neutrinoless

Neutrinoless refers to a class of processes that occur without emission of neutrinos. The most studied example is neutrinoless double beta decay (0νββ), a hypothetical nuclear transition in which two neutrons in a nucleus convert into two protons with the emission of two electrons but without any neutrinos. In contrast, the standard (two-neutrino) variant emits two electrons and two antineutrinos. Neutrinoless double beta decay would require that neutrinos be Majorana particles, meaning they are their own antiparticles, and it would violate lepton number conservation by two units.

Observation of 0νββ would have profound implications for particle physics and cosmology: it would establish lepton-number violation and demonstrate the Majorana nature of neutrinos, offering insight into the origin of neutrino masses and potentially the matter-antimatter asymmetry of the universe. The decay rate is governed by the effective Majorana mass, a combination of neutrino masses and mixing angles, and by nuclear matrix elements that are subjects of theoretical uncertainty.

Experiments search for a discrete energy peak at the Q-value of the decay in the sum of the energies of the two emitted electrons. Achieving ultra-low background and excellent energy resolution is essential. Isotopes widely used include 76Ge, 136Xe, 130Te, 100Mo, and 48Ca. Detector approaches range from high-purity germanium detectors to xenon time-projection chambers and cryogenic calorimeters.

To date no confirmed observation has been made. Lower limits on the half-life of 0νββ extend beyond 10^26 years in several isotopes, corresponding to upper limits on the effective Majorana mass that depend on nuclear-model assumptions. Next-generation experiments such as LEGEND, nEXO, CUPID, and upgrades to KamLAND-Zen and CUORE aim to improve sensitivity by an order of magnitude or more.