A new benchmark in determining neutron lifetime

Outside the stabilizing environment of the atomic nucleus, a free neutron survives for an average of less than a quarter hour before decaying into a proton, an electron, and an electron antineutrino. The value of the neutron lifetime, which is a crucial parameter in calculating the abundances of small nuclei in the primordial plasma minutes after the Big Bang, has proven difficult to pin down (see the article by Michael Snow, Physics Today, March 2013, page 50 ). One method entails tracking and counting decay protons as a beam of cold neutrons passes through a set of magnetic and electric fields; it yields a lifetime of 888 seconds . Another technique requires trapping even colder neutrons for minutes at a time in a bottle and counting the ones that don’t decay; those experiments yield a value of 879 seconds . Both techniques come with considerable potential for systematic error. Yet the difference of nearly four standard deviations has scientists wondering whether exotic physics is to blame for the discrepancy.

Now Robert Pattie Jr at Los Alamos National Laboratory and colleagues have made the most precise measurement of the neutron lifetime. The team employed the bottle method but introduced crucial improvements. First, the researchers used magnetic fields and gravity to trap the neutrons, which prevented any particles from crashing into a wall. They also dipped a sheet of plastic partway into the bottle to absorb neutrons that would have had enough kinetic energy to escape during the storage phase of the experiment. Following that phase, rather than emptying the enduring neutrons into an external detector, the researchers created a neutron-absorbing scintillator (shown in the photo) that they gradually dipped into the container to conduct a census of surviving neutrons at various energies.

Based on more than 600 trials, Pattie and colleagues came up with a neutron lifetime of between 876.8 and 878.8 seconds. The ultraprecise result is the first bottle measurement in which the systematic correction (+0.1 second, caused by interactions with residual gases in the storage chamber) is less than the statistical uncertainty (±0.7 second). It also reinforces the discrepancy between the beam and bottle approaches. Researchers participating in separate beam experiments at NIST and the Japan Proton Accelerator Research Complex will try to make similarly definitive measurements over the next few years. If the discrepancy holds, then expect theorists to add to the already diverse array of explanations that involve revisions to the standard model. (R. W. Pattie Jr et al., Science, in press, doi:10.1126/science.aan8895 .)