The knowledge of the nature and properties of neutrinos is one of the most relevant researches on fundamental physics nowadays. Neutrinos are among the most abundant particles in the Universe. They are also the lightest of all the known subatomic particles that have mass. However, they tend to cross matter undetected, which makes it extremely challenging to have information on their properties, as their mass or their feature to be at the same time matter and anti-matter. One of the most fascinating hypotheses on neutrinos, indeed, is that neutrino and antineutrino are the same particle, as proposed by Ettore Majorana in 1937. If this were true, the basic principles on which our understanding of the Universe is based would be violated, opening new horizons to knowledge.
A key role in this research is played by a very rare radioactive decay, neutrinoless double beta decay. If experimentally observed, it would confirm the hypothesis of Majorana and would give an answer to the neutrino absolute mass scale. Unfortunately, the experimental observation of such decay is very difficult and has not been measured yet. The NURE project aims to make an innovative contribution in this research field. The idea is to use nuclear reactions, in particular double charge exchange reactions, as surrogate process of neutrinoless double beta decay.
While being mediated by different interactions, the two processes - double beta decay and double charge exchange reaction - have many common aspects. The advantage of using nuclear reactions, as proposed by NURE, is that they can be explored in laboratory by the use of specific accelerated ion beams and detectors.
NURE carried out a campaign of experiments at INFN-Laboratori Nazionali del Sud (Italy) using accelerated beams impinging on different target nuclei candidates for neutrinoless double beta decay. The main objective of NURE is to extract data-driven information on the nuclear terms involved in neutrinoless double beta, by measuring for the first time the surrogate of neutrinoless double beta decay.
The research infrastructures involved in NURE are unique in the world and the experiments are being performed for the first time. When a new experimental challenge is faced continuous improvements and upgrades of the system are necessary to guarantee the results. Taking into account the great technology involved in the NURE experiments, also the development of new technologies play an important role. An aspect of NURE involving technological outcomes regards the opportunity to increase the beam intensity in order to improve the statistical significance of the measurements of the much-suppressed double charge exchange processes. This demand has led to a project of upgrade of the INFN-LNS experimental infrastructure to increase the beam current by two orders of magnitude. The increase of the beam intensity and consequently of the rate of particles at the detectors requires a specific care on the development of the front end and readout electronics which should be fast enough and tolerant to high rate. Other examples where the fundamental research of NURE could lead to applications are the study of radiation hard materials, such as the isotopically enriched target foils, the gas tracker detector and the solid-state particle identification wall. Such developments have been triggered by NURE and have lead to collaborations with academic and research institutions as well as specialized companies.
The involvement of students and young researchers in a project with deep scientific and technological outcomes is of great educational impact, contributing to create a center of development of knowledge not only in the field of nuclear and neutrino physics. In this context NURE plays an important role in generating knowledge, forming critical and analytical thinking and enabling unexpected long-term applications.
At the conclusion of the project one can say that the objectives proposed by NURE have been achieved. In particular, the first experimental measurements of double charge exchange (DCE) cross section for transitions involving nuclei candidates for neutrinoless double beta decay have been performed with success. A theory to describe the reaction mechanisms contributing to the observed DCE cross section has been developed for the first time, highlighting the similarities with the neutrinoless double beta decay process. A further important step to gather the two scientific communities of neutrino and nuclear physics, that were often separated and weakly interacting among them, have been done thanks to NURE. Moreover, thanks to the dissemination activities organized within NURE, we have contributed to the involvement of the general public, with the aim to make it aware of the scientific objectives and challenges of the NURE project, but also about the scientific research in general, the research institutions and the role of Europe and of the ERC programme in supporting frontier research.
