The quark condensate <0|qbar q|0> represents the most natural order parameter of chiral symmetry breaking, whence the push, in order to measure it. This parameter characterizes the realization of QCD at low energy, which is presently not yet strictly determined, providing a handle to understand the way chiral symmetry is broken.

The quark condensate can be determined through a measurement of pion pion scattering lengths. There are different ways to access experimentally the pion pion -> pion pion scattering amplitude near threshold:

1) the decay kaon -> pion pion electron neutrino, i.e. the so-called Ke4 decay;

2) the pionic atom (pionium);

3) the pion production reaction in nuclear matter pion nucleus -> pion pion nucleus'.
 
 

Theoretical and experimental results and prospects, along the three lines above, will be discussed, with talks by members of EURODAPHNE Collaboration (theory), E865 Collaboration at BNL, DIRAC Collaboration at CERN, Chaos Collaboration at TRIUMF.