Measurement of the decay rate and form factor parameter alpha(K)* in the decay KL -> e+ e- gamma

The decay rate of the neutral K meson K_L --> e+ e- gamma has been measured with the NA48 detector at the CERN SPS. A total of 6864 events has been observed with an estimated background of 10 events. The branching ratio is Gamma(K_L --> e+ e- gamma)/ Gamma(K_L --> all) = (1.06 +- 0.02 stat +- 0.02 sys +- 0.04 calc) * 10^-5. The parameter alphaK* describing the relative strength of the two contributing amplitudes to this decay through intermediate pseudoscalar or vector mesons was measured to be alphaK* = (-0.36 +- 0.06 stat +- 0.02 sys)