NSF EPSCoR: BL3 Neutron Lifetime Apparatus

Grants and Contracts Details


We propose to construct and commission a next-generation beam method experiment, BL3, to measure the neutron lifetime. It is similar in design and operation to previous recent beam neutron lifetime experiments, but will be significantly larger to accommodate a large area neutron beam and will incorporate many technical improvements, such as much higher counting statistics, a more uniform magnetic field in the trapping region; a large, segmented, ultrathin window silicon proton detector; and a sophisticated neutron time of flight spectrometer. BL3 has two main scientific goals: 1. To thoroughly investigate, test, and probe systematic effects in the beam method that may contribute to the 8.7 s (4 sigma) disagreement between the beam method and ultracold neutron storage experiments, and possibly resolve the discrepancy. 2. To improve the ultimate uncertainty of the beam method to < 0.3 s. This proposal is a collaboration of six institutions: Tulane U., U. Tennessee, Indiana U. Drexel U., U. Kentucky and Hamilton College. This team has decades of experience in neutron lifetime and neutron decay measurements. Intellectual Merit: The neutron lifetime is an important and fundamental physical parameter. The value of the neutron lifetime relates to many other important processes in nuclear physics, solar physics, and astrophysics that involve the charged weak interaction between free neutrons and protons. The neutron lifetime establishes the time scale and temperature of nucleon "freeze out" shortly after the Big Bang, which sets the neutron to proton ratio during the era of light element nucleosynthesis and hence the primordial helium abundance. With the neutron beta asymmetry (A), it can be used to determine Vud, the first element of the CKM matrix, test CKM unitarity, and constrain new physics beyond the Standard Model. We expect BL3 to play a key role in resolving the neutron lifetime discrepancy and pushing the uncertainty to the <0.3 s level. Broader Impacts: The neutron lifetime has broad implications for nuclear physics, particle physics, astrophysics, and cosmology. This project is an excellent opportunity to train undergraduate and graduate students and postdocs on the methods and theory of neutron science which are applicable not only to neutron decay research but also to diverse fields in physics, chemistry, materials science, and biology at existing and emerging neutron sources around the world. Throughout all phases of the experiment we will emphasize the professional development of young scientists on the project, including drafting papers, presenting reports and results at scientific meetings, and participating in important collaboration decisions. BL3 will impact science and society by contributing to the resolution of the neutron lifetime discrepancy. 2013030
Effective start/end date5/15/224/30/24


  • Tulane University


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