Neutron Lifetime Measurements Give Clues on Origin of the Universe

Precise measurements of the neutron lifetime are yielding clues about how subatomic particles coalesced into the elements that formed our universe after the so-called Big Bang.

Physicists M. Scott Dewey and Geoffrey Greene and their co- workers at the National Institute of Standards and Technology, in collaboration with scientists at the Institute Laue Langevin in France, and the Petersburg Nuclear Physics Institute in Russia, are reducing the uncertainty of neutron lifetime measurements. The NIST experiments are supported by NIST and the Department of Energy.

Future experiments to be carried out at the NIST National Cold Neutron Research Facility could improve the accuracy of neutron lifetime measurements by another factor of 10 or more. These experiments will be carried out as a collaborative effort between NIST, John Doyle of Harvard University, Steven Lamoreaux of the University of Washington and Robert Golub of the Hahn Meitner Institute of Germany. Additional support will come from the National Science Foundation and the DoE.

Dewey described recent experiments and plans for more precise measurements of the neutron lifetime at the annual meeting of the American Physical Society and American Association of Physics Teachers in Washington, D.C. Greene organized a session of talks on "Precision Measurements and Tests of Fundamental Laws in Beta Decay" which concerned related experiments carried out at a variety of laboratories worldwide.

Among the benefits of this research is a clearer view of one of the forces acting on subatomic particles as they cooled after the Big Bang. This so-called "weak force" is one of four forces in the "Standard Model" which physicists use to explain the behavior of particles. The weak force is responsible for the phenomenon of radioactivity. One of the simplest examples of radioactivity occurs when an isolated neutron decays into a proton, an electron and an anti-neutrino.

The neutron, which together with protons and electrons forms atoms, is stable when encased in an atomic nucleus. When neutrons exist outside an atom, as they did at the birth of the universe, they decay with an average lifetime of about 15 minutes. Just how fast these early neutrons decayed determined, in part, the ratio of abundances of elements in the universe.

According to the Standard Model, the weak force acts on particles in only one "handedness"--left. The observation of this universal "parity violation", one of the most important discoveries in particle physics in the last half-century, was first made at NIST (then the National Bureau of Standards) in 1957. Many physicists, however, believe that, while parity violation is now a universal phenomenon, during the first instants of the Big Bang there was equal probability of the weak force acting from the right or left, or in other words being either left-handed or right-handed.

If this is true, then a "spontaneous symmetry breaking event" turned the weak force to the left. One may consider a simple analogy to gain some insight into this phenomenon. It is as though a dinner guest at a round table with symmetrically placed water glasses chose a water glass on his left, thereby forcing everyone else to take a glass from the left.

Dewey says recent experiments seem to support the Standard Model and the notion that the weak force was always left-handed. More sensitive measurements using neutrons and other radioactive decays will be required to provide further insight into this question.

Since neutrons have no charge, they are somewhat difficult to manipulate. Specially designed instruments at NIST's Cold Neutron Research Facility will allow Dewey, Greene and their collaborators to make the most precise and accurate measurements ever of the neutron lifetime.

Neutrons are liberated from uranium atoms inside a 20 megawatt research reactor at NIST. These very energetic neutrons are chilled and slowed by passing through a hydrogenous "moderator" cooled to a very low temperature. Glass tubes with nickel-coated interiors guide the neutrons to various experiment stations in the Cold Neutron Research Facility.

Dewey and Greene rely on a magnetic trap at the end of a long neutron guide coming from the reactor. If a neutron happens to decay in the trap's cylinder, the resulting proton is caged within the trap's magnetic field. Then an electric field around the trap deflects the positively charged protons into a detector. The vast majority of neutrons do not decay and slam straight through the trap into another detector used to monitor the total flow of neutrons passing through.

The NIST team is presently carefully tracking possible sources of errors in their latest experimental data from both the proton trap and the neutron monitor.

The current world average value for the neutron lifetime is 14 minutes, 47.0 seconds, give or take 2.0 seconds. NIST expects to contribute a new, independent result to the world average within the next few months.

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