Observation of $B_c$ mesons in $p\overline{p}$ collisions at $\sqrt{s}=1.8\mathrm{}\mathrm{TeV}$

We report the observation of bottom-charmed mesons $B_c$ in 1.8 TeV $p\overline{p}$ collisions using the CDF detector at the Fermilab Tevatron. The $B_c$ mesons were found through their semileptonic decays, $B_c^{\pm }\to J/\psi l^{\pm }X.\mathrm{}$ A fit to the $J/\psi l$ mass distribution yielded ${20.4}_{-5.5}^{+6.2\mathrm{}}$ events from $B_c$ mesons. A test of the null hypothesis, i.e., an attempt to fit the data with background alone, was rejected at the level of 4.8 standard deviations. By studying the quality of the fit as a function of the assumed $B_c$ mass, we determined ${M(B}_c)=6.40\mathrm{}\pm 0.39\mathrm{}(\mathrm{stat}.\mathrm{})\mathrm{}\pm 0.13\mathrm{}\mathrm{}(\mathrm{syst})\mathrm{}\mathrm{}\mathrm{GeV}{/c}^2.$ From the distribution of trilepton intersection points in the plane transverse to the beam direction we measured the $B_c$ lifetime to be $\tau {(B}_c{)=0.46\mathrm{}}_{-0.16}^{+0.18\mathrm{}}\mathrm{}\left(\mathrm{stat}\right)\mathrm{}\pm 0.03\mathrm{}\mathrm{}\left(\mathrm{syst}\right)\mathrm{}\mathrm{}\mathrm{ps}.$ We also measured the ratio of production cross section times branching fraction for $B_c^{+}\to J/\psi l^{+}\nu$ relative to that for $B^{+}\to J/\psi K^{+}$ to be $\frac{\sigma {(B}_c)\times {B(B}_c\to J/\psi l\nu )}{\sigma \mathrm{}\left(B\right)\mathrm{}\times B(\overrightarrow{B}J/\psi K)}{=0.132\mathrm{}}_{-0.037}^{+0.041\mathrm{}}{\mathrm{}\left(\mathrm{stat}\right)\mathrm{}\pm 0.031\mathrm{}\mathrm{}\left(\mathrm{syst}\right)\mathrm{}}_{\mathrm{-}0.020}^{+0.032\mathrm{}}\mathrm{}\left(\mathrm{lifetime}\right)\mathrm{}.$