Uncovering the Origin of Neutrino Masses through Direct Searches and Global Fits

Neutrinos are the second most abundant particle in our Universe. They are produced in nuclear and particle physics processes, and affect both the microscopic subatomic world and the evolution of astrophysical objects. Even if neutrinos play a key role in our life they typically go unnoticed. They are extremely elusive. Being able to interact only through gravity and the weak force, they can fly undisturbed through entire galaxies. Neutrinos eluded detection until 1956 and their properties are still being investigated. The 2015 nobel prize was awarded for the discovery that neutrinos have mass. This empirical fact cannot be explained by the Standard Model of particle physics and it is probably the strongest evidence that there are deeper and still hidden laws in Nature.

This proposal aims at advancing our understanding of particle physics by uncovering the origin of neutrino masses through direct measurements and a global analysis.

I will search for a nuclear transition (called neutrinoless double-beta decay) with the LEGEND experiment. The discovery of neutrinoless double-beta decay would establish that neutrinos are their own antiparticles, a property required by many theories to explain their masses. Searching for neutrinoless double-beta decay is the only feasible way to prove this neutrino property but it is challenging. The current experimental constraints are already extremely stringent: a nucleus does not undergo this transition not event in a million billion times the age of the Universe. To observe such a rare process a large number of nuclei must be observed in an ultralow-background environment.

LEGEND aims at improving the sensitivity of the current searches by orders of magnitude by operating hundreds of semiconductor detectors (up to 1e27 nuclei) in a bath of ultrapure liquid argon. The liquid argon will provide shielding against natural radioactivity that can mimic a signal. Furthermore, semiconductor devices provide topological information about the location of the events that can also be used to increase the signal-to-noise ratio. Thanks to the combination of these two technologies, the total number of background events expected per year is less than 0.001. This will make LEGEND sensitive to the occurrence of even a single neutrinoless double-beta decay event.

I am currently preparing the first phase of the LEGEND experiment. With the fellowship, I will lead the commissioning of the experimental infrastructure and the subsequent data taking of the experiment. To handle the large number of semiconductor detectors operated in LEGEND, I will develop automatic monitoring and calibration tools as well as the offline event reconstruction algorithms. I will also model the data of the experiment to track the origin of background events and develop the statistical analysis for the signal extraction. By the end of this fellowship, I will either discover this nuclear transition or set word-leading constraints on its rate. At the same time I will explore innovative semiconductor technologies for future searches able to further increase the experimental sensitivity.

The results of LEGEND will be combined with data from neutrino oscillation experiments and cosmological surveys to test the theoretical models proposed to explain neutrino masses. By joining forces with experts in different experimental and theoretical areas that are present at University College London, I will develop a comprehensive analysis framework that makes direct use of the data of each experiment. By exploiting the synergy between data sets and considering all sources of systematic uncertainties, various classes of models will be ruled out or corroborated, leading to a significant advancement in our understanding of neutrino masses.