Systematic Uncertainties in the Measurement of Neutron lifetime Using Lunar Prospector Neutron Spectrometer

TL;DR

This study analyzes systematic uncertainties in neutron lifetime measurements using Lunar Prospector data, highlighting the impact of surface temperature and composition models, and emphasizes the need for optimized data and modeling for competitive results.

Contribution

It provides a detailed assessment of systematic errors in space-based neutron lifetime measurements, focusing on lunar surface temperature and composition effects.

Findings

Different surface models yield neutron lifetime estimates ranging from 739.6 to 767.3 seconds.

Systematic uncertainties from temperature and composition choices significantly affect the measured neutron lifetime.

Current measurements are not competitive with laboratory results due to large unaccounted systematics.

Abstract

The lifetime of free neutrons measured in the lab has a long standing disparity of $\sim$9~s. A space-based technique has recently been proposed to independently measure the neutron lifetime using interactions between the galactic cosmic rays and a low atmosphere planetary body. This technique has not produced competitive results yet due to constraints of non-optimized data that contain large systematic errors. We use data from the neutron spectrometer on-board NASA's Lunar Prospector, and study two large systematics in the measurement of neutron lifetime: the lunar sub-surface temperature and the lunar surface composition. We use the HeCd and HeSn neutron spectrometer data when the spacecraft was in a highly elliptical orbit during the orbit insertion period. We report the neutron lifetime using four different models that each have different choices of surface temperature and composition. 5$^{\circ}$ \cite{prettyman2006elemental} and 2$^{\circ}$ re-binned \cite{wilson2021measurement} maps result in 777.6$\pm$11.7~s and 739.6$\pm$10.8~s respectively. For the 20$^{\circ}$ map, constant equatorial and a latitude-dependent temperature model result in 738.6$\pm$10.8~s and 767.3$\pm$11.2~s respectively. Increasing the complexities of the models accounting for the systematic effects increase the measured lifetime. However, the reported measurements are not competitive with the laboratory results due to large unaccounted systematics resulting from non-optimized measurements and modeling assumptions. This work serves as a study of systematic uncertainties for future neutron lifetime measurements using the space-based technique. We estimate the effect on the lifetime from the choice of temperature model to be to be 28.7 $\pm$ 15.5~s, and choice of compositional map (for 20$^\circ$ and 5$^\circ$ maps) to be 10.3 $\pm$ 12.2~s.