A Measurement of the Kl-ks Mass Difference from the Charge Asymmetry in Semi-leptonic Kaon Decays

Introduction. The experiment reported in this paper is part of an extensive study aiming principally at a more precise determination of #+_ , the phase of q+_ , the parameter describing CP-violation in K” + 7~+trdecays. New values of K” + n+ndecay parameters have been reported previously [ 1 ] . The data on semi-leptonic kaon decays recorded simultaneously and in the same apparatus as the pionic decays, are presented here to the extent that they improve the accuracy of the phase $I+_. A measurement of the KL-KS mass difference from leptonic kaon decays is of interest for the following reason: the determination of 4+_ by an observation of the interference term -lq+_l cos(Amr--#+_) in the time distribution of rr+ndecays suffers from the strong correlation between the phase and the mass difference Am = mL ms. On the other hand, the charge asymmetry of semi-leptonic decays exhibits an interference term cos(Amr). In principle it is, therefore, the difference in phase in the interference terms of pionic and leptonic decays which gives a measure of 4+_ without explicit, accurate knowledge of the mass difference Am. In other words, a determination of $+_ using the mass difference obtained from leptonic decays has the advantage of reduced sensitivity to systematic errors in common to the two decay modes. The charge asymmetry in semi-leptonic decay K” + nTI1’ u is defined as: