Probing the nature of the neutrino with large scale dark matter detectors
PT-CERN Call 2022/2
Claudio Frederico Pascoal da Silva
Universidade de Coimbra
Laboratório de Instrumentação e Física Experimental de Partículas
Neutrinoless double-beta decay (0𝜈𝛽𝛽) is one of the most important topics in modern particle physics, offering a unique opportunity to discover physics beyond the Standard Model. In a neutrinoless double-beta decay, a nucleus with mass number A and charge Z undergoes the decay (A, Z) → (A, Z+2) + 2e- but no neutrinos are emitted. This is not allowed in the Standard Model of particle physics as it violates the conservation of the lepton number, hinting that leptons play a part in the observed Universe matter/antimatter asymmetry. It would also probe the Majorana nature of neutrinos, i.e. if neutrinos are their own antiparticles, and would provide information about the neutrino mass hierarchy and effective mass. As such the observation of this process would be a breakthrough in modern physics. Xe-136, which comprises 9% of natural xenon, is one of the few isotopes that are expected to undergo this decay. This makes xenon detectors designed towards the search for dark matter particles extremely competitive in the search for this process, given their large masses and extremely low backgrounds, reaching sensitivities similar to dedicated 0𝜈𝛽𝛽 experiments. LUX-ZEPLIN (LZ), which started operating in late 2021, is one of such detectors. Employing ten tonnes of liquid xenon, it contains a total of 630 kg of Xe-136 in its sensitive region. A preliminary study using simulated data showed that it could reach a half-life sensitivity of 1.06x10^26 years after 1000 days of operation, similar to the current best results. This study used conservative assumptions on the background assessment, energy resolution and background discrimination that can be significantly improved with real data. A next generation (G3) detector is already being planned, with a total mass up to 10x larger, which can lead to a half-life sensitivity 100x higher than LZ, leading to the confirmation (in case of an observation) or exclusion of the inverted hierarchy for the neutrino masses.