UCNb&[chi]: a proposal for a high-precision neutron decay branching ratio experiment using ultracold neutrons.

Neutron lifetime results confirm a discrepancy between counting protons, and bottled ultracold neutrons (UCN). One explanation is n-decay into a dark matter fermion, χ, via an intermediate leptoquark scalar, Φ = (3, 1)_−1/3. Such a scalar may also interfere with Standard Model (SM) β-decay, in the form of a spectral shape parameter b, the Fierz interference term. First constraints on b for the free neutron were set with the UCNA experiment, limited by systematic energy response uncertainty. To overcome this, a UCNbχ (“ultracold neutron branch”) experiment is proposed as a successor to the UCNb prototype, developed at Los Alamos National Laboratory. Data and modeling from UCNb will be presented to motivate UCNbχ’s design. An upgrade will include a larger 4π PTFE Ulbricht sphere and a DPS/Eu:CaF₂ scintillator storage volume. Calibration of UCNbχ can be greatly improved using in situ activated xenon isotopes and volatile stannic compounds. To suppress neutron generated background, dominant in UCNb, a 4π, half-ton, active gamma veto surrounding the decay chamber, will measure radiative branches, both from SM and aprotonic dark-matter model decays.