Measurement of the Free-Neutron Lifetime Using Space-based Neutron Data from NASA's MESSENGER Mission*

Precise knowledge of the free neutron lifetime, $\tau_n$, is required to test the consistency of the standard model and uncertainties in $\tau_n$ dominate those in predicted primordial $^{\rm 4}$He abundance from Big Bang nucleosynthesis. Presently, there exist two classes of experiments that have successfully made measurements of $\tau_n$. The `Beam' class involves measuring the activation of cold neutron beams and the `Bottle' class uses storage (material, magnetic and/or gravitational) to trap neutrons and measure the rate of decay during storage. However, there currently exists a 4\ $\sigma$ disagreement between the `beam' and `bottle' measurements. We have developed a new technique for using space-based neutron spectroscopy measurements to determine $\tau_n$. Under this technique the change in planet-originating neutron flux with planet-to-spacecraft distance yields a measure of $\tau_n$. Here, we will present an analysis of data from the neutron spectrometer on NASA's MESSENGER mission as a proof-of-principle demonstration of a space-based $\tau_n$ measurement. In this talk, we will discuss the basis of the technique, statistical and systematic errors of the measurement, and preliminary results will be presented.