We present a calculation of the standard model \Delta S=2 matrix element
relevant to indirect CP violation in K->\pi\pi decays which uses Neuberger's
chiral formulation of lattice fermions. The computation is performed in the
quenched approximation on a 16^3x32 lattice that has a lattice spacing a\sim
0.1 fm. The resulting bare matrix element is renormalized non-perturbatively.
Our main result is B_K^{RGI}=0.87(8)^{+2+14}_{-1-14}, where the first error is
statistical, the second is systematic and the third is an estimate of the
uncertainty associated with the quenched approximation and with the fact that
our kaons are composed of degenerate s and d quarks with masses \sim m_s/2.Comment: 5 pages (revtex4), 2 figure