Theory of neutrinoless double beta decay (1205.0649v2)

Abstract: Neutrinoless double beta decay, which is a very old and yet elusive process, is reviewed. Its observation will signal that lepton number is not conserved and the neutrinos are Majorana particles. More importantly it is our best hope for determining the absolute neutrino mass scale at the level of a few tens of meV. To achieve the last goal certain hurdles have to be overcome involving particle, nuclear and experimental physics. Nuclear physics is important for extracting the useful information from the data. One must accurately evaluate the relevant nuclear matrix elements, a formidable task. To this end, we review the sophisticated nuclear structure approaches recently been developed, which give confidence that the needed nuclear matrix elements can be reliably calculated. From an experimental point of view it is challenging, since the life times are long and one has to fight against formidable backgrounds. If a signal is found, it will be a tremendous accomplishment. Then, of course, the real task is going to be the extraction of the neutrino mass from the observations. This is not trivial, since current particle models predict the presence of many mechanisms other than the neutrino mass, which may contribute or even dominate this process. We will, in particular, consider the following processes: (i)The neutrino induced, but neutrino mass independent contribution. (ii)Heavy left and/or right handed neutrino mass contributions. (iii)Intermediate scalars (doubly charged etc). (iv)Supersymmetric (SUSY) contributions. We will show that it is possible to disentangle the various mechanisms and unambiguously extract the important neutrino mass scale, if all the signatures of the reaction are searched in a sufficient number of nuclear isotopes.

• The paper presents a theoretical framework for 0νββ-decay, highlighting mechanisms like light and heavy Majorana neutrino exchanges.
• It details advanced nuclear models such as ISM, QRPA, and EDF to compute matrix elements that connect decay rates to lepton number violation.
• The study outlines experimental challenges and future directions necessary to achieve sensitivity limits for probing neutrino properties beyond the Standard Model.

Insights into the Theory of Neutrinoless Double Beta Decay

Introduction

Neutrinoless double beta decay (0νββ-decay) is a theoretical process with profound implications in particle physics, particularly regarding the nature of neutrinos and the conservation of lepton number. The observation of this decay mode would imply that neutrinos are Majorana particles (i.e., they are their own antiparticles) and that lepton number is not conserved, thus providing insights beyond the Standard Model of particle physics.

Significance of Neutrinoless Double Beta Decay

The discovery of 0νββ-decay would be revolutionary in that it suggests the existence of lepton number violation, a key ingredient in explaining the matter-antimatter asymmetry in the universe. Additionally, it offers a unique laboratory for probing the absolute scale of neutrino masses, a parameter not directly accessible through neutrino oscillation experiments.

Theoretical Framework and Mechanisms

The paper provides a comprehensive theoretical overview of 0νββ-decay, highlighting the key nuclear physics challenges involved in calculating the relevant nuclear matrix elements (NMEs). Accurate NMEs are crucial as they form the bridge between the experimentally observable decay rates and the underlying lepton number violating (LNV) parameters that indicate physics beyond the Standard Model.

Several decay mechanisms are considered:

1. Light Majorana Neutrino Exchange: Traditionally the most discussed mechanism where the effective Majorana mass term of the neutrino mediates the decay.
2. Heavy Majorana Neutrino Contribution: A mechanism involving high-mass neutrino states which can also drive the decay, albeit with distinct nuclear matrix element contributions.
3. Right-Handed Currents and Supersymmetry: Mechanisms that invoke right-handed weak interactions or supersymmetric particles as mediators, further expanding the parameter space and complexity of LNV processes.

Nuclear Physics Challenges

Evaluating precise NMEs is challenging due to the complex nuclear structure of participating isotopes. Various sophisticated nuclear models are employed for these calculations, each with limitations:

• Interacting Shell Model (ISM): Provides full treatment of nuclear correlations but is limited by the number of basis states.
• Quasiparticle Random Phase Approximation (QRPA): Offers a broader configuration space at the cost of certain approximations.
• Energy Density Functional Approach (EDF) and Interacting Boson Model (IBM): Provide alternative descriptions with varying degrees of success and applicability.

Experimental Considerations and Current Status

The experimental landscape of 0νββ-decay searches is rich and varied. Technologies employed range from semiconductor detectors like germanium (GERDA, MAJORANA) to scintillator-based setups (KamLAND-Zen, SNO+). Each experiment faces the challenge of isolating the rare 0νββ-decay signals from background events, necessitating advancements in detector technology, background reduction, and isotope enrichment.

Future Directions

Future directions in both theory and experiment aim to achieve or surpass sensitivity limits required to probe the inverted neutrino mass hierarchy. Multimodal approaches, testing various isotopes and employing different detection techniques, will be essential in confirming and studying 0νββ-decay.

Conclusion

The quest for neutrinoless double beta decay remains a cornerstone in the search for physics beyond the Standard Model. Success in this endeavor promises not only to elucidate the nature of neutrinos but also to unlock deeper symmetries and mechanisms governing the universe. The interplay between refined theoretical models and cutting-edge experiments enhances the prospect of discovery, paving the way for transformational insights into particle physics and cosmology.