Implications of Isospin Conservation in Λ_b Decays and Lifetime

We consider isospin predictions for the semi-leptonic and non-leptonic decays of the $\Lambda_b$ baryon. Isospin conservation of the strong interactions constrains the possible final states in $\Lambda_b$ decays. Since the baryon hyperfine splittings depend upon light quark dynamics and do not decrease with increasing heavy quark mass, this leads in general to to phase space enhancements in $\Lambda_b$ decays relative to $B$ meson decays for the same underlying quark transitions. Consequently the $\Lambda_b$ lifetime is smaller than the $B$ lifetime. Phase space enhancements in $\Lambda_b$ decays relative to $B$ decays can be understood in terms of hyperfine interactions in the heavy quark system.