  We present experimental and theoretical results on the momentum distribution
and the quasiparticle renormalization factor in sodium. From an x-ray
Compton-profile measurement of the valence-electron momentum density, we derive
its discontinuity at the Fermi wavevector. This yields an accurate measure of
the renormalization factor that we compare with quantum Monte Carlo and GW
calculations performed both on crystalline sodium and on the homogeneous
electron gas. Our calculated results are in good agreement with the experiment.
