Measurements ofKLbranching fractions and theCPviolation parameter|η+−|

We present new measurements of the six largest branching fractions of the $K_L$ using data collected in 1997 by the KTeV experiment (E832) at Fermilab. The results are $B(K_L\to {\pi }^{\pm }{\mathrm{e}}^{\mp }\nu )=0.4067\pm 0.0011$, $B(K_L\to {\pi }^{\pm }{\mu }^{\mp }\nu )=0.2701\pm 0.0009$, $B(K_L\to {\pi }^{+}{\pi }^{-}{\pi }^0)=0.1252\pm 0.0007$, $B(K_L\to {\pi }^0{\pi }^0{\pi }^0)=0.1945\pm 0.0018$, $B(K_L\to {\pi }^{+}{\pi }^{-})=(1.975\pm 0.012)\times {10}^{-3}$, and $B(K_L\to {\pi }^0{\pi }^0)=(0.865\pm 0.010)\times {10}^{-3}$, where statistical and systematic errors have been summed in quadrature. We also determine the $CP$ violation parameter $|{\eta }_{+-}|$ to be $(2.228\pm 0.010)\times {10}^{-3}$. Several of these results are not in good agreement with averages of previous measurements.