The precise value of the
neutron lifetime plays an important role in nuclear and particle physics and
cosmology. It is a key input for
predicting the ratio of hydrogen to helium nuclei in the primordial universe,
it is used to estimate the rate of nuclear fusion and energy production in the
sun, and it can be used to search for physics beyond the Standard Model. There exists, however, a 10-second
discrepancy in the neutron lifetime as determined by two modern techniques. By counting the decay rate of free neutrons
in a beam of cold neutrons, the lifetime is 887.7 ± 2.2 s; by counting
surviving ultracold neutrons stored in a material bottle, it is 878.5 ± 0.8
s. I will describe the challenges to
perform these measurements and the innovations used to overcome these
challenges.