Beyond the Brink-Axel Hypothesis

Experiments routinely measure the response of the nuclear ground state to external fields, electromagnetic, weak, or other. But in hot environments, in astrophysics and elsewhere, excited states can be populated. The response, or transition strength, of those states are much harder to measure. Often one assumes the so-called Brink-Axel hypothesis, that the response of excited states is the same as that of the ground state, only shifted in energy. While for electric dipole (E1) transitions this can be somewhat justified, overall it is an act of desperation. So is Brink-Axel true, false, or somewhere in between? With detailed microscopic calculations I can answer this question. It turns out that the nuclear response does evolve with initial energy--but states in a narrow energy window have the same, statistically speaking, response. Furthermore the evolution can be understood by general mathematical arguments. While there is still work to be done, the modified Brink-Axel put excited state response--and applications to astrophysics--on a more certain footing.