UCNτ: An Experimental Method for High-Precision Determination of the Free Neutron Lifetime

The neutron is an unstable but long-lived neutral baryon, characteristics that conspire to make it a versatile laboratory for testing the Standard Model (SM) at the precision frontier. β-decay of the free neutron, for example, plays a central role in tests of symmetries that underlie fundamental particle interactions. τ_n, the mean lifetime for free neutron decay, is an interesting empirical target because, in combination with other β-decay observables, it can provide sensitive tests for physical phenomena not currently included in the SM. The interesting precision band for measurements of τ_n is between 0.1% and 0.01%, and current experimental techniques have demonstrated the capability to reach precisions in this range. A clear application of the resulting measurements is hampered by a nearly four-sigma discrepancy between the two prevalent techniques, called "beam" and "bottle," which could be due to a mundane experimental issue or itself an indication of new physics. Taken on its own, however, the most recent bottle result from UCNτ has for the first time demonstrated a level of precision sufficient to probe current theoretical understanding of neutron decay. We will describe the new UCNτ result and discuss the precision achieved in relation to tests of CKM matrix unitarity.