  The topological superconductor UPt3, has three distinct vortex phases, a
strong indication of its unconventional character. Using small-angle neutron
scattering we have probed the vortex lattice in the UPt3 B phase with the
magnetic field along the crystal c-axis. We find a difference in the vortex
lattice configuration depending on the sign of the magnetic field relative to
the field direction established upon entering the B phase at low temperature in
a field sweep, showing that the vortices in this material posses an internal
degree of freedom. This observation is facilitated by the discovery of a field
driven non-monotonic vortex lattice rotation, driven by competing effects of
the superconducting gap distortion and the vortex-core structure. From our bulk
measurements we infer that the superconducting order parameter in the UPt3 B
phase breaks time reversal symmetry and exhibits chiral symmetry with respect
to the c-axis.
