The ${\Lambda }_b(udb)$ baryon is observed in the decay ${\Lambda }_b\to J/\psi \Lambda$ using $6.1{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions collected with the D0 detector at $\sqrt{s}=1.96\mathrm{TeV}$. The production fraction multiplied by the branching fraction for this decay relative to that for the decay $B^0\to J/\psi K_s^0$ is measured to be $0.345\pm 0.034(\mathrm{stat})\pm 0.033(\mathrm{syst})\pm 0.003(\mathrm{PDG})$. Using the world average value of $f(b\to B^0)·\mathcal{B}(B^0\to J/\psi K_s^0)=(1.74\pm 0.08)\times {10}^{-5}$, we obtain $f(b\to {\Lambda }_b)·\mathcal{B}({\Lambda }_b\to J/\psi \Lambda )=(6.01\pm 0.60(\mathrm{stat})\pm 0.58(\mathrm{syst})\pm 0.28(\mathrm{PDG}))\times {10}^{-5}$. This measurement represents an improvement in precision by about a factor of 3 with respect to the current world average.

• Received 5 May 2011

DOI:https://doi.org/10.1103/PhysRevD.84.031102