Status of the UCNτ experiment upgrade to UCNτ+ for finer neutron lifetime precision*

UCNτ experiment at the Los Alamos National Laboratory uses a magneto-gravitational trap [1] for ultra-cold neutrons (UCN). It counts surviving UCNs after a given hold time using a 10B-coated ZnS scintillator detector [2]. The experiment has produced the world's most precise neutron lifetime [3]. The precision of the lifetime and the asymmetry (A) in the beta-decay of a free neutron are crucial to improving the determination of the Cabibbo-Kobayashi-Maskawa (CKM) matrix element (V_ud). A constrained error in V_ud is needed to probe physics beyond the Standard Model and understand a 3-σ deviation from unitarity in the top row of the matrix.

In the experiment, UCNs are loaded into the trap and cleaned of the high-energy (≥ 35 neV) UCNs by absorbing them on a boron-coated plate lowered into the trap. The precision of the lifetime result depends on the number of cleaned UCNs stored in the trap. To achieve higher precision (∼ 0.1 sec), the experiment is being upgraded to UCNτ+ by incorporating an elevator to load the UCNs in the trap and gain more cleaned UCNs via adiabatic cooling and improved loading efficiency. The elevator has an attached bin with Teflon lining carrying UCNs. To accommodate the addition of the elevator, the trap has now been modified with vacuum extensions and lowered relative to the UCN delivery guide.

In this contribution, I will explain the principle behind the loading method of UCNs in the trap and UCNτ+ geometry. I will also discuss plans for a commissioning run with the next beam cycle at LANSCE.