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.”


Features

More Features »

Announcements

More Announcements »

Subject Areas

Nuclear Physics

Previous Synopsis

Nonlinear Dynamics

Picking the Brain

Read More »

Next Synopsis

Superconductivity

Catching Dark Matter Red Handed

Read More »

Related Articles

Synopsis: Starting Fluid for Laser Fusion
Energy Research

Synopsis: Starting Fluid for Laser Fusion

A laser-based fusion experiment demonstrates that liquid fuel capsules could rectify problems encountered with ice-based fuel capsules. Read More »

Focus: More Hints of Exotic Cosmic-Ray Origin
Astrophysics

Focus: More Hints of Exotic Cosmic-Ray Origin

New Space Station data support a straightforward model of cosmic-ray propagation through the Galaxy but also add to previous signs of undiscovered cosmic-ray sources such as dark matter. Read More »

Synopsis: Neutron Stars in a Petri Dish
Nuclear Physics

Synopsis: Neutron Stars in a Petri Dish

Simulations of the dense matter in a neutron star’s crust predict the formation of structures that resemble those found in biological membranes. Read More »

More Articles