 and 3-point functions in the ENJL-model
We discuss the extended Nambu-Jona-Lasinio model as a low energy expansion,
all two-point functions and an example of a three-point function to all orders
in momenta and quark masses. The model is treated at leading level in
but otherwise exact. Some comments about the QCD flavour anomaly and Vector
Meson Dominance in this class of models is made. Uses epsf.sty and rotate.sty.
One postscript figure included at the end.Comment: NORDITA 94/43 N,P, talk given by JB at QCD94, July 7-13, 1994,
Montpellier, Franc
The Hadronic Light-by-Light Contribution to the Muon Anomalous Magnetic Moment: Where do we stand?
We review the status of the hadronic light-by-light contribution to the muon
anomalous magnetic moment and critically compare recent calculations. We also
study in detail which momentum regions the pi^0 exchange main contribution
originates. We also argue that a_\mu^{light-by-light} = (11 \pm 4) \times
10^{-10} encompasses the present understanding of this contribution and comment
on some directions to improve on that.Comment: 16 pages, 9 figure
Towards a determination of the chiral couplings at NLO in 1/N(C): L_8(mu) and C_38(mu)
We present a dispersive method which allows to investigate the low-energy
couplings of chiral perturbation theory at the next-to-leading order (NLO) in
the 1/N(C) expansion, keeping full control of their renormalization scale
dependence. Using the resonance chiral theory Lagrangian, we perform a NLO
calculation of the scalar and pseudoscalar two-point functions, within the
single-resonance approximation. Imposing the correct QCD short-distance
constraints, one determines their difference Pi(t)=Pi_S(t)-Pi_P(t) in terms of
the pion decay constant and resonance masses. Its low momentum expansion fixes
then the low-energy chiral couplings L_8 and C_38. At mu_0=0.77 GeV, we obtain
L_8(mu_0)^{SU(3)} = (0.6+-0.4)10^{-3} and C_{38}(mu_0)^{SU(3)}=(2+-6)10^{-6}.Comment: Extended version published at JHEP01(2007)039. A NLO prediction for
the O(p6) chiral coupling C_38 has been added. The original L_8 results
remain unchange
Scattering in Three Flavour ChPT
We present the scattering lengths for the processes in the three
flavour Chiral Perturbation Theory (ChPT) framework at next-to-next-to-leading
order (NNLO). The calculation has been performed analytically but we only
include analytical results for the dependence on the low-energy constants
(LECs) at NNLO due to the size of the expressions. These results, together with
resonance estimates of the NNLO LECs are used to obtain constraints on the
Zweig rule suppressed LECs at NLO, and . Contrary to
expectations from NLO order calculations we find them to be compatible with
zero. We do a preliminary study of combining the results from
scattering, scattering and the scalar form-factors and find only a
marginal compatibility with all experimental/dispersive input data.Comment: 23 page
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