  Since the strange quark has a light mass of order Lambda_QCD, fluctuations of
sea s-s bar pairs may play a special role in the low-energy dynamics of QCD by
inducing significantly different patterns of chiral symmetry breaking in the
chiral limits N_f=2 (m_u=m_d=0, m_s physical) and N_f=3 (m_u=m_d=m_s=0). This
effect of vacuum fluctuations of s-s bar pairs is related to the violation of
the Zweig rule in the scalar sector, described through the two O(p^4)
low-energy constants L_4 and L_6 of the three-flavour strong chiral lagrangian.
In the case of significant vacuum fluctuations, three-flavour chiral expansions
might exhibit a numerical competition between leading- and
next-to-leading-order terms according to the chiral counting, and chiral
extrapolations should be handled with a special care. We investigate the impact
of the fluctuations of s-s bar pairs on chiral extrapolations in the case of
lattice simulations with three dynamical flavours in the isospin limit.
Information on the size of the vacuum fluctuations can be obtained from the
dependence of the masses and decay constants of pions and kaons on the light
quark masses. Even in the case of large fluctuations, corrections due to the
finite size of spatial dimensions can be kept under control for large enough
boxes (L around 2.5 fm).
