Excited heavy baryons and their symmetries III: Phenomenology

Phenomenological applications of an effective theory of low-lying excited states of charm and bottom isoscalar baryons are discussed at leading and next-to-leading order in the combined heavy-quark and large-Nc expansion. The combined expansion is formulated in terms of the counting parameter λ∼1/mQ,1/Nc; the combined expansion is in powers of λ1/2. We work up to next-to-leading order. We obtain model-independent predictions for the excitation energies, the semileptonic form factors and electromagnetic decay rates. At leading order in the combined expansion these observables are given in terms of one phenomenological constant which can be determined from the excitation energy of the first excited state of Λc baryon. At next-to-leading order an additional phenomenological constant is required. The spin-averaged mass of the doublet of the first orbitally excited state of Λb is predicted to be approximately 5920 MeV. It is shown that in the combined limit at leading and next-to-leading order there is only one independent form factor describing Λb→Λclνbar; similarly, Λb→Λc*lνbar and Λb→Λc1lνbar decays are described by a single independent form factor. These form factors are calculated at leading and next-to-leading order in the combined expansion. The value of the Λb→Λclνbar form factor at zero recoil is predicted to be 0.998 at leading order which is very close to HQET value of unity. The electromagnetic decay rates of the first excited states of Λc and Λb are determined at leading and next-to-leading order. The ratio of radiative decay rates Γ(Λc*→Λcγ)/Γ(Λb1→Λbγ) is predicted to be approximately 0.2, greatly different from the heavy-quark effective theory value of unity