1,029 research outputs found
Pion Physics at Low Energy and High Accuracy
The role of the quark condensate for the low energy structure of QCD is
discussed in some detail. In particular, the dependence of M_pi on m_u and m_d
and the low energy theorems for the pi-pi scattering amplitude are reviewed.
The new data on Ke4 decay beautifully confirm the standard picture, according
to which the quark condensate is the leading order parameter of the
spontaneously broken chiral symmetry.Comment: Talk given at the Workshop QCD@Work, Martina Franca, June 200
The Masses of the Light Quarks
Talk given at the Conference on Fundamental Interactions of Elementary
Particles, ITEP, Moscow, Oct. 1995. The paper reviews the current status of
knowledge concerning m_u, m_d and m_s. Qualitative aspects of the resulting
picture for the breaking of isospin and eightfold way symmetries are discussed.
At a more quantitative level, the review focuses on the chiral perturbation
theory results for the masses of the Goldstone bosons. The corresponding bounds
and estimates for the ratios m_u/m_d and m_s/m_d are described in some detail.Comment: 23 pages, 3 figure
Light Quark Effective Theory
Lecture Notes, Summer School on Effective Theories and Fundamental
Interactions, Erice, 1996. The application of effective field theory methods to
the low energy structure of QCD is discussed. I emphasize the universal
structure of the corresponding effective Lagrangian and then discuss the
physics of the effective coupling constants. The implications of the effective
theory for the masses of the light quarks are analyzed in some detail.Comment: 30 pages, 3 postscript figure
pi-pi scattering: theory is ahead of experiment
I draw attention to a recent breakthrough in the field of low energy pion
physics: the consequences of the hidden symmetry of the QCD Hamiltonian have
successfully been incorporated in the general dispersive framework for the
pi-pi scattering amplitude, which is due to Roy. The meagre experimental
information about the imaginary parts at and above 0.8 GeV suffices to
unambiguously and accurately pin down the scattering amplitude at lower
energies. The recent Brookhaven data on the reaction K -> pi pi e nu provide a
significant test of the theory. They imply that the Gell-Mann-Oakes-Renner
relation is approximately valid -- the bulk of the pion mass indeed originates
in the quark condensate.Comment: Talk given in honour of Arnulfo Zepeda at the X Mexican School of
Particles and Fields, Playa del Carmen, Mexico, 2002, 8 p
On the dispersion theory of scattering
Recent developments in low energy pion physics are reviewed, emphasizing the
strength of dispersion theory in this context. As an illustration of the
method, I discuss some consequences of the forward dispersion relation obeyed
by the isoscalar component of the scattering amplitude.Comment: Contribution to the proceedings of the workshop "Quark Confinement
and the Hadron Spectrum VII", Ponta Delgada, Azores Islands, Portugal,
September 200
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