The Isgur-Wise function in the BPS limit

From sum rules in the heavy quark limit of QCD, using the non-forward amplitude, we demonstrate that if the slope � 2 = � ′ (1) of the Isgur-Wise function �(w) attains its lower bound 3 (as happens in the BPS limit proposed by Uraltsev), the IW function is completely determined, given by the function �(w) = � 2 w + 1 � 3/2