Precise Determination of theKL−KSMass Difference by the Gap Method (University of Chicago-University of Illinois Chicago Circle Collaboration)

The $K_L-K_S$ mass difference, $\Delta m\equiv \frac{(M_L-M_S)c^2}{\hslash }$, has been redetermined in a high-statistics experiment performed at the zero-gradient synchrotron of Argonne National Laboratory, using the "gap method" originally developed at Princeton. Two independent measurements, using ${\mathrm{B}}_4$C and Cu, respectively, as regenerators, were performed and gave consistent results. Their mean is $\Delta m=(0.542\mathrm{}\pm 0.006)\times {10}^{10}$ ${\sec }^{-1\mathrm{}}$. With this value, the result of our recent determination of arg ${\eta }_{+-}$ by "vacuum regeneration" is recalculated to be (44.7 ± 5.0)°, where the error allows for the uncertainty in $\Delta m$ given above.