Excited heavy baryons and their symmetries III: Phenomenology

Phenomenological applications of an effective theory of low-lying excited s tates of charm and bottom isoscalar baryons are discussed at leading and ne xt-to-leading order in the combined heavy-quark and large-N-c expansion. Th e combined expansion is formulated in terms of the counting parameter lambd a similar to l/m(Q) . l/N-c; the combined expansion is in powers of lambda (1/2). We work up to next-to-leading order. We obtain model-independent pre dictions for the excitation energies, the semileptonic form factors and ele ctromagnetic decay rates. At leading order in the combined expansion these observables are given in terms of one phenomenological constant which can b e determined from the excitation energy of the first excited state of Lambd a (c) baryon. At next-to-leading order an additional phenomenological const ant is required. The spin-averaged mass of the doublet of the first orbital ly excited state of Lambda (b) is predicted to be approximately 5920 MeV. I t is shown that in the combined limit at leading and next-to-leading order there is only one independent form factor describing Lambda (b) --> Lambda (c)l(v)over bar; similarly, Lambda (b) --> Lambda (c)*l(v)over bar and Lamb da (b) --> Lambda (c) (/) l(v)over bar decays are described by a single ind ependent form factor. These form factors are calculated at leading and next -to-leading order in the combined expansion. The value of the Lambda (b) -- > Lambda (c)l(v)over bar form factor at zero recoil is predicted to be 0.99 8 at leading order which is very close to HQET value of unity. The electrom agnetic decay rates of the first excited states of Lambda (c) and Lambda (b ) are determined at leading and next-to-leading order. The ratio of radiati ve decay rates Gamma(Lambda (c)* --> Lambda (c)gamma)/Gamma(Lambda (b/) --> A(b)gamma) is predicted to be approximately 0.2, greatly different from th e heavy. quark effective theory value of unity. (C) 2001 Elsevier Science B .V. All rights reserved.