A new field theoretic method for the virial expansion

We develop a graphical method for computing the virial expansion coefficients for a nonrelativistic quantum field theory. As an example we compute the third virial coefficient b3 for unitary fermions, a nonperturbative system. By calculating several graphs and performing an extrapolation, we arrive at b3 =-0.2930, within 0.7% of a recent computation b3 = -0.29095295 by Liu, Hu and Drummond, which involved summing 10,000 energy levels for three unitary fermions in a harmonic trap.