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

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The decay of the free neutron is the simplest nuclear beta decay. Theparameters that describe the decay of the free neutron, especially theneutron lifetime, provide essential inputs to all investigations thatinvolve the weak interaction. A precise value for the neutron lifetimeis required for several internal consistency tests of the StandardModel. The neutron lifetime is also an essential parameter in thetheory of Big Bang Nucleosynthesis (BBN). At present, the uncertaintyin the neutron lifetime currently provides the largest contribution tothe uncertainty in the value of the helium to hydrogen ratio predictedby BBN theory. Intercomparison between the neutron lifetime, theobserved cosmic light element abundances and the known number of lightneutrino species provides a critical consistency check for BBN theory.This experiment will measure the neutron lifetime using magneticallytrapped ultracold neutrons (UCN). A magnetic trap is ``filled'' byscattering a beam of low-energy neutrons from superfluid helium. Someneutrons lose enough energy to become trapped, with the remainderleaving the trapping region. The beam is then turned off and the rateof neutron decay, and thus their lifetime, is measured as these trappedneutrons undergo beta-decay. This technique should allow theresearchers to substantial improve the precision that the neutron betadecay lifetime is known. Using the method of magnetic trapping, itshould be possible to improve the accuracy of the neutron lifetime by afactor of between 2 and 50 using existing and next-generation neutronsources.