The Neutron Lifetime “Problem”
Within a nucleus, the neutron may be unconditionally stable. However, when liberated from a nucleus, the neutron is unstable and beta decays with a lifetime of approximately 10 minutes. Since neutron decay is the simplest example of nuclear beta decay, the value of the neutron lifetime is a parameter of considerable importance to a wide variety of physical systems. These range from astrophysics to particle physics to cosmology. Particularly noteworthy is role played by the neutron lifetime in the Big Bang where it sets the time scale for nucleosynthesis and thus determines the cosmic abundance distribution of light elements. Given the interest in the neutron lifetime, it is somewhat distressing to observe that the measurements of the neutron lifetime having the lowest quoted uncertainties are in substantial disagreement with one another. After a brief overview, I will discuss this “neutron lifetime problem,” outline the experimental landscape, and conclude with the prospects for future experiments.