  We demonstrate that the application of an external magnetic field could lead
to an improved background rejection in neutrinoless double-beta (0nbb) decay
experiments using a high pressure xenon (HPXe) TPC. HPXe chambers are capable
of imaging electron tracks, a feature that enhances the separation between
signal events (the two electrons emitted in the 0nbb decay of 136Xe) and
background events, arising chiefly from single electrons of kinetic energy
compatible with the end-point of the 0nbb decay (Qbb ). Applying an external
magnetic field of sufficiently high intensity (in the range of 0.5-1 Tesla for
operating pressures in the range of 5-15 atmospheres) causes the electrons to
produce helical tracks. Assuming the tracks can be properly reconstructed, the
sign (direction) of curvature can be determined at several points along these
tracks, and such information can be used to separate signal (0nbb) events
containing two electrons producing a track with two different directions of
curvature from background (single-electron) events producing a track that
should spiral in a single direction. Due to electron multiple scattering, this
strategy is not perfectly efficient on an event-by-event basis, but a
statistical estimator can be constructed which can be used to reject background
events by one order of magnitude at a moderate cost (approx. 30%) in signal
efficiency. Combining this estimator with the excellent energy resolution and
topological signature identification characteristic of the HPXe TPC, it is
possible to reach a background rate of less than one count per ton-year of
exposure. Such a low background rate is an essential feature of the next
generation of 0nbb experiments, aiming to fully explore the inverse hierarchy
of neutrino masses.
