Excited heavy baryons and their symmetries III: Phenomenology

Abstract

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 λ 1/2 . We work up to next-toleading 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 sate of Λ b is predicted to be approximately 5920 M eV . It is shown that in the combined limit at leading and next-to-leading order there is only one independent form factor describing Λ b → Λcℓν; similarly, Λ b → Λ * c ℓν and Λ b → Λc1ℓν 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 Λ b → Λcℓν 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 Λ b are determined at leading and next-to leading order. The ratio of radiative decay rates Γ(Λ * c → Λcγ)/Γ(Λ b1 → Λ b γ) is predicted to be approximately 0.2, greatly different from the heavy quark effective theory value of unity.

On charm baryon resonances and chiral symmetry

“…Recently three J P = 1 2 − states were observed by the CLEO Collaboration [8,9,10]. Open charm baryons have been studied extensively in the literature [11,12,13,14,15,16,17,18,19,20,21]. For review articles on the heavy-quark effective theory approach we refer to [12,13].…”

Excited baryons in large QCD

“…For excited baryons this symmetry is broken. However, based on the observation that excited states can be approximately classified as SU(2N f ) multiplets, considerable efforts have previously been made to analyze the excited states belonging to the [70, 1 − ] multiplet (N = 1 band) in the large N c limit [ 10,11,12,13,14,15,16,17,18,19,20], with obvious success.…”

“…coupled to a ground state core [ 10,11,12,13,14,15,16,17,18,19,20]. The second term will be treated here as an excited quark coupled to an excited core 5 .…”

Negative parity 70-plet baryon masses in the1/Ncexpansion

“…In the resonance domain, besides the light quark masses, one such expansion parameter is provided by 1/N c . The expansion in 1/N c for excited baryons has been proposed and applied in several works [7,8,9,10,11,12,13] with encouraging success.…”