UCNbX:a high-precision ultracold neutron decay branching ratio experiment.

It is now established that the neutron lifetime has a discrepancy between counting decay protons, and bottled ultracold neutrons (UCN) disappearance.
New experiments are needed to find out why.
Possible explanations include systematic problems in proton detection efficiency, neutron counting, and leaky traps. More exotic proposals call on new physics like low mass dark decays and neutron oscillations.

Further, the exact decay branching, capture and leak modes of free neutrons are not fully constrained experimentally to exactly match the Standard Model (SM) past the percent level, also on the scale of the lifetime discrepancy.

New interactions can interfere with SM decay measured as a spectral shape parameter $b$, called the Fierz interference term.
Current constraints on $b$ for the free neutron are limited by systematic energy response uncertainty and could not constrain other elements like synchronized proton detection, UCN fiducial density, and neutron loss events.

Based on the UCNb prototype developed at Los Alamos National Laboratory (LANL), UCNbX will constrain these branches and losses, and measure beta and gamma spectra,
to help pin point the nature of the lifetime anomaly.