Detecting phase transitions from the high-temperature phase in systems with a small parameter

The critical temperature of a dilute Bose gas is obtained in terms of the associated small gas parameter, by a unifying approach which treats three and two spatial dimensions on equal footing, even though the superfluid phase transition is known to be driven by different mechanisms in three and two dimensions. Good agreement is found with the critical temperature obtained by recent Monte Carlo data in three dimensions and by previous analytic methods in two dimensions, respectively. This approach is expected to apply more generally to systems where the many-body theory is controlled by a small physical parameter in the high-temperature phase.