these fits overestimate the experimental value while those employing $f$ underestimate it. This uncertainty clearly indicates the importance of $p^6$ terms in the ChPT interpretation of our form factor data at the considered range of pion mass.

Comparison of results using $f$ and $f_\pi$ for the SU(2) case at 411 MeV are made in Fig. \[fig:R2\_ChPT\_NLO\_411\_Qsq010\]. The left panels (a) and (c) are results obtained with $f$ while those using $f_\pi$ are shown in the right panels (b) and (d). In both cases, NLO ChPT fits do not explain the upward curvature of the form factor data at $Q^2=0.08$ and 0.10 GeV$^2$.

We should note that the SU(2) ChPT analysis requires tuning of the strange quark mass $m_s$ to the physical value, or alternatively, the dependence of the SU(2) LEC’s on $m_s$ around its physical value has to be determined from data. For the Wilson-clover quark action, there is an additional subtlety that the strange quark mass, as defined [*via*]{} the PCAC relation, varies with changing up-down quark hopping parameter even if the strange quark hopping parameter is kept fixed. Our data taken for only one value of the strange quark hopping parameter, however, is not detailed enough to fully 