Measurement of the mass difference ${m(D}_s^{+}\left)-{m(D}^{+}\right)$ at CDF II

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