Pattern recognition of $^{136}$Xe double beta decay events and background discrimination in a high pressure Xenon TPC

High pressure xenon gas time projection chambers (TPC) for the detection of the neutrinoless double beta decay of 136Xe provide good energy resolution and detailed topological information of events. 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 in two larger energy depositions. With a properly pixelized readout, this topological information is invaluable for performing powerful background discrimination. In this study, we carry out detailed simulations of the signal topology, as well as of 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 the region of interest by around three orders of magnitude, while keeping signal efficiency of 40%. This result supports the competitiveness of current or future ββ experiments based on gas TPCs, such as the Neutrino Xenon TPC (NEXT) currently under construction at the Laboratorio Subterraneo de Canfranc.