[Paper Review] Neutrino's Non-Standard Interactions; Another Eel under a Willow?

Hisakazu Minakata|ArXiv.org|May 9, 2009

Neutrino Physics Research3 citations

TL;DR

This paper investigates non-standard interactions (NSI) of neutrinos within a perturbative framework, focusing on parameter degeneracy and CP violation discrimination in neutrino oscillations. Using a neutrino factory with near (3000 km) and far (7000 km) detectors, it demonstrates sensitivity to NSI parameters $|\varepsilon_{e\mu}| \sim \text{several} \times 10^{-4}$ across most of the $\delta$ and $\phi_{e\mu}$ parameter space, with lower sensitivity to $|\varepsilon_{e\tau}| \sim 10^{-3}$, and high sensitivity to CP violation from NSI phases.

ABSTRACT

I report some progress that occurred since NO-VE08 in the field of non-standard interactions (NSI) of neutrinos. After briefly reviewing theoretical developments, I give a summary of the two works in which I was involved. Firstly, we have formulated a perturbative framework to illuminate the global features of neutrino oscillations with NSI, aiming at exploring method for determination of the standard mixing and the NSI parameters. We have recognized that the parameter degeneracy prevails with an extended form which involves the NSI elements. Furthermore, a completely new type of degeneracy is shown to exist. The nature of the former degeneracy is analyzed in detail in the second work. The work is primarily devoted to analyze the problem of discriminating the two CP violation, one due to the lepton Kobayashi-Maskawa phase and the other by phases of the NSI elements. We have shown that the near (3000 km)-far (7000 km) two detector setting in neutrino factory does have the discrimination capability and is sensitivities to CP violation due to NSI to |epsilon_{eμ}| to \simeq several x 10^{-4} in most of the region of δand ϕ.

Motivation & Objective

• To develop a perturbative framework for analyzing neutrino oscillations with non-standard interactions (NSI), addressing parameter determination and degeneracy.
• To investigate the discrimination capability between CP violation from the standard Kobayashi-Maskawa phase and that from NSI phases.
• To assess the sensitivity of a two-detector neutrino factory setup (3000 km and 7000 km baselines) to NSI parameters and CP violation.
• To clarify the nature of parameter degeneracy in the presence of NSI, particularly the emergence of a new type of degeneracy involving NSI elements.

Proposed method

• Formulates a perturbative approach to neutrino oscillations in the presence of NSI, using effective field theory with dimension-six operators.
• Applies the effective Lagrangian $\mathcal{L}_{\text{eff}}^{\text{NSI}} = -2\sqrt{2}\, \varepsilon_{\alpha\beta}^{fP} G_F (\bar{\nu}_\alpha \gamma_\mu P_L \nu_\beta)(\bar{f} \gamma^\mu P f)$ to model NSI effects in matter.
• Performs numerical simulations of neutrino factory experiments with two detectors at different baselines to evaluate sensitivity to NSI and CP violation.
• Analyzes parameter degeneracy by comparing oscillation probabilities under different combinations of $\delta$, $\phi_{e\mu}$, and NSI parameters $\varepsilon_{e\mu}$, $\varepsilon_{e\tau}$.
• Uses sensitivity contours to quantify the reach of the setup in probing $|\varepsilon_{e\mu}|$ and $|\varepsilon_{e\tau}|$, assuming both normal and inverted mass hierarchies.
• Evaluates the impact of non-zero NSI on CP violation discovery by comparing sensitivity with and without NSI in the fit.

Experimental results

Research questions

• RQ1 Can a perturbative framework effectively illuminate the global features of neutrino oscillations with non-standard interactions?
• RQ2 What types of parameter degeneracy arise in the presence of NSI, and how do they differ from standard degeneracies?
• RQ3 Can a neutrino factory with two detectors at 3000 km and 7000 km baselines discriminate between CP violation from the standard Kobayashi-Maskawa phase and that from NSI phases?
• RQ4 What is the sensitivity reach of such a setup for detecting $|\varepsilon_{e\mu}|$ and $|\varepsilon_{e\tau}|$ in the context of TeV-scale new physics?
• RQ5 How do systematic errors and backgrounds affect the sensitivity estimates for NSI and CP violation?

Key findings

• The perturbative framework reveals a new type of degeneracy involving NSI elements, extending the standard parameter degeneracy in neutrino oscillations.
• The near (3000 km) and far (7000 km) detector setup in a neutrino factory achieves sensitivity to $|\varepsilon_{e\mu}| \simeq \text{several} \times 10^{-4}$ across most of the $\delta$ and $\phi_{e\mu}$ parameter space.
• Sensitivity to $|\varepsilon_{e\tau}|$ is lower but still reaches $\simeq 10^{-3}$, indicating detectable reach for this channel.
• The setup demonstrates high sensitivity to CP violation from NSI phases, with similar reach to $|\varepsilon_{e\mu}| \simeq \text{several} \times 10^{-4}$ in most of the $\delta$ and $\phi_{e\mu}$ region.
• Sensitivity contours are unstable to systematic errors and backgrounds, suggesting that the optimistic estimates may be reduced in realistic experimental settings.
• The study highlights that effects from NSI in neutrino production and detection—previously ignored—could significantly improve sensitivity, though their realization remains challenging.