  High pressure gas detectors offer advantages for the detection of rare
events, where background reduction is crucial. For the neutrinoless double beta
decay of 136Xe a high pressure xenon gas Time Projection Chamber (TPC) combines
a good energy resolution and a detailed topological information of each event.
The ionization topology of the double beta decay event of 136Xe in gaseous
xenon has a characteristic shape defined by the two straggling electron tracks
ending up in two higher ionization charge density blobs. With a properly
pixelized readout, this topological information is invaluable to perform
powerful background discrimination. In this study we carry out detailed
simulations of the signal topology, as well as the competing topologies from
gamma events that typically compose the background at these energies. We define
observables based on graph theory concepts and develop automated discrimination
algorithms which reduce the background level in around three orders of
magnitude while keeping signal efficiency of 40%. This result supports the
competitiveness of current or future double beta experiments based on gas TPCs,
like the Neutrino Xenon TPC (NEXT) currently under construction in the
Laboratorio Subterraneo de Canfranc (LSC).
