Measurement of Bottom-Quark Hadron Masses in Exclusive $J/\psi$ Decays with the CDF Detector

We measure the masses of $b$ hadrons in exclusively reconstructed final states containing a $J/\psi \to {\mu }^{-}{\mu }^{+}$ decay using $220{\mathrm{pb}}^{-1}$ of data collected by the CDF II experiment. We find: $m(B^{+})=5279.10\pm {0.41}_{(\mathrm{stat}.)}\pm {0.36}_{(\mathrm{sys}.)}\mathrm{MeV}/c^2$, $m(B^0)=5279.63\pm {0.53}_{(\mathrm{stat}.)}\pm {0.33}_{(\mathrm{sys}.)}\mathrm{MeV}/c^2$, $m(B_s^0)=5366.01\pm {0.73}_{(\mathrm{stat}.)}\pm {0.33}_{(\mathrm{sys}.)}\mathrm{MeV}/c^2$, $m({\Lambda }_b^0)=5619.7\pm {1.2}_{(\mathrm{stat}.)}\pm {1.2}_{(\mathrm{sys}.)}\mathrm{MeV}/c^2$. $m(B^{+})-m(B^0)=-0.53\pm {0.67}_{(\mathrm{stat}.)}\pm {0.14}_{(\mathrm{sys}.)}\mathrm{MeV}/c^2$, $m(B_s^0)-m(B^0)=86.38\pm {0.90}_{(\mathrm{stat}.)}\pm {0.06}_{(\mathrm{sys}.)}\mathrm{MeV}/c^2$, $m({\Lambda }_b^0)-m(B^0)=339.2\pm {1.4}_{(\mathrm{stat}.)}\pm {0.1}_{(\mathrm{sys}.)}\mathrm{MeV}/c^2$. The measurements of the $B_s^0$, ${\Lambda }_b^0$ mass, $m(B_s^0)-m(B^0)$ and $m({\Lambda }_b^0)-m(B^0)$ mass difference are of better precision than the current world averages.

Figure 1

The reconstructed mass of dimuons from $J/\psi$ decays, as a function of transverse momentum. The three sets represent various stages of corrections: the solid circles indicate no correction, the triangles add the material tuning and the squares show all corrections including the magnetic field scale.

Figure 2

The invariant mass distribution for $J/\psi K^{+}$, $J/\psi K^{0*}$, $J/\psi \varphi$, and $J/\psi {\Lambda }^0$ candidates. The results of the log-likelihood fits are superimposed. The fit probability obtained from a ${\chi }^2$ test is shown. The left shoulder in Fig. 2 originates from partially reconstructed decays, as explained in the text.