Eighty years after Chadwick discovered the neutron, physicists today still cannot agree on how long the neutron lives. Measurements of the neutron lifetime have achieved the 0.1% level of precision (~ 1 s). However, results from several recent experiments are up to 7 s lower than the PDG value before 2010. Experiments using the trap technique produce lifetime results lower than those using the beam technique. The PDG urges the community to resolve this 6.5 sigma discrepancy.

It is difficult to measure the absolute neutron lifetime because of several limitations: the low energy of the neutron decay products, the inability to track slow neutrons, and the fact that the neutron lifetime is long (880.1 ± 1.1 s, PDG2012). Slow neutrons are susceptible to many loss mechanisms other than beta-decay, such as upscatterings and absorptions on material surface. Often times, these interactions act on a time-scale comparable to the neutron beta-decay, making the extraction of the beta-decay lifetime particularly challenging. We will revisit this measurement by trapping ultracold neutrons (UCN) in a hybrid magnetic-gravitational trap. The trap is made of a Halbach array of permanent magnets, which can levitate UCN up to 50 neV. These neutrons are also confined vertically up to 0.5 m by gravity. Such a trap minimizes the chance of neutron interactions with material walls. In addition, the open-top geometry allows room to implement novel schemes to detect neutrons and decay particles in-situ. The UCNτ experiment aims to improve on the uncertainty of the neutron lifetime measurement beyond 1 second. In this talk, I will report results of our first attempt to trap UCN earlier this year and discuss plans to quantify systematic effects.

Argonne Physics Division Colloquium Schedule