Measurements of semileptonicKLdecay form factors

We present new measurements of $K_L$ semileptonic form factors using data collected in 1997 by the KTeV (E832) experiment at Fermilab. The measurements are based on $1.9\times {10}^6$ $K_L\to {\pi }^{\pm }{e}^{\mp }\nu$ and $1.5\times {10}^6$ $K_L\to {\pi }^{\pm }{\mu }^{\mp }\nu$ decays. For $f_{+}(t)$, we measure both a linear and quadratic term: ${\lambda }_{+}^{\prime }=(20.64\pm 1.75)\times {10}^{-3}$ and ${\lambda }_{+}^{\prime \prime }=(3.20\pm 0.69)\times {10}^{-3}$. For $f_0(t)$, we find ${\lambda }_0=(13.72\pm 1.31)\times {10}^{-3}$. These form factors are consistent with $K^{\pm }$ form factors, suggesting that isospin symmetry breaking effects are small. We use our measured values of the form factors to evaluate the decay phase space integrals, $I_K^e=0.15350\pm 0.00105$ and $I_K^{\mu }=0.10165\pm 0.00080$, where errors include uncertainties arising from the form factor parametrizations.