In recent years extensive theoretical and experimental studies of universal few-body physics have led to advances in our understanding of universal Efimov physics. Whereas theory had been the driving force behind our understanding of Efimov physics for decades, recent experiments have contributed an unexpected discovery. Specifically, measurements have found that the so-called threebody parameter determining several properties of the system is universal, even though fundamental assumptions in the theory of the Efimov effect suggest that it should be a variable property that depends on the precise details of the short-range two-and three-body interactions. The present Letter resolves this apparent contradiction by elucidating previously unanticipated implications of the two-body interactions. Our study shows that the three-body parameter universality emerges because a universal effective barrier in the three-body potentials prevents the three particles from simultaneously getting close together. Our results also show limitations on this universality, as it is more likely to occur for neutral atoms and less likely to extend to light nuclei. PACS numbers: 31.15.ac,31.15.xj, In the early 70's, Vitaly Efimov predicted a strikingly counterintuitive quantum phenomenon [1], today known as Efimov effect: in three-body systems for which the two-body s-wave scattering length a is much larger than the characteristic range r 0 of the two-body interaction, an infinite number of three-body bound states can be formed even when the short-range two-body interactions are too weak to bind a two-body state (a < 0). The Efimov effect, once considered a mysterious and esoteric effect, is today a reality that many experiments in ultracold quantum gases have successfully observed and continued to explore [2][3][4][5][6][7][8][9][10][11][12][13][14]. One of the most fundamental assumptions underlying our theoretical understanding of this peculiar effect is that the weakly bound three-body energy spectrum, and other low-energy three-body scattering observables, should depend on a three-body parameter that encapsulates all details of the interactions at short distances [15]. For this reason, the three-body parameter has been viewed as nonuniversal since its value for any specific system would depend on the precise details of the underlying two-and three-body interactions [16][17][18].In nuclear physics, this picture seems be consistent, i.e., three-body weakly bound state properties seem to be sensitive to the nature of the two-and three-body short-range interactions [17]. More recently, however, Berninger et al. [3] have directly explored this issue for alkali atoms whose scattering lengths are magnetically tuned near different . Even though the short-range physics can be expected to vary from one resonance to another, Efimov resonances were found for values of the magnetic field at which a=a − 3b =−9.1(2)r vdW , where r vdW is the van der Waals length [20, 21]. Therefore, in each of these cases, the three-body parameter was approximatel...

A general method to study classical scattering in n-dimension is developed. Through classical trajectory calculations, the three-body recombination is computed as a function of the collision energy for helium atoms, as an example. Quantum calculations are also performed for the J(Π) = 0(+) symmetry of the three-body recombination rate in order to compare with the classical results, yielding good agreement for E ≳ 1 K. The classical threshold law is derived and numerically confirmed for the Newtonian three-body recombination rate. Finally, a relationship is found between the quantum and classical three-body hard hypersphere elastic cross sections which is analogous to the well-known shadow scattering in two-body collisions.

We theoretically study the superfluid phase of a strongly correlated 173 Yb Fermi gas near its orbital Feshbach resonance, by developing a quantitative pair-fluctuation theory within a two-band model. We examine the density excitation spectrum of the system and determine a stability phase diagram. We find that the 173 Yb Fermi gas is intrinsically metastable and has a peculiar equation of state, due to the small but positive singlet scattering length. The massive Leggett mode, arising from the fluctuation of the relative phase of two order parameters, is severely damped. We discuss the parameter space where an undamped Leggett mode may exist.