  The appearance of tracks, close to classical orbits, left by charged quantum
particles propagating inside a detector, such as a cavity periodically
illuminated by light pulses, is studied for a family of idealized models. In
the semi-classical regime, which is reached when one considers highly energetic
particles, we present a detailed, mathematically rigorous analysis of this
phenomenon. If the Hamiltonian of the particles is quadratic in position- and
momentum operators, as in the examples of a freely moving particle or a
particle in a homogeneous external magnetic field, we show how symmetries, such
as spherical symmetry, of the initial state of a particle are broken by tracks
consisting of infinitely many approximately measured particle positions and
how, in the classical limit, the initial position and velocity of a classical
particle trajectory can be reconstructed from the observed particle track.
