Heinrich Leutwyler

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Low energy precision experiments provide significant tests of the laws of nature that can reveal physics beyond the Standard Model. A good theoretical understanding of the low energy properties of QCD is required for this purpose. The recent developments at the interface between lattice and effective field theory methods provide an excellent basis for pion(More)
The available experimental information is consistent with the Weinberg predictions for the threshold parameters of ππ-scattering. The data, however, only test those relations that are insensitive to the quark masses m u and m d. Recent theoretical progress leads to remarkably sharp predictions for the behaviour of the Sand P-wave phase shifts in the elastic(More)
It is pointed out that there are several advantages in abstracting properties of hadrons and their currents from a Yang-Mills gauge model based on colored quarks and color octet gluons. In the discussion of hadrons, and especially of their electromagnetic and weak currents, a great deal of use has been made of a Lagrangian field theory model in which quark(More)
The talk 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(More)
In the large N c limit, the variables required to analyze the low energy structure of QCD in the framework of an effective field theory necessarily include the degrees of freedom of the η ′. We evaluate the decay constants of the pseu-doscalar nonet to one loop within this extended framework and show that, as a consequence of the anomalous dimension of the(More)
The remarkable properties of certain tops are due to a conservation law which originates in the geometry of the frictional forces. It is shown that spherical tops form three distinct families which perform qualitatively different motions and which are characterized by inequalities involving the moments of interia In contrast to many serious scientific(More)
The scalar radius of the pion plays an important role in χPT, because it is related to one of the basic effective coupling constants, viz. the one which controls the quark mass dependence of F π at one loop. In a recent paper, Ynduráin derives a robust lower bound for this radius, which disagrees with earlier determinations. We show that such a bound does(More)