First Direct Measurement of the Parity-Violating Coupling of the ${\mathit{Z}}^0$ to the $\mathit{s}$ Quark

We present the first direct measurement of $A_s$, the parity-violating coupling of the $Z^0$ boson to the strange quark, using $\sim 550000$ $e^{+}{e}^{-}\to Z^0\to \mathrm{hadrons}$ events recorded by the SLC Large Detector with a polarized $e^{-}$ beam. We tagged $Z^0\to s\overline{s}$ events by the absence of $B$ or $D$ hadrons and the presence in each hemisphere of a high momentum $K^{\pm }$ or $K_s^0$. Fitting the polar angle distributions of the strangeness-signed thrust axis gave $A_s=0.895\mathrm{}\pm 0.066\left(\mathrm{stat}\right)\pm 0.062\left(\mathrm{syst}\right)$. The analyzing power and $u\overline{u}+d\overline{d}$ background were constrained using the data, greatly reducing any model dependence.