Comments on Recent Measurements of R_c and R_b

Abstract

Discrepancies between Standard Model predictions and experimental measurements of the fractions R_c and R_b of hadronic Z decays to charm and bottom are investigated. We show that there exists a discrepancy in two complementary determinations of B(Bbar --> D X). Reducing the branching ratio B(D0 --> K- Pi+) by about 15% from currently accepted values to 3.50+/-0.21 removes the discrepancy. Since B(D0 --> K- Pi+) calibrates most charmed hadron yields, the reduced value also eliminates the discrepancy between the predicted and measured values of R_c and mitigates a problem in semileptonic B decays. A reduction in B(D0 --> K- Pi+) would also mean that roughly 15% of all D0 and D+ decays have not been properly taken into account. It is shown that if the missing decay modes involve multiple charged particles, they would be more likely to pass the requirements for lifetime B tagging at LEP and SLC. This would mean that the charm tagging efficiency in (Z --> c cbar) has been underestimated. As a consequence R_b would need to be revised downward, potentially bringing it in line with the Standard Model prediction.