Space-based measurement of the neutron lifetime using data from the neutron spectrometer on NASA's MESSENGER mission

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 $780\pm {60}_{\text{stat}}\pm {70}_{\text{syst}}$ s, thereby demonstrating the viability of this new approach.

Figure 1

PDG and more recent measurement of ${\tau }_n$ [9, 10, 11, 12]. The shaded regions represent the standard error on the uncertainty-weighted mean lifetime in each class.

Figure 2

Modeled and measured data taken by MESSENGER's neutron spectrometer when the spacecraft was within ${10}^4$ km of Venus' surface for the Li-glass detectors with their surface normals approximately (a) aligned with and (b) opposite the direction of spacecraft motion.

Figure 3

(a) ${\chi }^2$ comparison of the models with differing ${\tau }_n$ values and ${\mathrm{N}}_2$ abundances. The contours show the 68%, 95%, and 99% confidence intervals, which were calculated assuming a Gaussian likelihood. (b) The probability distribution of ${\tau }_n$ using the likelihoods derived from (a). The red curve is a Gaussian fit to the points, with the fit parameters shown.

Figure 4

Change in ${\tau }_n$ inferred from the Venus flyby data when changing ${\mathrm{\Sigma }}_a$ for the Mercury flyby.

Figure 5

The neutron flux at several altitudes above Mercury. The solid curves show neutrons detected with ${\mu }_R=1$ and dashed curves ${\mu }_R=0.7$.

Figure 6

(a) Change in data-to-model normalization factor with solar ACE-derived modulation parameter. (b) Change in the MESSENGER NS triples count (a proxy for GCR flux) with ACE-derived solar modulation parameter. The red curves in both panels are linear fits to the points.