Observation of the $B_c$ Meson in the Exclusive Decay $B_c\to J/\psi \pi$

A fully reconstructed $B_c\to J/\psi \pi$ signal is observed with the D0 detector at the Fermilab Tevatron $p\overline{p}$ collider using $1.3{\mathrm{fb}}^{-1}$ of integrated luminosity. The signal consists of $54\pm 12$ candidates with a significance that exceeds 5 standard deviations, and confirms earlier observations of this decay. The measured mass of the $B_c$ meson is $6300\pm 14(\mathrm{stat})\pm 5(\mathrm{syst})\mathrm{MeV}/c^2$.

Figure 1

$J/\psi \pi$ invariant mass distribution of $B_c$ candidates after the final selection. A projection of the unbinned maximum likelihood fit to the distribution is shown overlaid.