Breakdown of the Born-Oppenheimer description explains neutron Compton-scattering anomaly

Several neutron Compton scattering experiments reveal an apparent drop of the proton cross section when the collision time tau(q) is around 1 fs. Such small tau(q) corresponds to a large energy spread of the proton wave packet after collision, allowing it to access excited electronic levels. This nonadiabatic excitation of electrons leads to a distortion of the shape of the neutron scattering response function with some redistribution of intensity at energies higher than the nuclear recoil energy and a slight shift of the main neutron intensity peak to lower energies.