In view of recent experimental trends we investigate the weak nonleptonic decays of charm baryons within the framework of the constituent quark model. Branching ratios and asymmetry parameters for all $++i++O-charm-changing modes are calculated with appropriate QCD corrections. The effect of flavor dependence on the scale is found to be quite significant.

Motivated by the observation of the decay B → K * γ by the CLEO collaboration, we have systematically analyzed the weak radiative decay Λ b → Λ γ, evaluating the confined effects of quarks in the covariant oscillator quark model. This decay process receives both short distance (electromagnetic penguins at the one loop level) and long distance contributions in the quark level. The long distance contributions are analyzed using the vector meson dominance (VMD) method. The estimated branching ratio is found to be 0.23 × 10 −5 . §1. IntroductionWeak radiative (flavor changing neutral current) decays of hyperons have attracted the interest of physicists during the last three decades. In the Standard Model (SM) these processes are forbidden at the tree level and are strongly suppressed by the GIM mechanism. Hence they offer a unique possibility to test the CKM sector of the SM and possibly open a door to physics beyond it. Experimental data are now available for the light baryon sector, i.e., Σ + → p γ, Λ → n γ, Ξ − → Σ − γ, Ξ 0 → Σ 0 γ and Ξ 0 → Λ γ, 1) which involve transitions of the type s → dγ. Recently the rare decay B → K * γ has been observed by the CLEO collaboration, 2) which is dominated by the quark level process b → sγ. Therefore, one may expect that there is also some possibility for the rare decay of heavy baryons. For the heavy hadron decay processes of this type, it is considered that the confined effects of quarks play generally an important role, since there is a large difference between the initial and final hadron masses. In a preceding paper 3) we investigated the rare B → K * γ process, evaluating the confined effects in the framework of the covariant oscillator quark model (COQM). 4) In this paper we study the weak radiative decay Λ b → Λ γ, which results from the interplay of electroweak and gluonic interactions. At the quark level, there are two essential mechanisms responsible for the weak radiative decays (b → sγ): the short distance (SD) electroweak penguin and the long distance (LD) contributions. Recently, an investigation 5) of long distance contributions to these decays was made using vector meson dominance (VMD) at the quark level as b → s[ψ] followed by the conversion [ψ] → γ. We estimate the contributions to the decay Λ b → Λ γ arising

Using a theoretical framework based on the next-to-leading order QCD improved effective Hamiltonian, we have estimated the branching ratios and asymmetry parameters for the two body charmless nonlep- The baryonic form factors at maximum momentum transfer (q 2 m ) are evaluated using the nonrelativistic quark model and the extrapolation of the form factors from q 2 m to the required q 2 value is done by assuming the pole dominance. The obtained branching ratios for Λ b → pπ, pK processes lie within the present experimental upper limit .

Cabibbo allowed two-body hadronic weak decays of Λ b baryons are analyzed in the factorization approximation. We use the covariant oscillator quark model to evaluate the heavy → heavy and heavy → light form factors. When applied in the heavy quark limit, our form factors satisfy all the constraints imposed by heavy quark symmetry. The decay rates and up-down asymmetries for Λ b baryon decaying into Λ c + P (V ) are calculated. It is found that the up-down asymmetry is negative in all these decay modes. Furthermore, the prediction Br(Λ b → Λ + J/ψ) = 2.49 × 10 −4 is consistent with the recent experimental data. Finally it is pointed out that the CKM-Wolfenstein parameter ρ 2 + η 2 , where η is the CP phase, can be determined from the ratio of the widths Λ b → ΛD 0 and Λ b → ΛJ/ψ, independent of the QCD parameter. The value of (ρ 2 + η 2 ) 1/2 calculated in our model agrees very well with the value recently predicted by Rosner. by guest on March 23, 2015 http://ptp.oxfordjournals.org/ Downloaded from

No abstract