UCNtau Experiment

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A precision neutron lifetime measurement using Ultracold neutrons in a magneto-gravitational trap

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 (pre-2010) particle data group (PDG) value. Experiments using the trap technique yield lifetime results lower than those using the beam technique. The PDG urges the community to resolve this discrepancy, now 6.5 sigma.

Measuring the absolute neutron lifetime is difficult 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 (880.0 +/- 0.9 s). Slow neutrons are susceptible to many loss mechanisms other than beta-decay, such as upscattering and absorption on material surfaces. Often, these interactions act on time scales 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 consists 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 UCNtau experiment aims to reduce the uncertainty of the neutron lifetime measurement to below 1 second.

Current neutron lifetime in pdgLive. Other Neutron data in pdgLive

Progress & Current Status

2014 Commissioned the V in-situ detector. Enhance UCN statistics inside the trap. Anticipate 1 s statistical uncertainty this year. 
2013 Commissioned the prototype trap. First UCN trapped, measurement of a neutron "storage time" inside the trap : 860 +/- 20 sec, consistent with the PDG value within 1 sigma.
2011 Official collaboration formed
2010 - 2012 Completed the prototype construction with the IU NSF support
2005 - 2009 Ideas initiated, engineer design completed & construction started with the LANL LDRD support

updated by C.-Y. Liu, 04/16/2014