Neutrino-less Double Beta Decay and Particle Physics (1106.1334v3)

Abstract: We review the particle physics aspects of neutrino-less double beta decay. This process can be mediated by light massive Majorana neutrinos (standard interpretation) or by something else (non-standard interpretations). The physics potential of both interpretations is summarized and the consequences of future measurements or improved limits on the half-life of neutrino-less double beta decay are discussed. We try to cover all proposed alternative realizations of the decay, including light sterile neutrinos, supersymmetric or left-right symmetric theories, Majorons, and other exotic possibilities. Ways to distinguish the mechanisms from one another are discussed. Experimental and nuclear physics aspects are also briefly touched, alternative processes to double beta decay are discussed, and an extensive list of references is provided.

• The paper reviews key mechanisms of neutrino-less double beta decay, detailing both the standard light Majorana neutrino exchange and several non-standard scenarios.
• It establishes experimental constraints with half-lives over 10^25 years and effective neutrino masses between 0.2 and 0.5 eV based on nuclear matrix elements.
• It explores implications for particle physics by highlighting possible lepton number violation and insights into neutrino mass hierarchies relevant for theories beyond the Standard Model.

Neutrino-less Double Beta Decay and Particle Physics: Review and Potential Implications

The paper in question provides a comprehensive review of neutrino-less double beta decay ($\betadecay$) focusing on its implications and interpretations in particle physics. The discussion is centered on potential mechanisms that might mediate this decay process and the significant role neutrino physics plays in these interpretations.

Mechanisms of Neutrino-less Double Beta Decay

The primary interpretation of $\betadecay$ is via the exchange of light, massive Majorana neutrinos, a mechanism often referred to as the standard scenario. This process directly tests lepton number violation, a critical phenomenon not predicted within the Standard Model (SM) but anticipated by most theories extending beyond it. The amplitude for this mechanism is proportional to the effective electron neutrino mass, encapsulated in the parameter $\meff$.

Beyond the standard interpretation, the paper elaborates on various non-standard interpretations where $\betadecay$ might be mediated by alternative processes or particles. These include:

1. Heavy Majorana Neutrinos: If neutrinos are Majorana particles with masses much heavier than the typical energies involved in $\betadecay$ ($\sim$ 0.1 GeV), the decay rate would depend inversely on neutrino mass.
2. Supersymmetric and Left-Right Symmetric Models: Here, the decay could be mediated by superpartners or via right-handed currents, respectively. Each of these scenarios introduces additional particle physics parameters and potential experimental signatures.
3. Emission of Exotic Particles: Processes involving Majoron emission add extra particles into the final state, modifying the spectrum of emitted electrons and providing a distinctive experimental signature.

Experimental and Numerical Aspects

The paper reviews both experimental and theoretical efforts to determine the half-life of $\betadecay$. Current experiments limit the $\betadecay$ half-life to $T^{0\nu}_{1/2} > 10^{25}$ years, setting constraints on $\meff$ between 0.2 and 0.5 eV, depending on the chosen nuclear matrix elements (NMEs) and isotopes. The paper emphasizes the importance of NMEs in interpreting experimental results, acknowledging significant uncertainties that complicate the extraction of particle physics parameters from experimental data.

Implications and Future Directions

Detecting $\betadecay$ would have profound theoretical implications by confirming the Majorana nature of neutrinos and providing insights into neutrino mass hierarchies and absolute mass scales. It would directly impact theories on leptogenesis and baryogenesis, offering clues about the matter-antimatter asymmetry in the universe.

Future experimental developments aim to reduce backgrounds and enhance sensitivity to reach effective mass limits below the inversion hierarchy predictions, thus potentially differentiating between normal and inverted mass ordering.

Conclusions

The pursuit of $\betadecay$ spans beyond neutrino physics, intertwining with a broader quest to uncover physics beyond the Standard Model, probing the very nature of neutrino masses, lepton number violation, and related grand unification theories. As experimental techniques advance, the intricate interplay of phenomenology and theory in understanding $\betadecay$ continues to inspire and challenge the field of particle physics, highlighting the need for precise measurements and innovative theoretical frameworks. The research holds promise for illuminating fundamental aspects of our universe, potentially reshaping contemporary understanding of particle physics and cosmology.