All the structures that we see in the Universe like stars, galaxies, clusters, consist of matter. There is no antimatter in appreciable quantities and the Standard Model (SM) of particle physics alone cannot account for it. Why there is such more matter than antimatter in the Universe is one of the most important open questions in particle physics nowadays.
The discovery of the neutrino as Majorana particle means that it is indistinguishable from its antiparticle. This will have profound implications for cosmology, as Majorana neutrinos violate lepton number and could explain matter-antimatter difference in the Universe.
Currently, the most sensitive experimental method to establish that neutrinos are Majorana particles is the search for neutrinoless double beta decay (0nubb). The NEXT experiment is seeking for this decay in a high pressure xenon gas (HPGXe) time projection chamber (TPC) with electroluminescent (EL) amplification. This technology is one of the most promising as it can uniquely image the track of the two electrons emitted in a bb0nu event, together with high energy resolution. However, the current imaging performance is limited by the diffusion of the ionization electrons during drift. The goal of the Marie Sklodowska Curie project MELODIC is to reduce this diffusion by a factor 3 at 10 bar pressure by adding additives like He or CH4 to the pure xenon gas. This could enhance the sensitivity of the HPGXe-EL technology to the bb0nu decay by a factor 2 at 15 bar pressure.
The main goal of the project has been achieved. We have demonstrated experimentally that adding 15% of Helium to the pure xenon can reduce the transverse diffusion of the pure xenon configuration a factor of 3 at a total pressure of 10 bar.
