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gion of p$_{T}$ for which both approximations are applicable for bottom quarks. The disadvantage of studying the more robust R cb observable is that the AdS momentum reach is limited by the much smaller charm quark mass; an over- lapping momentum region of validity for both coupling limits may not exist for this observable. Further careful study of the “speed limit” and higher order corrections to the AdS/CFT result must be done in order to fruitfully compare R c AA and R cb results to experiment. Or, turning this around, experiment might inform theory as to the correct scales for which these theoretical predictions 

