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Title: Are neutrinos Majorana particles, their own antiparticles?
Plain: If a neutrino is its own antiparticle, certain nuclei could undergo a double beta decay that emits no neutrinos at all. Its observation would show that lepton number is not conserved.
Precise: Determine whether lepton number is violated by Majorana neutrino masses via neutrinoless double beta decay, with half-life T_1/2 ~ 1e26 to 1e28 yr for $m_{bb} \sim 10\ \text{to}\ 50\ \mathrm{meV}$. KamLAND-Zen (2024) gives T_1/2(Xe-136) > 3.8e26 yr, $m_{bb} < \sim 28\ \text{to}\ 122\ \mathrm{meV}$ (spread from nuclear matrix elements).
Settled by: Ton-scale double beta decay experiments (LEGEND-1000, nEXO, CUPID, KamLAND2-Zen) covering the inverted-ordering band $m_{bb} \sim 15\text{ to }50\,\mathrm{meV}$.
Kind: well-posed
Answer type: yes-no
References: (empty)
Literature status: open
Status note: No signal through 2025; the inverted-ordering band is partly probed.
Posed since: 1937