Are Neutrinos Majorana Particles?

This is an elementary account of neutrinos as Majarona particles and the search for neutrinoless double beta decays. It also includes some more ideas about neutrinos.

💡 Research Summary

The talk provides a comprehensive overview of the theoretical motivation, experimental status, and future roadmap for determining whether neutrinos are Majorana particles—a question that lies at the heart of particle physics and cosmology. It begins by recalling Ettore Majorana’s 1937 proposal that a fermion can be its own antiparticle, a property only possible for electrically neutral particles. Within the Standard Model, the neutrino is the sole neutral fermion, making it the prime candidate for a Majorana nature. The author explains that when neutrinos were thought to be massless, the “confusion theorem” rendered Dirac and Majorana descriptions indistinguishable. The discovery of non‑zero neutrino masses in the last decade, however, re‑opens the distinction and demands experimental verification.

If neutrinos are Majorana, the seesaw mechanism naturally explains why their masses are tiny: heavy right‑handed Majorana states induce tiny effective masses for the observed left‑handed neutrinos. Moreover, lepton number (L) violation becomes possible, providing a pathway for leptogenesis—an early‑universe process that could generate the observed baryon asymmetry. These deep theoretical connections make the search for lepton‑number‑violating processes a priority.

The most direct probe is neutrinoless double‑beta decay (0νββ). In this rare nuclear transition, two electrons are emitted without accompanying neutrinos, violating lepton number by two units. The decay rate is proportional to the effective Majorana mass ⟨mₑₑ⟩, a coherent sum of the three neutrino masses weighted by the PMNS mixing matrix elements and the CP‑violating Majorana phases. Observation of 0νββ would therefore confirm Majorana neutrinos and give access to the absolute mass scale and CP phases that oscillation experiments cannot provide. The author stresses that extracting ⟨mₑₑ⟩ requires precise nuclear matrix element (NME) calculations, a major source of theoretical uncertainty.

Experimentally, two‑neutrino double‑beta decay (2νββ) has been observed in many isotopes, confirming the standard weak interaction picture. In contrast, 0νββ remains unobserved. The 2004 claim by Klapdor‑Kleingrothaus of a signal in ⁷⁶Ge sparked intense debate but has not been reproduced. Current large‑scale experiments—GERDA, MAJORANA, EXO‑200/nEXO, KamLAND‑Zen, CUORE, LEGEND, and others—are pushing background levels down, improving energy resolution, and scaling up detector mass to explore ⟨mₑₑ⟩ down to the tens of meV range.

The talk then shifts to the Indian context. The India‑based Neutrino Observatory (INO) is primarily focused on the magnetised iron calorimeter (ICAL) for atmospheric and accelerator neutrinos, but the author argues that a parallel, well‑funded 0νββ program is essential. He outlines a roadmap: (1) strengthen the dedicated 0νββ team, (2) recruit talent across particle physics, nuclear physics, materials science, chemistry, and engineering, (3) develop R&D for ultra‑low‑background detectors and high‑purity isotopic sources, (4) collaborate internationally on NME calculations, and (5) eventually link the 0νββ effort to a broader dark‑matter search program.

Beyond conventional 0νββ, the author discusses two innovative ideas. First, a Mössbauer‑like resonant capture of antineutrinos from tritium beta decay (³H → ³He + e⁻ + ν̄ₑ) embedded in a solid lattice, which could enhance the capture cross‑section by many orders of magnitude and enable tabletop neutrino experiments, including gravitational red‑shift measurements and short‑baseline oscillation studies. Second, by polarising tritium nuclei and electrons in a strong magnetic field, one could produce a mono‑energetic (18.6 keV), unidirectional antineutrino beam, exploiting the fact that only S‑wave antineutrinos are emitted. Such a beam could be used to probe lepton‑number‑violating processes like μ⁻ → e⁺ conversion with a different mass combination than 0νββ, potentially circumventing cancellations that could render ⟨mₑₑ⟩ very small.

The talk concludes with an analogy to Galileo’s struggle for acceptance, emphasizing that scientific breakthroughs often face skepticism, but persistent, collaborative effort can eventually overcome it. The author calls on the Indian and global community to invest in the necessary infrastructure, human resources, and interdisciplinary cooperation to finally answer whether neutrinos are Majorana particles, a discovery that would reshape our understanding of mass generation, matter‑antimatter asymmetry, and the fundamental symmetries of nature.