DOI 10.1007/s100520000303
Nf dependence of the quark condensate from a chiral sum rule
B. Moussallam
I.P.N., Groupe de Physique Théorique,
Université Paris-Sud, 91406 Orsay Cédex, France
Received: 3 September / Published online: 6 April 2000 - © Springer-Verlag 2000
Abstract
How fast does the quark condensate in QCD-like theories
vary as a function of Nfis inferred from real QCD using chiral perturbation theory at
order one loop.
A sum rule is derived for the single relevant chiral
coupling constant, L6. A model independent lower bound is
obtained.
The spectral function satisfies a Weinberg-type
superconvergence relation. It is discussed how this, together
with
chiral constraints, allows a solid evaluation of L6, based
on
experimental 
-
S-wave T-matrix input.
The resulting value of L6 is compatible with a strong
Nf dependence possibly suggestive of the proximity of a chiral
phase transition.
Copyright Società Italiana di Fisica, Springer-Verlag 2000
