Collaborative Research: Determination of the Neutron Lifetime Using Magnetically Trapped Neutrons

North Carolina State University, Raleigh NC

Investigators

Abstract

The decay of the free neutron is the simplest nuclear beta decay. The parameters that describe the decay of the free neutron, especially the neutron lifetime, provide essential inputs to all investigations that involve the weak interaction. A precise value for the neutron lifetime is required for several internal consistency tests of the Standard Model. The neutron lifetime is also an essential parameter in the theory of Big Bang Nucleosynthesis (BBN). At present, the uncertainty in the neutron lifetime currently provides the largest contribution to the uncertainty in the value of the helium to hydrogen ratio predicted by BBN theory. Intercomparison between the neutron lifetime, the observed cosmic light element abundances and the known number of light neutrino species provides a critical consistency check for BBN theory. This experiment will measure the neutron lifetime using magnetically trapped ultracold neutrons (UCN). A magnetic trap is "filled" by scattering a beam of low-energy neutrons from superfluid helium. Some neutrons lose enough energy to become trapped, with the remainder leaving the trapping region. The beam is then turned off and the rate of neutron decay, and thus their lifetime, is measured as these trapped neutrons undergo beta-decay. This technique should allow the researchers to substantial improve the precision that the neutron beta decay lifetime is known. Using the method of magnetic trapping, it should be possible to improve the accuracy of the neutron lifetime by a factor of between 2 and 50 using existing and next-generation neutron sources.