which are normalized to one, and thus, when contracted with the position or two positions describe the probability or joint probability densities of finding one or two particles at particular positions in space (occasionally called the distribution functions). From these quantities, the correlation coefficient, τ , can be obtained, which is the ratio between the covariance with respect to the position vectors of two particles, ⃗ r 1 and ⃗ r 2 , and the scalar variance with respect to the position of one particle. Given the indistinguishability and choice of origin at the center of mass, we can write the correlation coefficient as