Space-based Measurement of the Free Neutron Lifetime

Knowledge of the free neutron lifetime, $\tau_n$, is important as, along with other neutron decay parameters, it enables the determination of the quark-mixing matrix element $|V_{ud}|$ and therefore the testing of the unitarity of the CKM matrix. Additionally, uncertainties in $\tau_n$ dominate those in predictions of primordial $^4$He abundance from Big Bang nucleosynthesis. Historically, there have been two classes of high-precision experiments that measure $\tau_n$. ``Beam'' experiments measure the activation of a cold neutron beam passing through a volume capable of trapping the created protons or electrons. The second method is the more precise ``bottle'' technique that involves trapping ultra-cold neutrons and measuring their decay curves directly. However, a significant disagreement, currently $> 4.5~\sigma$, exists between the average results of these two techniques and has persisted for more than 15 years. This disagreement might be explained by an unidentified or improperly quantified systematic. However, given the direction of the disagreement the decay of neutrons to dark matter particles could also explain the observations. Recent analyses of data from the neutron spectrometers onboard NASA’s Lunar Prospector and MESSENGER spacecraft have demonstrated the possibility of using a third space-based technique that might one day break the stalemate. In this talk, I will review the neutron lifetime puzzle and the space-based measurements of $\tau_n$, describe the improvements made to the space-based technique and how it might be used in the future.