  A new method to track the motion of a single particle in the field of a
high-finesse optical resonator is described. It exploits near-degenerate
higher-order Gaussian cavity modes, whose symmetry is broken by the phase shift
on the light induced by the particle. Observation of the spatial intensity
distribution behind the cavity allows direct determination of the particle's
position with approximately wavelength accuracy. This is demonstrated by
generating a realistic atomic trajectory using a semiclassical simulation
including friction and diffusion and comparing it to the reconstructed path.
The path reconstruction itself requires no knowledge about the forces on the
particle.
