THE UNIVERSITY OF CHICAGO A MEASUREMENT OF THE BRANCHING RATIO OF THE 0 DALITZ DECAY USING KL! 30 DECAYS FROM KTEV A DISSERTATION SUBMITTED TO THE FACULTY OF THE DIVISION OF THE PHYSICAL SCIENCES IN CANDIDACY FOR THE DEGREE OF

We present a measurement of B(0! e+e¡°)=B(0! °°) using data taken in 1999 by the E832 KTeV experiment at Fermilab. The 0s were produced by KL decays in °ight that are fully reconstructed. We ¯nd 63; 693 KL! 30! ° ° ° ° e+e¡° decays in KTeV data (an increase of a factor of » 20 in event statistics over previous experiments), and normalize to KL! 30! 6°, to extract B(0! e+e¡°;me+e ¡> 15MeV=c2)=B(0! °°) = (3:920 § 0:016 § 0:036)£10¡3; where the ¯rst error is statistical and the second is systematic. Using the Mikaelian and Smith prediction for the e+e ¡ mass spectrum as implemented in the KTeV Monte Carlo to correct to the full e+e ¡ mass range yields B(0! e+e¡°)=B(0! °°) = (1:1559 § 0:0046 § 0:0107)%: This result is consistent with previous measurements and with theoretical predictions, and the uncertainty is a factor of three smaller than any previous measurement. iii ACKNOWLEDGMENT