Synopsis: Discrepancy in Neutron Lifetime Still Unresolved

Precision measurements of the neutron lifetime differ by about 8 seconds.

Outside of the nucleus, the proton remains stable for at least 1034 years, but an isolated neutron survives just 15 minutes before it decays into a proton, electron, and an antineutrino. Astrophysicists rely on a precise value of the free neutron lifetime to calculate the rate of nucleosynthesis during the big bang, while particle physicists use it to constrain fundamental parameters of the standard model. Yet measured lifetimes have varied by about a percent, depending on the experimental technique. As reported in Physical Review Letters, the latest refinement of the neutron lifetime in one type of experiment has left this discrepancy unresolved.

Researchers have relied on two experimental strategies to measure the neutron lifetime. In the “bottle” method, low-energy neutrons are confined in a trap constructed from magnetic fields or wall materials like beryllium that reflect neutrons. The neutron lifetime can be determined by simply counting the number of particles that survive after a fixed storage time. The alternative is the “in-beam” method, in which a beam of neutrons with a precisely known flux travels through a well-defined volume, and the lifetime is determined by counting the number of decay products.

The two types of experiments give a neutron lifetime that differs by 8 seconds (or 2.6 standard deviations)—a discrepancy that researchers have chalked up to unresolved systematic errors. As a first step, scientists working at the NIST Center for Neutron Research in Gaithersburg, Maryland, have now refined their earlier in-beam measurement from 2005 with a more accurate value of the neutron flux (the largest uncertainty in the original experiment) by recalibrating their original neutron detector. The NIST researchers now report an in-beam neutron lifetime of 887.7±2.3s, in agreement with their previously measured value, but raising the significance of the discrepancy to 3.8 standard deviations. – Kevin Dusling

Correction (2 December 2013): Paragraph 3, sentence 1, “2.9 standard deviations” changed to “2.6 standard deviations.”


More Features »


More Announcements »

Subject Areas

Nuclear Physics

Previous Synopsis

Nonlinear Dynamics

Picking the Brain

Read More »

Next Synopsis


Catching Dark Matter Red Handed

Read More »

Related Articles

Focus: <i>Video</i>—Nuclear Fusion in Hi-Def
Nuclear Physics

Focus: Video—Nuclear Fusion in Hi-Def

A new model provides a detailed visualization of the clustering of protons and neutrons within the excited nuclear compound formed just after two nuclei collide and fuse. Read More »

Viewpoint: Out of Neutron Star Rubble Comes Gold
Nuclear Physics

Viewpoint: Out of Neutron Star Rubble Comes Gold

New calculations show that the accretion flows that form after a neutron star collision can eject large amounts of matter that is rich in gold and other heavy elements. Read More »

Viewpoint: Doubly Magic Nickel
Nuclear Physics

Viewpoint: Doubly Magic Nickel

Two independent experiments on the isotope copper-79 confirm that its nuclear neighbor nickel-78 is indeed a doubly magic nucleus. Read More »

More Articles