  The pion masses and the pion decay constant are calculated to 1-loop order in
twisted mass Wilson chiral perturbation theory, assuming a large pion mass
splitting and tuning to maximal twist. Taking the large mass splitting at
leading order in the chiral expansion leads to significant modifications in the
chiral logarithms. For example, the result for the charged pion mass contains a
chiral logarithm that involves the neutral pion mass instead of the charged
one. Similar modifications appear in the results for the neutral pion mass and
the decay constant. These new results are used in fits to lattice data obtained
recently by the European twisted mass collaboration. The data can be fitted
well, in general better than with the standard chiral perturbation theory
expressions that ignore the mass splitting. The impact on the extraction of
low-energy couplings is briefly discussed.

High Energy Physics - Lattice