ππ 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 ππ 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 → ππeν 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. Excerpts from a talk given in honour of Arnulfo Zepeda at the X Mexican School of Particles and Fields, Playa del Carmen, México, 2002.