nEXO's science

The search for neutrinoless double beta decay is one of the most promising avenues for discovering physics beyond the Standard Model. Its observation would have profound implications for our understanding of fundamental physics and cosmology.

Neutrinos stand out among elementary particles. While we know that they have non-zero masses, these masses have never been measured; we only know that they are at least 1,000,000 times lighter than any of the other Standard Model fermions. This prompts an intriguing question: while the Higgs mechanism imbues the other particles with mass, could there be a new mechanism at play in the neutrino sector?

Masses of the fundamental fermions — Spectrum of fermion masses. Neutrinos are approximately 1,000,000 times lighter than any of the others. Figure from https://arxiv.org/abs/1109.5515.

The most compelling possibility is that neutrinos may be Majorana fermions – meaning, they may not have distinct particle/anti-particle states. This would only be possible for neutrinos, which unlike quarks and charged leptons (the electrons, muons, and tau particles) possess no electric charge. While this possibility is intriguing by itself, it would also provide a new mechanism by which the observable neutrino masses could be extremely small without fine-tuning their coupling to the Higgs field.

While Majorana neutrinos would be a relatively straightforward modification to the theory of elementary particles, this change would have profound implications for fundamental physics. It would introduce a new mechanism for generating masses of elementary particles, and would permit new ways of measuring the absolute mass scale of neutrinos. It would also introduce new interactions that violate CP symmetry and the conservation of lepton number, which is currently thought to be a symmetry of the Standard Model. Both CP violation and lepton number violation (LNV) could be key ingredients for creating the baryon asymmetry, potentially explaining how matter was originally created in the big bang – one of the biggest outstanding questions in cosmology.

Double beta decay feynman diagrams — Feynman diagrams of 2νββ (left) and 0νββ (right). Figure taken from M. Jewell thesis.

The most sensitive experimental test of the Majorana neutrino hypothesis is the study of double beta decay, a form of radioactive decay where a nucleus turns two neutrons into two protons. In normal double beta decay (denoted 2nuBB), this process emits two electrons and two anti-neutrinos, conserving lepton number. If neutrinos are Majorana particles, the two antineutrinos could co-annihilate without leaving the nucleus, leading to a neutrinoless double beta decay (denoted 0nuBB). This decay mode violates conservation of lepton number, and its observation would herald an immediate discovery of new physics.