Precision Measurement of the Neutron Scattering Length of ^{4}He Using Neutron Interferometry.

We report a 0.08% measurement of the bound neutron scattering length of ^{4}He using neutron interferometry. The result is b=(3.0982±0.0021[stat]±0.0014[syst])  fm. The corresponding free atomic scattering length is a=(2.4746±0.0017[stat]±0.0011[syst])  fm. With this result the world average becomes b=(3.0993±0.0025)  fm, a 2% downward shift and a reduction in uncertainty by more than a factor of six. Our result is in disagreement with a previous neutron interferometric measurement but is in good agreement with earlier measurements using neutron transmission.