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Its observation would show that lepton number is not conserved.","posed_since":"1937","precise":"Determine whether lepton number is violated by Majorana neutrino masses via neutrinoless double beta decay, with half-life T_1/2 $\\sim 10^{26}$ to $10^{28}$ yr for $m_{bb} \\sim 10\\ \\text{to}\\ 50\\ \\mathrm{meV}$. KamLAND-Zen (2024) gives T_1/2(Xe-136) > $3.8 \\times 10^{26}$ yr, $m_{bb} < \\sim 28\\ \\text{to}\\ 122\\ \\mathrm{meV}$ (spread from nuclear matrix elements).","problem_ref":"f9794321318da5a15c677d4d95fcca9b9be6737584554573c57eaed87bfb4769","references":"","settled_by":"Ton-scale double beta decay experiments (LEGEND-1000, nEXO, CUPID, KamLAND2-Zen) covering the inverted-ordering band $m_{bb} \\sim 15\\text{ to }50\\,\\mathrm{meV}$.","status_note":"No signal through 2025; the inverted-ordering band is partly probed.","title":"Are neutrinos Majorana particles, their own 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