Evidence for the Exclusive Decay $B_c^{\pm }\to J/\psi {\pi }^{\pm }$ and Measurement of the Mass of the $B_c^{\pm }$ Meson

We report the first evidence for a fully reconstructed decay mode of the $B_c^{\pm }$ meson in the channel $B_c^{\pm }\to J/\psi {\pi }^{\pm }$, with $J/\psi \to {\mu }^{+}{\mu }^{-}$. The analysis is based on an integrated luminosity of $360{\mathrm{pb}}^{-1}$ in $p\overline{p}$ collisions at 1.96 TeV center of mass energy collected by the Collider Detector at Fermilab. We observe $14.6\pm 4.6$ signal events with a background of $7.1\pm 0.9$ events, and a fit to the $J/\psi {\pi }^{\pm }$ mass spectrum yields a $B_c^{\pm }$ mass of $6285.7\pm 5.3(\mathrm{stat})\pm 1.2(\mathrm{syst})\mathrm{MeV}/c^2$. The probability of a peak of this magnitude occurring by random fluctuation in the search region is estimated as 0.012%.

Figure 1

The invariant mass distribution of the $B^{\pm }\to J/\psi K^{\pm }$ candidates. The curve is a fit to the data.

Figure 2

The invariant mass distribution of the $J/\psi {\pi }^{\pm }$ candidates and results of an unbinned likelihood fit in the search window. The inset shows the peak section of the distribution. The broad enhancement below $6.2\mathrm{GeV}/c^2$ is attributable to partially reconstructed $B_c^{\pm }$ mesons.

Figure 3

Impact parameter of the third track relative to the $J/\psi$ vertex for the lower sideband region, after subtraction of the same distribution for the upper side band: (top panel) the curve is the sum of two Gaussians, fitted to the $B^{\pm }$ data points; (bottom panel) the $B_c^{\pm }$ data points, overlaid with the above curve, rescaled. In both cases the selection criteria were relaxed.