Abstract
We establish the feasibility of measuring the neutron lifetime via an alternative, space-based class of methods, which use neutrons generated by galactic cosmic ray spallation of planets' surfaces and atmospheres. Free neutrons decay via the weak interaction with a mean lifetime of around 880 s. This lifetime constrains the unitarity of the CKM matrix and is a key parameter for studies of Big-Bang nucleosynthesis. However, current laboratory measurements, using two independent approaches, differ by over 4$\sigma$. Using data acquired in 2007 and 2008 during flybys of Venus and Mercury by NASA's MESSENGER spacecraft, which was not designed to make this measurement, we estimate the neutron lifetime to be $780\pm60_\textrm{stat}\pm70_\textrm{syst}$ s, thereby demonstrating the viability of this new approach.
Highlights
Measurement of the neutron lifetime τn via space-based observation was first proposed in 1990 by Feldman et al [1]
Using data taken by MESSENGER’s neutron spectrometer during its flybys of Venus and Mercury we found τn = 780 ± 60stat ± 70syst s
This result establishes the feasibility of making a measurement of τn from space
Summary
Measurement of the neutron lifetime τn via space-based observation was first proposed in 1990 by Feldman et al [1]. Unlike Venus’ atmosphere, Mercury’s surface is not spatially uniform and contains a large number of elements present at levels high enough to affect the planet-originating neutron flux. Many of these elements were mapped by MESSENGER during its 4-year orbital mission [29,30,31,32,33]; we have a good understanding of Mercury’s composition on large scales. Constant composition was required as the parameter space of potential surface compositions is too large to explore fully
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