Determination of the Free Neutron Lifetime
  We present the status of current US experimental efforts to measure the lifetime of the free neutron by the "beam" and "bottle" methods. BBN nucleosynthesis models require accurate measurements with 1 second uncertainties, which are currently feasible. For tests of physics beyond the standard model, future efforts will need to achieve uncertainties well below 1 second. We outline paths achieve both. 
J. David Bowman, L. J. Broussard, S. M. Clayton, M. S. Dewey, N.   Fomin, K. B. Grammer, G. L. Greene, P. R. Huffman, A. T. ...
nucl-ex physics.ins-det