Figure 1:
Schematic plot of the typical momentum distribution n(k) of a fermion gas with short-range interactions, showing the characteristic behavior of n(k) approaching a constant value below unity at k→0 and exhibiting a rapid variation near the Fermi wave vector k_{F} (that would be a sharp discontinuity at T→0 for a Fermi liquid). At asymptotically large k, as shown theoretically by Tan and confirmed experimentally by Stewart et al., one has n(k)∼C/k^{4} (characterizing the fermions as they undergo collisions, seen in the inset), with the contact C governing thermodynamic quantities like the total energy of the gas.