Using 230 Â 10 6 B " B events recorded with the BABAR detector at the e þ e À storage rings PEP-II, we reconstruct approximately 4100 B 0 ! J= K þ % À and 9930 B þ ! J= K þ decays with J= ! " þ " À and e þ e À . From the measured B-momentum distributions in the e þ e À rest frame, we determine the mass difference mðB 0 Þ À mðB þ Þ ¼ ðþ0:33 AE 0:05 AE 0:03Þ MeV=c 2 .
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