Experimental lifetime of the a^{1}Δ electronically excited state of CH^{−}

By repeatedly probing the a^{1}Δ excited state and the X^{3}Σ^{−} ground-state populations in a beam of CH^{−} ions stored in a cryogenic ion-beam storage ring for 100 s, we extract an intrinsic lifetime of 14.9±0.5 s for this excited state. This is far longer than all earlier experimental and theoretical results, exposing large difficulties in measuring and calculating slow decays and the need for benchmark quality experiments.