  We present the first result for the hyperon vector form factor f_1 for Xi^0
-> Sigma^+ l bar{nu} and Sigma^- -> n l bar{nu} semileptonic decays from fully
dynamical lattice QCD. The calculations are carried out with gauge
configurations generated by the RBC and UKQCD collaborations with (2+1)-flavors
of dynamical domain-wall fermions and the Iwasaki gauge action at beta=2.13,
corresponding to a cutoff 1/a=1.73 GeV. Our results, which are calculated at
the lighter three sea quark masses (the lightest pion mass down to
approximately 330 MeV), show that a sign of the second-order correction of
SU(3) breaking on the hyperon vector coupling f_1(0) is negative. The tendency
of the SU(3) breaking correction observed in this work disagrees with
predictions of both the latest baryon chiral perturbation theory result and
large N_c analysis.
