Two-body charmless nonleptonic decays of B u and B d mesons are studied within the framework of generalized factorization in which the effective Wilson coefficients c eff i are renormalization-scale and -scheme independent while factorization is applied to the tree-level hadronic matrix elements. Contrary to previous studies, our c eff i do not suffer from gauge and infrared problems. Nonfactorizable effects are parametrized in terms of N eff c (LL) and N eff c (LR), the effective numbers of colors arising from (V − A)(V − A) and (V − A)(V + A) four-quark operators, respectively. Tree and penguin transitions are classified into six different classes. The data of B − → ρ 0 π − andThe first measurement of the b → u mode B − → ρ 0 π − and the experimental information on the tree-dominated mode B − → ωπ − all imply that N eff c (LL) is less than 3, whereas the CLEO measurement of B − → φK − shows N eff c (LR) > 3. For given input parameters, the prediction of B(B → η ′ K) is largely improved by setting N eff c (LL) ∼ 2 and N eff c (LR) > N eff c (LL); in particular, the charm content of the η ′ contributes in the right direction. The decay rate of B → φK * is very sensitive to the form-factor ratio A 2 /A 1 ; the absence of B → φK events does not necessarily invalidate the factorization approach. If the branching ratio of B − → ωK − is experimentally found to be significantly larger than that of B − → ρ 0 K − , we argue that inelastic final-state rescattering may account for the disparity between ωK − and ρ 0 K − . By contrast, if B(B − → ρ 0 K − ) ∼ B(B − → ωK − ) is observed, then W -annihilation and/or spacelike penguins could play a prominent role. The decay modes B 0 d → φπ 0 , φη, φη ′ , φρ 0 , φω, B − → φπ − , φρ − involving a vector meson φ are dominated by electroweak penguins. We show that a unitarity angle γ larger than 90 • is helpful for explaining the π + π − , πK and η ′ K data. The relative magnitudes of tree, QCD penguin and electroweak penguin amplitudes are tabulated for all charmless B → P P, V P, V V decays. Our favored predictions for branching ratios are those for N eff c (LL) ≈ 2 and N eff c (LR) ∼ 5.
(ii) nonfactorizable W -exchange effects are generally important; they can be comparable to and somtimes even dominate over factorizable contributions, depending on the decay modes under consideration, (iii) large-N c approximation for factorizable amplitudes also works in the heavy baryon sector and it accounts for the color nonsuppression of Λ + c → pK 0 relative to Λ + c → Λπ + , (iv) a measurement of the decay rate and the sign of the α asymmetry parameter of certain proposed decay modes will help discern various models; especially the sign of α in Λ + c → Σπ 1 decays can be used to unambiguously differentiate recent theoretical schemes from current algebra, and (v) p waves are the dominant contributions to the decays Λ + c → Ξ 0 K + and Ξ 0A c → Σ + K − , but they are subject to a large cancellation; this renders present theoretical predictions on these two channels unreliable. *
Two-body nonleptonic weak decays of charmed baryons are analyzed in the framework of the pole model, which is more general and pertinent than current algebra since its use is not restricted to the soft meson limit and to the pseudoscalar-meson final state. The s-wave amplitudes are dominated by the f baryon resonances. Special attention is paid to the parity-violating tp-;-baryon matrix elements, --which are evaluated using the MIT bag model. For definiteness we compute the a asymmetry parame--0 -do ter and the branching ratios for the decay modes &+pK ( K 1, i\.rr+(p+), ZO.n+(p+), Z+.rro(po), p d , and find a good agreement with experiment. We conclude that (i) there is no color suppression in -0 -*o A: -pK ( K ) and p d , (ii) nonspectator contributions are in general smaller than the factorizable ones for the decay modes which receive contributions from the factorizable diagram, and (iii) the predicted branching ratios will be too small if the wave functions of the heavy-quark bag model are employed. A comparison of our work with current algebra and other theoretical calculations is made.
The experimental hint of large B ! h 0 1 X s is linked to the b ! s penguins via the gluon anomaly. Using running a s in the h 0 -g-g coupling, the standard b ! sg ء penguin alone seems insufficient, calling for the need of dipole b ! sg at the 10% level from new physics, which could also resolve the semileptonic branching ratio and charm counting problems. The interference of standard and new physics contributions may result in direct CP asymmetries at the 10% level, which could be observed soon at B factories. [S0031-9007(97)
Weak current-induced baryonic form factors at zero recoil are evaluated in the rest frame of the heavy parent baryon using the nonrelativistic quark model. Contrary to previous similar work in the literature, our quark model results do satisfy the constraints imposed by heavy quark symmetry for heavy-heavy baryon transitions at the symmetric point v · v ′ = 1 and are in agreement with the predictions of the heavy quark effective theory for antitriplet-antitriplet heavy baryon form factors at zero recoil evaluated to order 1/m Q . Furthermore, the quark model approach has the merit that it is applicable to any heavy-heavy and heavy-light baryonic transitions at maximum q 2 . Assuming a dipole q 2 behavior, we have applied the quark model form factors to nonleptonic, semileptonic and weak radiative decays of the heavy baryons. It is emphasized that the flavor suppression factor occurring in many heavy-light baryonic transitions, which is unfortunately overlooked in most literature, is very crucial towards an agreement between theory and experiment for the semileptonic decay Λ c → Λe + ν e . Predictions for the decay modes Λ b → J/ψΛ, Λ c → pφ, Λ b → Λγ, Ξ b → Ξγ, and for the semileptonic decays of Λ b , Ξ b,c and Ω b are presented.
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