On the size of the chiral condensate, generalized chiral perturbation theory and the DIRAC experiment

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In the near future, the DIRAC collaboration will measure pi pi scattering lengths with great precision. Those measurements are likely to shed some light on the problem of the size of the chiral [0\(q) over bar q\0] condensate. Although it is usually assumed to be as large as similar to (-225 MeV)(3), in the last years a more general approach has been developed to accommodate either a large or a small alternative similar to (-100 MeV)(3). Such a low value would also modify the standard temperature estimate at which the chiral phase transition occurs. In this work we briefly review the basic theoretical ideas related to this issue as well as the experiment that could help to establish any of the two scenarios.
© Acta Physica Hungarica. A.D. would like to thank Julia Nyiri for giving him the opportunity to know better, in his very last days, the great man that Gribov was and, at the same time, to meet a great woman. J.R.P. thanks B. Adeva for suggesting him the preparation of the lectures that lead to part of this review and for explaining him in detail the DIRAC experiment, as well as J. Prades for his explanations of sum rules and quark masses. He would also like to thank the Theory Group at SLAC for their kind hospitality and the Spanish Ministerio de Educaci´on y Cultura for a Fellowship. This work has been partially supported by the Spanish CICYT under contract AEN93-0776 and by the U.S. Department of Energy under contract DE-AC03-76SF00515.
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