Measurement of Kμ 30 form factors

This Letter reports on a new high precision measurement of the form factors of the KL → π ± μ∓ νμ decay. The data sample of about 2.3 × 10^6 events was recorded in 1999 by the NA48 experiment at CERN. Studying the Dalitz plot density we measured a linear, λ+ = (20.5 ± 2.2stat ± 2.4syst ) × 10−3 , and a quadratic, λ+ = (2.6 ± 0.9stat ± 1.0syst ) × 10−3 term in the power expansion of the vector form factor. No evidence was found for a second order term for the scalar form factor; the linear slope was determined to be λ0 = (9.5 ± 1.1stat ± 0.8syst ) × 10−3 . Using a linear fit our results were: λ+ = (26.7 ± 0.6stat ± 0.8syst ) × 10−3 and λ0 = (11.7 ± 0.7stat ± 1.0syst ) × 10−3 . A pole fit of the form factors yields: mV = (905 ± 9stat ± 17syst ) MeV/c2 and mS = (1400 ± 46stat ± 53syst ) MeV/c2 .