Beam-based Neutron Lifetime Measurements at NIST: BL2 and BL3

Neutron beta decay is the simplest example of nuclear beta decay and is crucial in our understanding of weak processes. The neutron lifetime, when combined with other neutron decay parameters, provides a test of the unitarity of the CKM matrix in the Standard Model; is an important input in Big Bang Nucleosynthesis models; and plays a role in other areas including solar physics and the detection of reactor antineutrinos. Competitive tests of unitarity in the CKM matrix require determination of the neutron lifetime to better than 0.3 s. While the most precise neutron lifetime experiments have reached uncertainties of less than 1 s, the current 10 s discrepancy between bottle and beam-based determinations presents a challenge.

In this talk I will discuss our efforts to address this discrepancy via the ongoing BL2 beam-based neutron lifetime experiment at NIST. I will also provide an update on the next-generation beam lifetime experiment, BL3, currently under construction. The goal of BL3 is to achieve < 0.3 s uncertainty utilizing the beam method.