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Energy dependent $\ell$-wave confinement-induced resonances

The universal aspects of two-body collisions in the presence of a harmonic confinement are investigated for both bosons and fermions. The main focus of this study are the confinement-induced resonances (CIR) which are attributed to different angular momentum states $\ell$ and we explicitly show that in alkaline collisions only four universal $\ell$-wave CIRs emerge. Going beyond the single mode regime the energy dependence of $\ell$-wave CIRs is studied. In particular we show that all the $\ell$-wave CIRs may emerge even when the underlying two-body potential cannot support any bound state. We observe that the intricate dependence on the energy yields resonant features where the colliding system within the confining potential experiences an effective free-space scattering. Our analysis is done within the framework of the generalized $K$-matrix theory and the relevant analytical calculations are in very good agreement with the corresponding ab initio numerical scattering simulations.