Multiple scattering dynamics of fermions at an isolated p-wave resonance

Abstract

The wavefunction for indistinguishable fermions is anti-symmetric under particle exchange, which directly leads to the Pauli exclusion principle, and hence underlies the structure of atoms and the properties of almost all materials. In the dynamics of collisions between two indistinguishable fermions, this requirement strictly prohibits scattering into 90° angles. Here we experimentally investigate the collisions of ultracold clouds fermionic 40K atoms by directly measuring scattering distributions. With increasing collision energy we identify the Wigner threshold for p-wave scattering with its tell-tale dumb-bell shape and no 90° yield. Above this threshold, effects of multiple scattering become manifest as deviations from the underlying binary p-wave shape, adding particles either isotropically or axially. A shape resonance for 40K facilitates the separate observation of these two processes. The isotropically enhanced multiple scattering mode is a generic p-wave threshold phenomenon, whereas the axially enhanced mode should occur in any colliding particle system with an elastic scattering resonance.

Crossover from the Ultracold to the Quasiclassical Regime in State-Selected Photodissociation

“…Approaches to exploring the formation and breaking of molecular bonds across the low energy regime include a variety of techniques such as photoassociation [1], ultracold atomic collisions [2,3], ultracold molecular collisions [4], state-selected atomic and molecular beams with relative velocity control [5][6][7][8][9][10], and molecule collisions with cold trapped ions [11]. Each method has its benefits such as high tunability of collision energies, the ability to access ultralow energies, or the possibility to handle diverse molecular species.…”

Homogeneous Atomic Fermi Gases

“…Consequences of Pauli blocking have been observed in ultracold gases, for example, in nondegenerate samples, the reduction of collisions in spin-polarized gases below the p-wave threshold [2,37] and, upon entering degeneracy, Pauli pressure [38,39], reduced collisions [40,41], antibunching in noise correlations [42], and the reduction of density fluctuations [43,44]. In optical lattices under microscopes, Pauli blocking has been observed in real space through observations of band insulating states [16,17,45] and of the Pauli hole in pair correlations [20].…”

Quantum Scattering in an Optical Collider for Ultracold Atoms

“…In this collisionally opaque regime, multiple scattering events become very important. Near the peak of the resonance, higher-order scattering events are most likely to result in p-wave collision halos that are slightly rotated from the collision axis, and an average over all these rotations gives rise to an apparent isotropic scattering halo that is superimposed on the expected p-wave halo [23]. At energies well above the shape resonance, multiple scattering events give rise to an enhancement in the number of atoms scattered close to the collision axis.…”

“…At low energies and for short-ranged interactions, all scattering is via the = 1 partial wave due to Wigner threshold suppresion [22] and the wavefunction for outgoing fermions in the center-of-mass frame must have a p-wave angular distribution proportional to cos 2 θ. Using our optical collider, we accelerated two clouds of fermionic 40 K atoms each in the |F = 9 2 , m F = 9 2 state towards each other at collision energies from 50 to 1800 μK [23]. Figure 2 shows a post-collision image of the outgoing clouds accompanied by a halo of scattered particles for an experiment conducted at an energy of 150 μK.…”