GLYPH<Minus> 4 GLYPH<Minus> 2 0 2 4 2 4 6 8 c for attractive fermions PSfrag replacements c µ/T FIG. 4: The scaling function c for attractive fermions as a function of µ/T . relativistic system in 2 spatial dimensions: where v is the average speed and ℓ$_{free}$ the mean free path. The mean free path is ℓ$_{free}$ = 1 / $_{(}$√$_{2}$$_{nσ}$$_{) where}$ σ is the total cross-section. The √$_{2 comes from the ratio}$ of the mean speed to the mean relative speed[28]. In the unitary limit, formally S = 1 + i IM = − 1, which implies the scattering amplitude M = 2 i/ I . (See section IV.) The cross-section in eq. (7) gives σ = m $^{2}$/k I 2 in 2 dimensions, where I = m/ 4[20]. Thus, in the unitary limit η = 1 2 nvmℓ$_{free}$ (69) σ = 16 | k | (70) η = m $_{16}$√$_{2}$ ( 1 2 mv 2 ) (71) where k is a single particle momentum. This gives 24