Observation of the B_c meson in pp¯ collisions at √s = 1.8 TeV

We have observed bottom-charm mesons via the decay mode B^± _c → J/ψl^±ν in 1.8 TeV pp¯ collisions using the CDF detector at the Fermilab Tevatron. A fit of background and signal contributions to the J/ψl mass distribution yielded 20.4^+6.2 _-5.5 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 ± 0.39(stat) ± 0.13(syst) GeV/c² and the B⁺ _c lifetime to be 0.46^+0.18 _-0.16(stat) ± 0.03(syst)ps. Our measured yield (production cross section times branching ratio) for B^± _c → J/ψl⁺ν relative to that for B⁺ → J/ψK⁺ is 0.132^+0.041 _-0.037(stat) ± 0.031(syst)^+0.032 _-0.020(lifetime).