BL3: The Next Generation Neutron Beam Lifetime Experiment*

Neutron beta decay is a fundamental process in nuclear physics that provides sensitive means to uncover the details of the weak interaction. A precise measurement of the neutron lifetime, along with neutron beta decay correlations, can provide tests of CKM unitarity and is needed for Big Bang Nucleosynthesis calculations of the primordial 4He abundance, which correlates with other cosmological parameters. A next generation beam method experiment, BL3 proposed at the NCNR, aims to improve the systematic precision of neutron lifetime measurements in order to resolve the discrepancy between beam and bottle method measurements. In BL3, the recoil protons from neutron beta decay are born in a quasi-Penning trap with magnetic and electric fields that transport them to a segmented silicon detector. The BL3 experiment will use a larger beam with higher neutron flux, which necessitates a larger apparatus with a larger proton trap and detector, an improved neutron flux monitor, and an upgraded version of the alpha-gamma device that provides the absolute calibration of the neutron flux monitor. This will allow BL3 to improve the precision of beam-based lifetime experiments to 0.3 s and perform thorough evaluation of systematic effects. In this talk, we will discuss the measurement techniques in BL3 as well as provide an overview and status update on the construction of the new apparatus, currently underway.