[Paper Review] General classification and analysis of neutron beta-decay experiments

Vladimir Gudkov, J. R. Calarco+1 more|arXiv (Cornell University)|Oct 3, 2005

Atomic and Subatomic Physics Research4 citations

TL;DR

This paper develops a comprehensive framework for analyzing neutron beta-decay experiments to detect new physics beyond the Standard Model, incorporating all known radiative and recoil corrections within the Standard Model to achieve ~10⁻⁵ theoretical accuracy. It demonstrates that the standard parametrization in terms of tree-level coefficients a, A, and B is inadequate for next-generation high-precision experiments, necessitating a full analysis in terms of fundamental coupling constants to distinguish between different new physics models based on their unique angular correlation signatures.

ABSTRACT

A method for the general analysis of the sensitivities of neutron beta-decay experiments to manifestations of possible deviations from the Standard model is proposed. In a consistent fashion, we take into account all known (radiative and recoil) corrections which are incorporated within the Standard Model to provide a description of angular correlations in neutron decay in the first order of approximation, or down to the level of $\sim 10^{-5}$. The contributions from models beyond the Standard model are, for low energy neutron decay, parameterized in terms of vector, axial-vector, scalar and tensor coupling constants and in terms of parameters related to specific models. For the present analysis we derive the exact expressions for the neutron beta decay probability which includes all possible manifestations models beyond the Standard Model down to level of $\sim 10^{-5}$ without time-reversal violation. Based on the general expressions for manifestation of the deviations from the standard model, we present analysis of the sensitivities for selected neutron decay experiments.

Motivation & Objective

• To provide a consistent theoretical description of neutron beta-decay observables that includes all known Standard Model radiative and recoil corrections.
• To identify the limitations of the conventional a, A, B coefficient parametrization when experimental sensitivities approach or exceed the level of these corrections.
• To develop a general analysis method that links experimental measurements of angular correlations to specific new physics models via low-energy effective field theory.
• To enable discrimination between different types of new physics (e.g., vector, scalar, tensor, or right-handed currents) based on their distinct signatures in decay correlations.
• To guide the design and optimization of next-generation neutron decay experiments by quantifying the sensitivity of different observables to various non-Standard Model couplings.

Proposed method

• The authors use an effective field theory approach to describe neutron beta decay via a general Hamiltonian involving low-energy constants (C_i) for all possible Lorentz structures (vector, axial-vector, scalar, tensor, pseudoscalar).
• They include all known radiative and recoil corrections within the Standard Model, reducing the description to a single parameter with ~10⁻⁵ theoretical accuracy.
• The framework extends the Standard Model description to include all possible time-reversal-conserving non-Standard Model interactions through additional coupling constants (e.g., a_LR, a_RR).
• The angular correlation coefficients a, A, and B are derived as functions of these coupling constants, allowing direct comparison with experimental data.
• The method enables the calculation of exact spectra for any given new physics model, including the interplay between radiative corrections and new physics contributions.
• The analysis is validated by comparing theoretical predictions with known experimental data and by visualizing the sensitivity of different observables (a, A, B) to specific new physics parameters through dedicated plots.

Experimental results

Research questions

• RQ1 How do radiative and recoil corrections in the Standard Model affect the interpretation of neutron beta-decay correlation coefficients at high precision?
• RQ2 Why is the conventional a, A, B coefficient parametrization insufficient for next-generation experiments with sensitivities at the 10⁻⁵ level?
• RQ3 What are the distinct angular correlation signatures of different types of new physics (e.g., left-right symmetric models, right-handed currents) in neutron decay?
• RQ4 How do the contributions of new physics to the a, A, and B coefficients differ in shape and energy dependence, enabling model discrimination?
• RQ5 To what extent do radiative corrections obscure or mimic the signals of new physics in neutron decay experiments?

Key findings

• The inclusion of all Standard Model radiative and recoil corrections reduces the description of neutron beta decay to a single parameter with a theoretical accuracy of approximately 10⁻⁵.
• The standard parametrization using tree-level coefficients a, A, and B is inadequate for high-precision experiments, as it fails to account for the full complexity of corrections and new physics effects.
• Different new physics models—such as left-right symmetric models—produce distinct, shape-dependent contributions to the a, A, and B coefficients, enabling model discrimination.
• The contributions of new physics to the A and B coefficients are significantly modulated by the value of the parameter (α/(2π))e_V^R, which is related to nuclear structure effects.
• Plots of the A and B coefficients show that radiative and recoil corrections must be included with high precision, as they can be comparable in magnitude to potential new physics signals.
• The framework enables the calculation of exact spectra for any new physics model, allowing for fine-tuned analysis and optimization of future high-precision neutron decay experiments.