Delta Gamma_d: A Forgotten Null Test of the Standard Model

The recent measurement of an anomalous like-sign dimuon asymmetry by the D0 collaboration has prompted theoretical speculation on possible sources of physics beyond the Standard Model that may affect lifetimes and lifetime differences in neutral B meson systems. One observable that deserves closer attention is the width difference in the B_d system, Delta Gamma_d. Since the Standard Model prediction for this quantity is well below 1%, it serves as a "null test" whereby the measurement of a larger value would cleanly reveal the presence of new physics. Methods to measure Delta Gamma_d at current and future experiments are reviewed and an attractive new approach is proposed.