Measurement of the mass difference<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msubsup><mml:mrow><mml:mi>m</mml:mi><mml:mo>(</mml:mo><mml:mi>D</mml:mi></mml:mrow><mml:mrow><mml:mi>s</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msubsup></mml:mrow><mml:mo>)</mml:mo><mml:mi>−</mml:mi><mml:mrow><mml:msup><mml:mrow><mml:mi>m</mml:mi><mml:mo>(</mml:mo><mml:mi>D</mml:mi></mml:mrow><mml:mrow><mml:mo>+</mml:mo></mml:mrow></mml:msup></mml:mrow><mml:mo>)</mml:mo></mml:math>at CDF II

We present a measurement of the mass difference ${m(D}_{s}^{+})\ensuremath{-}{m(D}^{+}),$ where both the ${D}_{s}^{+}$ and ${D}^{+}$ are reconstructed in the $\ensuremath{\varphi}{\ensuremath{\pi}}^{+}$ decay channel. This measurement uses 11.6 ${\mathrm{pb}}^{\mathrm{\ensuremath{-}}1}$ of data collected by CDF II using the new displaced-track trigger. The mass difference is found to be ${m(D}_{s}^{+})\ensuremath{-}{m(D}^{+})=99.41\ifmmode\pm\else\textpm\fi{}0.38(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.21(\mathrm{syst})\mathrm{MeV}{/c}^{2}.$