  The NEXT neutrinoless double beta decay experiment will use a high- pressure
gas electroluminescence-based TPC to search for the decay of Xe-136. One of the
main advantages of this technology is the possibility to reconstruct the
topology of events with energies close to Qbb. The rejection potential
associated to the topology reconstruction is limited by our capacity to prop-
erly reconstruct the original path of the electrons in the gas. This
reconstruction is limited by different factors that include the geometry of the
detector, the density of the sensors in the tracking plane and the separation
among them, etc. Ultimately, the resolution is limited by the physics of
electron diffusion in the gas. In this paper we present a series of molecular
additives that can be used in Xenon gas at very low partial pressure to reduce
both longitudinal and transverse diffusion. We will show the results of
different Monte-Carlo simulations of electron transport in the gas mixtures
from wich we have extracted the value of some important parameters like
diffusion, drift velocity and light yields. These results show that there is a
series of candidates that can reduce diffusion without affecting the energy
resolution of the detector and they should be studied experimentally. A
comparison with preliminary results from such an ongoing experimental effort is
given.

Instrumentation and Detectors;High Energy Physics - Experiment;Nuclear Experiment