Measurement of the branching ratio of pi(0) Dalitz decay using K-L -> pi(0)pi(0)pi(0) decays

We present a measurement of B(pi(0) -> e(+)e(-)gamma)/B(pi(0) -> gamma gamma), the Dalitz branching ratio, using data taken in 1999 by the E832 KTeV experiment at Fermi National Accelerator Laboratory. We use neutral pions from fully reconstructed K-L decays in flight; the measurement is based on similar to 60 thousand K-L -> pi(0)pi(0)pi(0) -> gamma gamma gamma gamma e(+)e(-)gamma decays. We normalize to K-L -> pi(0)pi(0)pi(0) -> 6 gamma decays. We find B(pi(0) -> e(+)e(-)gamma)/B(pi(0) -> gamma gamma) (m(e+e-) > 15 MeV/c(2)) = [3.920 +/- 0.016(stat) +/- 0.036(syst)] x 10(-3). Using the Mikaelian and Smith prediction for the e(+)e(-) mass spectrum, we correct the result to the full e(+)e(-) mass range. The corrected result is B(pi(0) -> e(+)e(-)gamma)/B(pi(0) -> gamma gamma) = [1.1559 +/- 0.0047(stat) +/- 0.0106(syst)]%. This result is consistent with previous measurements, and the uncertainty is a factor of 3 smaller than any previous measurement.