In the limit of ultra-low collision energies, the interactions between particles are dominated by s-wave scattering and higher partial waves are negligible. In this case, the two-body interaction can be characterized by a single parameter, the s-wave scattering length a, if the inter-particle potential is short-ranged. This simplification enables us to describe entirely different systems such as nuclei, molecules and neutral atoms universally. In this universal regime, quantum correlations between particles lead to exotic few-body phenomena, whose paradigm is the Efimovs prediction of a series of three-boson states. In the last decade, ultra-cold atomic gases have opened up the possibility to explore Efimovs scenario experimentally and to test further predictions of universal theory.

This thesis reports on a series of studies on Efimov physics performed with ultracold cesium gases. We first present a thorough study of the scattering properties of cesium atoms over a wide range of magnetic fields, where many magnetically induced Feshbach resonances enable us to tune the scattering length by varying the magnetic field. We then present a work in which three new triatomic Efimov resonances are observed at three broad Feshbach resonances. By comparing the four observed Efimov states in cesium, we find the value of the three-body parameter (3BP), which fixes the Efimov spectrum, to be approximately constant. This work has triggered the discovery of the ‘van der Waals universality, which attributes the constancy of the 3BP to the van der Waals interaction between neutral atoms. On the other hand, we also measure the atom-dimer recombination resonance caused by an Efimov state and observe a substantial deviation from the universal relation. But these non-universal results can still be explained by a coupled-channel model in which the van der Waals interaction is considered. Furthermore, we observe a resonance due to a universal five-body state associated to an Efimov state. This is an important step towards understanding the general nature of N-body cluster states and their implications for many-body physics.

One distinguishing feature of the Efimov states is the discrete scaling symmetry, which predicts that the positions of Efimov resonances caused by successive Efimov states are connected by a universal factor of 22.7 in scattering length in the case of identical bosons. We observe an excited-state Efimov resonance and compare it with the ground-state one, extracting a value of 21.0(1.3) for the scaling factor. The small difference between the measured value and the ideal one suggests non-universal effects caused by the finite interaction range, which may particularly influence the position of the ground-state Efimov resonance.

In our analysis of the excited-state Efimov resonance, finite-temperature effects are considered by a universal zero-range model. We extend this model to consider distinguishable atoms and use it to analyze the excited-state Efimov resonance of 6 Li in a previous work.

A precise value of the 3BP is obtained and compared with the one from cesium. The deviation between the 6 Li and cesium data shows that the van der Waals universality is only approximately fulfilled. Moreover, we perform finite-temperature measurements over a large temperature range on the ground-state Efimov resonance to test the universal zero-range theory directly. The prediction and observation are found to be generally consistent, while a small deviation between them is characterized by an empirical function and attributed to the finite-range of the van der Waals potential.