Periodic Reporting for period 2 - MELODIC (Molecule for low diffusion TPCs for rare event searches) Berichtszeitraum: 2018-09-18 bis 2019-09-17 Zusammenfassung vom Kontext und den Gesamtzielen des Projekts 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. Arbeit, die ab Beginn des Projekts bis zum Ende des durch den Bericht erfassten Berichtszeitraums geleistet wurde, und die wichtigsten bis dahin erzielten Ergebnisse During the outgoing phase at UTA (Texas) (18/09/2017 - 18/10/2018), I built the Teapot detector at the University of Texas at Arlington that is currently being upgraded to perform measurements of xenon plus helium mixtures. We published a paper regarding the concentration of CH4 needed (0.4%) to reduce diffusion by a factor of 5 without affecting the energy resolution. In addition, we published a paper on xenon plus helium mixtures. We found that adding 20% of helium to pure xenon can also decrease the transverse diffusion from 10 to 2 mm/√m. During the outgoing phase, I also published a paper with Prof. Nygren on a new detector concept for neutrinoless double beta decay searches: a high pressure selenium hexafluoride gas time projection chamber. Finally, I have been coordinating the re-design of the DEMO++ detector to be ready to hold both CH4 and He additives separately.Since my return to IFIC, (31/10/2018-20/10/2019) I constructed the DEMO++ detector together with the technical team and my PhD student. We operated DEMO++ with pure xenon and with the mixtures and finally achieved the main goal of Melodicwhich is the measurement of the transverse diffusion. We measured that the addition of 15 % of Helium to pure xenon decreases the transverse diffusion a factor of 3 with respect to the pure xenon configuration. This is the first direct measurementof the effect of using helium mixtures to reduce the transverse diffusion in a high pressure xenon gas time projection chamber with electroluminescent amplification. Fortschritte, die über den aktuellen Stand der Technik hinausgehen und voraussichtliche potenzielle Auswirkungen (einschließlich der bis dato erzielten sozioökonomischen Auswirkungen und weiter gefassten gesellschaftlichen Auswirkungen des Projekts) The current upper limits on the Majorana neutrino mass are in the range between 60-600 meV. The most stringent limits have been obtained using Ge-76 and Xe-136 isotopes by GERDA (using germanium diodes) and KAMLAND-Zen (Liquid scintillator Xe based) collaborations. However, the current generation of 0nubb experiments do not still have the potential to explore the neutrino inverted mass hierarchy region, which corresponds to the mass range of 15-50 meV. If no signal is found by the current generation of 0nubb experiments, isotope masses in the tonne scale and further background reduction will be required. The current background measurements suggest that the sensitivity to the 0nubb search by all proposed tonne-scale experiments is still limited by the amount of background events. Therefore, the improvement of the detector technologies is mandatory.The expected final result of the MELODIC project is to improve the HPGXe-EL technology to substantially enhance the expected sensitivity to the 0nubb decay. The increase on the expected sensitivity for a tonne-scale is of a factor two, increasing considerably with exposure. MELODIC has demonstrated that the helium mixtures can reduced a factor of 3 the transverse diffusion with respect to pure xenon. The work performed in MELODIC will be continue by the N. López-March, which is currently the coordinator of the DEMO++ experiment to explore further gas mixtures to reduce the diffusion of the electrons and hence improve the imaging capability of the NEXT technology. Me building the DEMO++ detector at IFIC.