The Beam Lifetime 3 experiment

Free neutron $eta-$decay provides a test of big bang nucleosynthesis through the abundance of light elements as well as a test of the standard model through a calculation of $V_{ud}$ that is free from nuclear structure effects. The purpose of the Beam Lifetime 3 (BL3) experiment at the National Institute of Standards and Technology (NIST) is to measure the free neutron lifetime with an uncertainty of $<$0.3 seconds and to investigate a range of systematic effects that may be responsible for a 4$sigma$ discrepancy between beam measurements and trapped UCN measurements of the free neutron lifetime. Improvements to the proton trap acceptance, proton detector, neutron flux, and a more uniform neutron acceptance allow a measurement with a statistical uncertainty of 1 second to be taken in less than a day.