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Nuclear Reactions for Neutrinoless Double Beta Decay

Periodic Reporting for period 2 - NURE (Nuclear Reactions for Neutrinoless Double Beta Decay)

Reporting period: 2018-10-01 to 2020-03-31

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 “comfortably” explored in laboratory by the use of accelerated ion beams and detectors.
NURE plans to carry 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.
A part of the work within the NURE activity has concerned the optimization of the existing apparatus to perform the experimental activity. It has focused on the improvement of the particle identification technique, of the resolution and sensitivity performances of the spectrometer and related detectors and of the tolerable detection rate. This work, that will continue during the full project duration, has led to the publication of technical articles.
The experimental activity with accelerated beams is started and the first systems of interest for neutrinoless double beta decay have been explored. A detailed study has been initiated on the elastic and inelastic scattering and on the multi-nucleon transfer channels. The study of these channels is important to characterize the reaction and model the mechanism and the conditions under which similarities or differences with neutrinoless double beta decay appear. Encouraging preliminary results show that the multi-nucleon transfer process, which is an alternative route leading to the same final nuclear states not present in the decay and thus not of interest for neutrino issues, gives a negligible contribution. The double charge exchange channel for a number of systems candidate for neutrinoless double beta decay has been measured and a preliminary analysis is already on going extracting the transition probabilities. Scientific papers have already been published on these items and others are planned in the next future. Moreover these preliminary results have been presented at international conferences and workshops.
The extraction from measured quantities of key nuclear structure information requires a complete theoretical description of the reaction mechanism, which was missing in the case of double charge exchange reactions before NURE. An effort, triggered by the NURE physics case, started in the past months and has already conducted to the publication of a review paper on the state of the art and new approaches on heavy ion charge exchange reactions as probes for nuclear beta decay.
An important progress obtained in the implementation of the NURE project concerns the first experimental measurement of the double charge exchange reaction process in some of the nuclei candidate for neutrinoless double beta decay. Not only the precise reaction channel has been identified, but also the precise transition to the ground state, which is the same transition that would be populated in the spontaneous decay, has been measured. Moreover the absolute cross section has been extracted from the data. This measurement represents an experimental challenge for the high request in terms of particle identification, resolution, treatment of the rare events over a large background. For these reasons, the measurement performed at INFN-LNS with the NURE setup can be considered a great progress beyond the state of the art.
The extraction from measured quantities of nuclear structure information helpful for neutrino physics requires a complete theoretical description of the reaction mechanism, which was missing in the case of double charge exchange reactions before NURE. An effort in this direction has started, triggered by the NURE physics case. It has conducted to the introduction of a new mechanism to describe the double charge exchange process, having formal analogies with neutrinoless double beta decay. The existence of such formal analogies is a first step toward the use of double charge exchange nuclear reactions for the extraction of data-driven information on neutrinoless double beta decay.
The next step is the comparison of the theoretically extracted quantities with the measured ones, in order to confirm the reliability of the calculations and estimate the contribution of the above-mentioned mechanism in the nuclear reaction. Preliminary results show that this newly introduced mechanism well describe the data and that the processes in competition with such direct double charge exchange are negligible. The confirmation of these preliminary results would be the achievement of the main goal of NURE. It would give a way to test the nuclear structure input necessary in neutrinoless double beta decay independently, in applications to a technically and physically accessible surrogate process of neutrinoless double beta decay like double charge exchange reactions.