Measurement and Interpretation of the Beta Decay Spectrum of Cadmium-113 for Investigating Axial-Vector Coupling Quenching

Core Concepts

Precise measurements of the beta decay spectrum of Cadmium-113, using a cryogenic calorimeter, suggest a possible quenching of the axial-vector coupling constant, a finding with implications for understanding nuclear models and processes like neutrinoless double beta decay.

Abstract

This research paper presents a precise measurement of the beta decay spectrum of Cadmium-113 (113Cd) using a cryogenic calorimeter.

Bibliographic Information: Bandac, I., Berg´e, L., Calvo-Mozota, J.M. et al. Precise 113Cd 𝛽decay spectral shape measurement and interpretation in terms of possible 𝑔𝐴quenching. Eur. Phys. J. C (2024).

Research Objective: The study aims to test nuclear models and investigate the potential quenching of the axial-vector coupling constant (g_A) by analyzing the spectral shape of the fourth-forbidden non-unique beta decay of 113Cd.

Methodology: The researchers used a CdWO4 crystal bolometer operating at low temperature in the Canfranc underground laboratory to measure the beta spectrum of 113Cd. They employed a Bayesian fit of the experimental data to three nuclear models: the microscopic interacting boson-fermion model (IBFM-2), the microscopic quasiparticle-phonon model (MQPM), and the nuclear shell model (NSM). The fit considered two free parameters: the effective axial-vector coupling constant (g^eff_A) and a small relativistic nuclear matrix element (s-NME).

Key Findings: The analysis revealed an effective axial-vector coupling constant (g^eff_A) between 1.0 and 1.2, suggesting a possible quenching of g_A. The measured half-life of the 113Cd beta decay, including systematic uncertainties, was found to be 7.73+0.60−0.57 × 10¹⁵ years, consistent with previous experimental results.

Main Conclusions: The study's findings indicate a potential renormalization of the axial-vector coupling strength in the nuclear medium, which could be attributed to nuclear medium effects not fully captured by current theoretical models. The results highlight the importance of studying highly forbidden beta decays to refine nuclear models and improve predictions for processes like neutrinoless double beta decay.

Significance: This research contributes significantly to the understanding of fundamental nuclear processes and the limitations of current nuclear models. The precise measurement of the 113Cd beta decay spectrum and the observation of possible g_A quenching provide valuable data for constraining theoretical calculations and enhancing the accuracy of predictions for other nuclear processes, including neutrinoless double beta decay.

Limitations and Future Research: The study acknowledges the simplicity of the background model used in the analysis and suggests further investigation into the potential presence of additional low-energy beta decay contamination. Future research could focus on refining the background model, exploring other nuclear models, and conducting similar studies on other highly forbidden beta decays to gain a more comprehensive understanding of axial-vector coupling quenching in nuclear processes.

Stats

The signal-to-background ratio in the energy region of interest ([15, 330] keV) was approximately 12.
The analysis achieved an energy threshold of 15 keV.
The measured half-life of the 113Cd beta decay was 7.73+0.60−0.57 × 1015 years.
The effective axial-vector coupling constant (geffA) was found to be between 1.0 and 1.2.

Quotes

Key Insights Distilled From

Precise $^{113}$Cd $\beta$ decay spectral shape measurement and interpretation in terms of possible $g_A$ quenching

by I. Bandac, L... at arxiv.org 11-06-2024

https://arxiv.org/pdf/2411.02944.pdf

Deeper Inquiries

How might these findings concerning gA quenching in 113Cd influence the experimental search for neutrinoless double beta decay in other isotopes?

Answer:
The findings concerning gA quenching in 113Cd have significant implications for the search for neutrinoless double beta decay (0νββ). Here's how:

Nuclear Matrix Element (NME) Calculations: The rate of 0νββ decay is directly proportional to the square of the NME, which quantifies the overlap between the initial and final nuclear states. Theoretical models are used to calculate NMEs, and these models rely on the value of the axial-vector coupling constant, gA. Quenching of gA directly impacts the calculated NMEs.
Impact on Predicted Half-lives: A quenched gA leads to smaller NMEs, which in turn results in longer predicted half-lives for 0νββ decay. This has a crucial consequence for experimental searches: if gA is indeed quenched, experiments need to achieve significantly higher sensitivities to have a chance of observing this rare decay.
Model Dependence: The extent of gA quenching can vary depending on the specific nuclear model used (e.g., the Interacting Boson-Fermion Model (IBFM-2), the Quasiparticle-Phonon Model (MQPM), or the Nuclear Shell Model (NSM)). This model dependence introduces uncertainty in the predicted 0νββ decay half-lives.
Extrapolation to Other Isotopes: While the 113Cd study provides evidence for gA quenching, it's essential to remember that the extent of quenching might differ for other isotopes used in 0νββ decay experiments (e.g., 76Ge, 130Te, 136Xe). Therefore, further studies of gA quenching in various nuclear systems are crucial.
In summary: The potential quenching of gA adds complexity to the search for 0νββ decay. It highlights the need for:

