Observation of theBcMeson inpp¯Collisions at√s=1.8TeV

We have observed bottom-charm mesons via the decay mode $B_c^{\pm }\to J/\psi {\ell }^{\pm }\nu$ in 1.8 TeV $p\overline{p}$ collisions using the CDF detector at the Fermilab Tevatron. A fit of background and signal contributions to the $J/\psi \ell$ mass distribution yielded ${20.4}_{-5.5\mathrm{}}^{+6.2\mathrm{}}$ events from $B_c$ mesons. A fit to the same distribution with background alone was rejected at the level of 4.8 standard deviations. We measured the $B_c^{+}$ mass to be $6.40\pm 0.39\left(\mathrm{stat}\right)\pm 0.13\left(\mathrm{syst}\right)\mathrm{GeV}{/c}^2$ and the $B_c^{+}$ lifetime to be ${0.46}_{-0.16\mathrm{}}^{+0.18\mathrm{}}\left(\mathrm{stat}\right)\pm 0.03\left(\mathrm{syst}\right)\mathrm{ps}$. Our measured yield (production cross section times branching ratio) for $B_c^{+}\to J/\psi {\ell }^{+}\nu$ relative to that for $B^{+}\to J/\psi K^{+}$ is ${0.132}_{-0.037\mathrm{}}^{+0.041\mathrm{}}\left(\mathrm{stat}\right)\pm 0.031\left(\mathrm{syst}{)}_{-0.020\mathrm{}}^{+0.032\mathrm{}}\right(\mathrm{lifetime})$.