A note on spontaneous symmetry breaking in the mean-field Bose gas

We consider the homogeneous Bose gas in the three-dimensional unit torus, where $N$ particles interact via a two-body potential of the form $N^{-1}v(x)$. The system is studied at inverse temperatures of order $N^{-2\mathrm{/}3}$, which corresponds to the temperature scale of the Bose–Einstein condensation phase transition. We show that spontaneous $U(1)$ symmetry breaking occurs if and only if the system exhibits Bose–Einstein condensation in the sense that the one-particle density matrix of the Gibbs state has a macroscopic eigenvalue.