Measurement of Ke3(0) form factors

The semileptonic decay of the neutral K meson, K_L --> pi(+/-) e(-/+) nu (Ke3), was used to study the strangeness-changing weak interaction of hadrons. A sample of 5.6 million reconstructed events recorded by the NA48 experiment was used to measure the Dalitz plot density. Admitting all possible Lorentz-covariant couplings, the form factors for vector (f+(q2)), scalar (fS) and tensor (fT) interactions were measured. The linear slope of the vector form factor λ+=0.0284±0.0007±0.0013 and values for the ratios |fs/f+(0)|=0.015+0.007-0.010+-0.012 and |fT/f+(0)|=0.05+0.03-0.04+-0.03 were obtained. The values for fS and fT are consistent with zero. Assuming only vector–axial-vector couplings, λ+=0.0288±0.0004±0.0011 and a good fit consistent with pure V–A couplings were obtained. Alternatively, a fit to a dipole form factor yields a pole mass of M=859±18 MeV, consistent with the K*(892) mass.