### Speaker

Guillaume Toucas
(Laboratoire de Physique Théorique d'Orsay)

### Description

Due to the intermediate position of the strange quark in the hierarchy of quark masses, a numerical competition may arise in chiral series between leading (LO) and next-to-leading order (NLO) in three-flavour Chiral Perturbation Theory (ChPT), as it was suggested by several studies performing a chiral extrapolation of Nf=2+1 lattice data. A way to study this question is to use a modified version of ChPT called Resummed Chiral Perturbation Theory, which does not assume the smallness of NLO terms compared to LO in the series expansion.
Working within this framework, we fit recent lattice results on masses of pseudoscalar mesons, decay constants and Kl3 form factors from several simulations using 2+1 dynamical quarks. We extract numerical values for the order parameters of chiral symmetry breaking, along with other quantities like the quark masses and decay constants ratio. We also deduce from those values a numerical estimate of some NLO low energy constants as well as other quantities of phenomenological interest like f+(0). We observe the previously discussed numerical competition between LO and NLO thus favoring the use of the resummed theory in three flavour calculations.
We then investigate within the same framework the topological susceptibility of QCD, another quantity of interest since the size of its LO term is directly linked to the size of the quark condensate in the chiral limit. Different sets of Nf=2+1 lattice data are used to perform the fits.

### Primary author

Guillaume Toucas
(Laboratoire de Physique Théorique d'Orsay)