Measurement of the Free Neutron Lifetime using the Neutron Spectrometer on NASA's Lunar Prospector Mission*

Knowledge of the free neutron lifetime, τ_n, is important as, along with other neutron decay parameters, it enables the testing of the unitarity of the CKM matrix in the Standard Model.  Additionally, uncertainties in τ_n dominate those in predictions of primordial He abundance from Big Bang nucleosynthesis. Historically, there have been two classes of high-precision experiments that measure τ_n. However, a 4.5 σ disagreement exists between the results of the two techniques. We recently demonstrated an alternative measurement method using space-based neutron spectroscopy measurements with data from NASA’s MESSENGER mission. However, due to the constraints of the mission this measurement had significant uncertainties.  Here, we use data from the Lunar Prospector Neutron Spectrometer to make the second space-based measurement of the free neutron lifetime finding τ_n = 887 ± 15 s. Improvements in modeling enabled the reduction of the systematic uncertainty from 70 s on the previous space-based lifetime measurement to 7 s here. This modeling moves space-based neutron lifetime measurement towards the ultimate goal of reducing the magnitude of the systematics on a future space-measurement to the level of those seen in laboratory-based experiments. In this talk, we will discuss the improvements made to the space-based technique, and the statistical and systematic errors of the measurement.