Systems, such as cold atoms and light halo nuclei, share universal features at low energies, for which details of inter-particle interactions are not essential. We study three-body physics in such systems in an effective-field-theory (EFT) framework.
The ground-state of Helium-6 can be treated as a two-neutron halo with an alpha core. This bound state is generated by the resonant neutron-neutron and neutron-alpha interactions. The latter is dominated by a shallow p-wave resonance. We present the first calculation of helium-6 in an EFT approach appropriate for halo systems. In our leading-order study, an n-n-alpha three-body counterterm is needed for proper renormalization. We adjust the coefficient of this counterterm to reproduce the Helium-6 ground-state energy, and present its running as a function of the cutoff.
We then discuss higher-order corrections beyond three-body universality. We study effective-range effects perturbatively up to N2LO for the case of three identical bosons. We apply this analysis to recombination features in cold atomic gases and properties of Helium trimers. The range effects also play an important role in the EFT study of few-nucleon systems and halo nuclei.
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