BL3: A next-generation free neutron lifetime experiment using the beam method

Recent measurements of the free neutron beta decay lifetime using the cold neutron beam method and the ultracold neutron storage method differ by four standard deviations. While new physics models have been proposed to explain the discrepancy, a more probable culprit is misunderstood systematic effects in one or both methods. BL3 is a next generation beam neutron lifetime experiment based on the Sussex-ILL-NIST technique. A cold neutron beam passes through a quasi-Penning trap. Recoil protons from neutron decay are trapped and periodically counted by a silicon detector and the neutron beam density is measured using a thin foil target. BL3 will employ a larger superconducting magnet, a larger high-flux beam, and a large diameter segmented silicon detector, giving a 100 times increase in proton trapping rate compared to previous experiments. A number of new features to test and explore systematic effects will be included. The goal of BL3 is twofold: 1) to search for an explanation of the neutron lifetime discrepancy and validate systematics in the beam method; and 2) reduce the ultimate precision in the beam method to <0.3 s. Plans for the design and operation of the experiment will be discussed.