Refined theoretical models that can more accurately account for nuclear structure effects and provide more precise NMEs.
Experimental measurements of gA quenching in a wider range of isotopes to better understand its systematics.
Increased sensitivity in 0νββ decay experiments to probe the longer half-lives predicted if gA quenching is significant.

Could alternative theoretical frameworks beyond the IBFM-2, MQPM, and NSM models potentially explain the observed data without invoking gA quenching?

Answer:
Yes, it's possible that alternative theoretical frameworks could explain the observed data without requiring gA quenching. Here are some possibilities:

Beyond the Impulse Approximation: The standard theoretical framework for calculating beta decay rates, including the models mentioned (IBFM-2, MQPM, NSM), often relies on the impulse approximation. This approximation assumes that the decaying nucleon interacts with the weak current independently of the other nucleons. More sophisticated calculations that go beyond this approximation, explicitly including meson-exchange currents or two-body currents, could potentially modify the predicted spectral shapes and rates, potentially resolving the discrepancy without gA quenching.
Non-nucleonic Degrees of Freedom: The models mentioned primarily focus on nucleonic degrees of freedom. However, at the energy scales relevant to beta decay, the substructure of nucleons (quarks and gluons) and the presence of virtual mesons within the nucleus could play a role. Theoretical frameworks that explicitly incorporate these non-nucleonic degrees of freedom might lead to different predictions.
Deficiencies in Nuclear Wave Functions: The accuracy of nuclear structure calculations heavily depends on the quality of the nuclear wave functions used. It's possible that the observed discrepancies arise not from gA quenching itself but from limitations in the wave functions employed in the IBFM-2, MQPM, or NSM calculations. Improved many-body techniques for calculating nuclear wave functions could potentially resolve the issue.
New Physics Beyond the Standard Model: While less likely, it's conceivable that the observed deviations from standard model predictions hint at new physics beyond our current understanding. For example, the existence of new particles or interactions could affect beta decay rates and potentially mimic the effects of gA quenching.
In conclusion: While gA quenching is a plausible explanation for the observed data, it's not the only possibility. Exploring alternative theoretical frameworks and refining our understanding of nuclear structure are essential to confirm or refute the need for gA quenching and to gain a more complete picture of beta decay processes.

What broader implications might a confirmed quenching of the axial-vector coupling constant have for our understanding of fundamental forces and particles in the universe?

Answer:
A confirmed quenching of the axial-vector coupling constant (gA) would have profound implications for our understanding of fundamental forces and particles:

Nuclear Structure and the Strong Force: gA quenching suggests that the simple picture of nucleons interacting as free particles within the nucleus is incomplete. The strong force, mediated by gluons, governs the interactions between quarks and gluons within nucleons and between nucleons themselves. Quenching implies that the nuclear medium significantly modifies the effective strength of the weak interaction, highlighting the complex interplay between the strong and weak forces within the nucleus.
Precision Tests of the Standard Model: The Standard Model of particle physics describes the fundamental forces and particles. Precise measurements of beta decay rates, including those sensitive to gA, provide stringent tests of the Standard Model. A confirmed quenching of gA would necessitate modifications or extensions to the Standard Model to account for the observed deviations.
Neutrino Physics: The axial-vector current plays a crucial role in neutrino interactions. A modified gA in the nuclear medium could affect our understanding of neutrino-nucleus scattering cross-sections, which are essential for interpreting data from neutrino oscillation experiments and studying the properties of neutrinos.
Astrophysical Processes: Beta decay and other weak interaction processes are fundamental to many astrophysical phenomena, including stellar evolution, supernova explosions, and nucleosynthesis (the creation of elements). A quenched gA could alter the rates of these processes, potentially impacting our models of stellar evolution and the chemical evolution of the universe.
In essence: A confirmed gA quenching would signify that our understanding of nuclear structure and the interplay of fundamental forces within the nucleus is incomplete. It would necessitate refinements to the Standard Model and could have far-reaching consequences for our understanding of neutrinos, astrophysical processes, and the evolution of the universe.