[Paper Review] Neutrinoless double beta decay: searching for new physics with comparison of different nuclei

Ahmed Ali, A. V. Borisov+1 more|arXiv (Cornell University)|Dec 17, 2011

Neutrino Physics Research4 references4 citations

TL;DR

This paper investigates neutrinoless double beta decay (0ν2β) across multiple isotopes—76Ge, 82Se, 100Mo, 130Te, and 136Xe—using a general Lorentz-invariant effective Lagrangian to disentangle new physics mechanisms. It shows that comparing half-lives and electron angular correlations across nuclei can reduce theoretical uncertainties in nuclear matrix elements and identify dominant decay mechanisms, with 100Mo and 82Se being most sensitive to specific new physics contributions.

ABSTRACT

The neutrinoless double beta decay is analyzed using a general Lorentz invariant effective Lagrangian for various decaying nuclei of current experimental interest: $^{76}$Ge, $^{82}$Se, $^{100}$Mo, $^{130}$Te, and $^{136}$Xe. We work out the half-lives and angular correlation coefficients of the outgoing electrons in several scenarios for new physics: the left-right symmetric models, the R-parity-violating SUSY and models with leptoquarks. The theoretical uncertainty in the nuclear matrix elements is discussed.

Motivation & Objective

• To reduce theoretical uncertainties in nuclear matrix elements for 0ν2β decay by comparing results across multiple isotopes.
• To identify the dominant new physics mechanism (e.g., leptoquarks, R-parity-violating SUSY, left-right symmetric models) responsible for 0ν2β decay.
• To assess the sensitivity of different nuclei to specific non-Standard Model contributions via half-lives and angular correlations.
• To evaluate the potential of future experiments to measure electron angular correlations and distinguish among competing mechanisms.
• To provide a framework for using multi-nucleus comparisons to extract information on neutrino masses and mixing from 0ν2β decay data.

Proposed method

• Uses a general Lorentz-invariant effective Lagrangian to describe 0ν2β decay, including contributions from Majorana neutrinos, leptoquarks, R-parity-violating SUSY, and right-handed W bosons.
• Applies the differential decay width formula dΓ/dcosθ = (ln 2/2)|M_GT|²𝒜(1−Kcosθ), where K is the angular correlation coefficient dependent on new physics parameters.
• Calculates ratios of half-lives (ℛ) and angular correlation coefficients (𝒦) relative to 76Ge, the best-studied isotope, to isolate model-independent signatures.
• Considers two scenarios: (A) non-zero effective Majorana mass |⟨m⟩| with SM + Majorana neutrino exchange, and (B) vanishing |⟨m⟩| with non-zero new physics coefficients ε.
• Performs numerical analysis using QRPA with and without p-n pairing to assess model dependence of results.
• Focuses on the V−A, V+A, and T_L,R current structures, isolating contributions from specific new physics operators.

Experimental results

Research questions

• RQ1 How do the half-lives of 0ν2β decay vary across different isotopes when different new physics mechanisms are considered?
• RQ2 To what extent can angular correlation coefficients help distinguish between competing new physics models in 0ν2β decay?
• RQ3 Which isotopic pairs are most sensitive to specific new physics contributions such as R-parity-violating SUSY or leptoquarks?
• RQ4 How do theoretical uncertainties in nuclear matrix elements affect the interpretation of 0ν2β decay data across different nuclei?
• RQ5 Can multi-nucleus comparisons reduce ambiguity in identifying the underlying mechanism of 0ν2β decay?

Key findings

• The ratio 𝒦_{V±A}^{V−A} for 100Mo is 1.08, indicating high sensitivity to V−A current contributions, while 82Se shows 𝒦_{V+A}^{V−A} = 1.11, making it sensitive to V+A terms.
• For half-lives, 100Mo exhibits the highest sensitivity to ε_{V−A}^{V+A} with ℛ_{V−A}^{V+A} = 52.87 (QRPA without pairing), while 82Se is most sensitive to ε_{V+A}^{V+A} with ℛ_{V+A}^{V+A} = 0.24.
• The ratio ℛ_{V−A}^{V−A} for 130Te is 0.24 (without pairing), indicating a significantly suppressed half-life compared to 76Ge, suggesting strong model dependence.
• Angular correlation ratios are largely independent of nuclear matrix elements, especially 𝒦_{V−A}^{V+A}, making them robust probes of new physics.
• The pair 76Ge–100Mo is most sensitive to ε_{V±A}^{V+A} via angular correlations, with 𝒦_{V±A}^{V−A} = 1.08 for 100Mo.
• The pair 100Mo–130Te is most sensitive to ε_{V−A}^{V+A} in half-lives, with ℛ_{V−A}^{V+A} = 1.11 for 130Te and 52.87 for 100Mo, indicating strong discrimination potential.