We present a measurement of the mass difference m(Ds +) -m(D+), where both the Ds+ and D+ are reconstructed in the φπ+ decay channel. This measurement uses 11.6 pb-1 of data collected by CDF II using the new displaced-track trigger. The mass difference is found to be m(D s+)-m(D+) = 99.41±0.38(stat) ±0.21(syst) MeV/c2. copy; 2003 The American Physical Society.
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