Observation of the B{sub c} Meson in the Exclusive Decay B{sub c}{yields}J/{psi}{pi}

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