Universality in strongly interacting bosonic clusters

We develop an effective field theory (EFT) for strongly interacting bosonic clusters, using $^4$He as a paradigmatic example of universality in systems with large scattering length.

At leading order (LO), two- and three-body zero-range interactions are entirely determined by the dimer and trimer ground-state energies.

We show that ground-state energies for up to $N=15$ particles converge to cutoff-independent limits with extrapolation coefficients of natural size.

At next-to-leading order (NLO), corrections stemming from the two-body interaction range and a four-body force, calibrated to the tetramer ground-state energy, reduce cutoff sensitivity.

Close agreement with results from a realistic potential is found at LO and improved at NLO, demonstrating systematic convergence with few parameters at each order.

The resulting EFT is directly applicable to larger clusters and bulk helium.