The state of the neutron lifetime

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. The value of the neutron lifetime is also an important input in Big Bang Nucleosynthesis models. Additionally, the value of the neutron lifetime is relevant in other areas including solar physics and the detection of reactor antineutrinos.

Two main methods have been utilized to measure the neutron lifetime: the “bottle” method and the “beam” method. In the bottle method, ultracold neutrons are confined in a material and/or magnetic trap. After varying lengths of storage time, the number of neutrons remaining in the trap are counted. In the beam method a cold neutron beam is passed through a fiducial volume. The absolute neutron beam flux is measured, as well as the absolute number of decay particles (protons or electrons) resulting from neutron decay inside the fiducial volume.

Despite the increasing precision of the individual methods, there has been a persistent 4 σ discrepancy between the bottle average value and the beam average value for the neutron lifetime. An overview of the measurement methods, including relevant challenges and planned/ongoing improvements, will be given. A variety of possible explanations for the discrepancy, ranging from experimental errors to new physics will be discussed.