Measurement of the Lepton Charge Asymmetry inKL0→π±l∓νDecays

We have made a measurement of the lepton charge asymmetry in ${K_L}^0$ decays. Magnetic analyses of the decay products in a spectrometer using multiwire proportional counters allowed kinematic reconstruction of the event where the particles traversed only 293 mg of matter. The leptons were differentiated from the pions only through their different transverse-momentum distributions. The asymmetry was measured to be (3.33 ± 0.50) × ${10}^{-3\mathrm{}}$, in accord with the superweak description of $\mathrm{CP}$ nonconservation.