Neutral-Kaon Mixing from (2+1)-Flavor Domain-Wall QCD

We present the first results for neutral-kaon mixing using ($2+1$)-flavors of domain-wall fermions. A new approach is used to extrapolate to the physical up and down quark masses from our numerical studies with pion masses in the range 240–420 MeV; only $SU(2{)}_L\times SU(2{)}_R$ chiral symmetry is assumed and the kaon is not assumed to be light. Our main result is $B_K^{\overline{\mathrm{MS}}}(2\mathrm{GeV})=0.524(10)(28)$ where the first error is statistical and the second incorporates estimates for all systematic errors.