Towards the discovery of neutrinoless double beta decay : first results from LEGEND

The sheer abundance of neutrinos all around us makes it hard to believe that we still don’t understand so much about them. It is this allusiveness that makes research into neutrino physics so fascinating, and within this unknown, many mysteries about the universe could be unfolded.

A discovery of the hypothetical neutrinoless double beta decay (0$\nu\beta\beta$) process would have profound implications. It would prove that lepton number is not a conserved quantity, prove that neutrinos are Majorana in nature, and constrain the mass hierarchy.

The LEGEND experiment is designed to search for 0$\nu\beta\beta$ using Ge-76 enriched high purity germanium detectors that are immersed in liquid argon. LEGEND-200 (L200), operating at LNGS in Italy, builds on the successes in background suppression and analysis techniques from the Majorana Demonstrator and GERDA experiments. L200’s first results are based on 61 kgyr of exposure with an estimated background index of $0.5^{+0.3}_{-0.2}$ cts/(keV ton yr). Data from GERDA and the Majorana Demonstrator were combined with L200’s for a joint analysis, yielding a 90% CL sensitivity of $2.8\times10^{26}$ yr and setting a new lower limit of > $1.9\times10^{26}$ yr, for the half-life of 0$\nu\beta\beta$. Assuming the decay mechanism is mediated by the exchange of a light Majorana neutrino, this half-life limit corresponds to an upper limit on the effective Majorana mass of $m_{\beta\beta}$ < 75-200 meV. In an attempt to encourage our nuclear theory colleagues, an additional limit has been set, with $m_{\beta\beta}$< 320 meV, utilising the recent developments in uncertainty-quantified nuclear matrix elements.

In this talk, I will present L200’s first results published in the recently released paper (https://arxiv.org/pdf/2505.10440) with some added detail about the detector technology. I will also briefly showcase the outlook for the next phase of the LEGEND experiment, L1000, with some context for the UK double beta decay community.