Are neutrinos Majorana particles, their own antiparticles?
In plain words
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 statement
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).
What would settle it
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 in the literature
Unverified note
No signal through 2025; the inverted-ordering band is partly probed.
Related problems
- Special case of What mechanism generates neutrino masses?