Abstract:
We study a dilute Bose gas of atoms whose scattering length
a is large compared to the range of their interaction. We
calculate the energy density of the homogeneous
Bose-Einstein condensate to second order in the low-density
expansion, expressing it in terms of a and a second
parameter Lambda_* that determines the low-energy
observables in the 3-body sector. The second-order
correction to the energy density has a small imaginary part
that reflects the instability due to 3-body recombination.
In the case of a trapped Bose-Einstein condensate with large
negative scattering length, we calculate the coefficient of
the 3-body mean-field term in the energy density in terms of
a and Lambda_*. It can be very large if there is an Efimov
state near threshold.