Extracting short-distance physics from KL, S→ π0e+e− decays

Abstract

We present a new analysis of the rare decay K L → π 0 e + e − taking into account important experimental progress that has recently been achieved in measuring K L → π 0 γγ and K S → π 0 e + e −. This includes a brief review of the direct CP-violating component, a calculation of the indirect CP-violating contribution, which is now possible after the measurement of K S → π 0 e + e −, and a reanalysis of the CP-conserving part. The latter is shown to be negligible, based on experimental input from K L → π 0 γγ, a more general treatment of the form factor entering the dispersive contribution, and on a comparison with the CP-violating rate, which can now be estimated reliably. We predict B( K L → π 0 e + e −)=(3.2 +1.2 −0.8)×10 −11 in the Standard Model, dominated by CP violation with a sizable contribution (∼40%) from the direct effect, largely through interference with the indirect one. Methods to deal with the severe backgrounds for K L → π 0 e + e − using Dalitz-plot analysis and time-dependent K L – K S interference are also briefly discussed.