Univ Toronto, Dept Phys, Toronto, ON M5S 1A7, Canada
; CCAST, World Lab, Beijing 100080, Peoples R China
; Acad Sinica, Inst High Energy Phys, Beijing 100039, Peoples R China
; Univ Hawaii, Dept Phys & Astron, Honolulu, HI 96822 USA
; ICTP, High Energy Sect, I-34014 Trieste, Italy
We study the flavor changing processes Y-->B/(B) over bar X-s and J/psi-->D/(D) over bar X-u in B factories and tau-charm factories. In the standard model, these processes are predicted to be unobservable; so they serve as a probe of new physics. We first perform a model-independent analysis, then examine the predictions of the models, such as the top-color models, the MSSM with R-parity violation, and the two-Higgs-doublet model, for the branching ratios of Y-->B/(B) over bar X-s and J/psi-->D/(D) over bar X-u. We find that these branching ratios could be as large as 10(-6) and 10(-5) in the presence of new physics. [S0556-2821(99)00513-5].