3,833 research outputs found
Anatomy of a Weak Matrix Element
Although the weak nonleptonic amplitudes of the Standard Model are
notoriously difficult to calculate, we have produced a modified weak matrix
element which can be analyzed using reliable methods. This hypothetical
nonleptonic matrix element is expressible in terms of the isovector vector and
axialvector spectral functions and , which
can be determined in terms of data from tau lepton decay and
annihilation. Chiral symmetry and the operator product expansion are used to
constrain the spectral functions respectively in the low energy and the high
energy limits. The magnitude of the matrix element thus determined is compared
with its `vacuum saturation' estimate, and in the future may be accessible with
lattice calculations.Comment: Standard Latex file, 10 pgs (figure not included), UMHEP-39
A Non-Perturbative Chiral Approach for Meson-Meson Interactions
A non-perturbative method which combines constraints from chiral symmetry
breaking and coupled channel unitarity is used to describe meson-meson
interactions up to \sqrt{s}\lesssim 1.2 GeV, extending in this way the range of
applicability of the information contained in Chiral Perturbation Theory (\chi
PT), since this perturbative series is typically restricted to \sqrt{s}\lesssim
500 MeV. The approach uses the O(p^2) and O(p^4) \chiPT Lagrangians. The seven
free parameters resulting from the O(p^4) Lagrangian are fitted to the
experimental data. The approach makes use of the expansion of T^{-1} instead of
the amplitude itself as done in \chiPT. The former expansion is suggested by
analogy with the effective range approximation in Quantum Mechanics and it
appears to be very useful. The results, in fact, are in good agreement with a
vast amount of experimental analyses.Comment: 4 pages, 2 figures, LaTeX, Talk given at PANIC99, Uppsala (Sweden),
June 10-16, 199
Threshold Photo/Electro Pion Production - Working Group Summary
We summarize the pertinent experimental and theoretical developments in the
field of pion photo- and electroproduction in the threshold region. We discuss
which experiments and which calculations should be done/performed in the
future.Comment: plain TeX (macro included), 6pp, summary talk presented at the
workshop on "Chiral Dynamics: Theory and Experiments", MIT, July 25-29, 199
Topics in Chiral Perturbation Theory
I consider some selected topics in chiral perturbation theory (CHPT). For the
meson sector, emphasis is put on processes involving pions in the isospin zero
S-wave which require multi-loop calculations. The advantages and shortcomings
of heavy baryon CHPT are discussed. Some recent results on the structure of the
baryons are also presented.Comment: 30 pp, TeX, Review talk, Third Workshop on High Energy Particle
Physics (WHEPP III), Madras, India, January 1994. 7 figures available upon
request. CRN--94/0
The eta' in baryon chiral perturbation theory
We include in a systematic way the eta' in baryon chiral perturbation theory.
The most general relativistic effective Lagrangian describing the interaction
of the lowest lying baryon octet with the Goldstone boson octet and the eta' is
presented up to linear order in the derivative expansion and its heavy baryon
limit is obtained. As explicit examples, we calculate the baryon masses and the
pi N sigma-term up to one-loop order in the heavy baryon formulation. A
systematic expansion in the meson masses is possible, and appearing divergences
are renormalized.Comment: 16 pages, 2 figure
Updated analysis of meson-nucleon sigma terms in the perturbative chiral quark model
We present an updated analysis of meson-baryon sigma terms in the
perturbative chiral quark model, which is based on effective chiral Lagrangian.
The new feature concerns the inclusion of excited states in the quark
propagator. Its influence on meson loops is shown to lead in particular for the
pion-nucleon sigma term to an enhancement relevant for the current evaluation
of this quantity. We also determine various flavor combinations of the scalar
nucleon form factors and their respective low-momentum transfer limits.Comment: 26 pages, 10 figures, to be published in Phys Rev
Meson Masses in High Density QCD
The low-energy effective theories for the two- and three-flavor
color-superconductors arising in the high density limit of QCD are discussed.
Using an effective field theory to describe quarks near the fermi surface, we
compute the masses of the pseudo-Goldstone bosons that dominate the
low-momentum dynamics of these systems.Comment: 13 pages, 3 figures, late
Can one see the number of colors in eta, eta-prime --> pi^+ pi^- gamma?
We investigate the decays eta, eta-prime --> pi^+ pi^- gamma up to
next-to-leading order in the framework of the combined 1/N_c and chiral
expansions. Counter terms of unnatural parity at next-to-leading order with
unknown couplings are important to acommodate the results both to the
experimental decay width and the photon spectrum. The presence of these
coefficients does not allow for a determination of the number of colors from
these decays.Comment: 8 pages, 2 figure
The reactions \gwl\ and \gzl\ in strongly interacting theories
Building on recent phenomenology of \gpi, we discuss the expectation for two
photon production of longitudinal gauge boson pairs in technicolor
theories. The treatment involves a matching of dispersive techniques with the
methodology of chiral perturbation theory.Comment: 25 LaTeX pages (incl. 10 figures on 7 pages, also in plain LaTeX,
also available from the authors in PostScript form via regular E_mail),
UMHEP-39
Chiral Perturbation Theory for the Quenched Approximation of QCD
[This version is a minor revision of a previously submitted preprint. Only
references have been changed.] We describe a technique for constructing the
effective chiral theory for quenched QCD. The effective theory which results is
a lagrangian one, with a graded symmetry group which mixes Goldstone bosons and
fermions, and with a definite (though slightly peculiar) set of Feynman rules.
The straightforward application of these rules gives automatic cancellation of
diagrams which would arise from virtual quark loops. The techniques are used to
calculate chiral logarithms in , , , and the ratio of
to . The leading
finite-volume corrections to these quantities are also computed. Problems for
future study are described.Comment: 14 page
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