[Paper Review] Neutrino Physics Now and in the Near Future

Hisakazu Minakata|arXiv (Cornell University)|Mar 13, 2014

Neutrino Physics Research58 references3 citations

TL;DR

This paper reviews the state of neutrino physics as of 2014, summarizing the precise measurement of mixing angles in the MNS matrix, the status of mass hierarchy and CP phase determination, and the emerging potential of high-energy neutrino astrophysics and precision cosmology. It emphasizes that while all mixing angles are now measured, the 2-3 mixing angle θ₂₃ remains the least precisely known, and highlights the critical role of future experiments in probing the Majorana phases and lepton CP violation.

ABSTRACT

The current status of neutrino physics is reviewed with some near future perspective. After recollecting the birth of modern neutrino physics with nonzero masses and flavor mixing, I summarize the present status of measurement of the mixing parameters in 2-3, 1-2, and 1-3 sectors of the MNS matrix. Then, I describe the attempts to uncover the regularities, if any, in the measured values of the mixing angles; mostly reviewing. Yet, a possible large deviation of θ_{23} to the second octant may trigger interests in the triangle relation of the lepton mixing angles. In the latter part of my lecture some perspective of determination of the mass hierarchy and measurement of lepton Kobayashi-Maskawa phase $δ$ are described. Finally, I discuss the prospects of the new, fast developing field of high-energy neutrino astrophysics, and the emerging new precision era of cosmology and particle physics. I conclude with optimistic speculations.

Motivation & Objective

• To summarize the current experimental status of neutrino mixing parameters, particularly θ₁₃, θ₂₃, and Δm²₃₂, as of 2013–2014.
• To assess the challenges and prospects for determining the neutrino mass hierarchy and the leptonic CP-violating phase δ.
• To evaluate the potential of future experiments in measuring the Majorana phases and probing the absolute neutrino mass scale.
• To explore the emerging role of high-energy neutrino astrophysics and cosmological precision measurements in advancing neutrino physics.

Proposed method

• Analysis of updated data from long-baseline accelerator experiments (T2K and MINOS) and atmospheric neutrino experiments (Super-K).
• Use of the standard parametrization of the MNS matrix to describe three-flavor neutrino mixing, including the Dirac phase δ and Majorana phases α₂₁, α₃₁.
• Application of statistical tools such as the CP exclusion fraction f_CPX to quantify sensitivity to CP violation and Majorana phases.
• Combination of constraints from future measurements of the sum of neutrino masses (Σ) and the effective Majorana mass in 0νββ decay (m₀νββ) to assess sensitivity to Majorana phases.
• Use of likelihood contours in the sin²θ₂₃–Δm²₃₂ plane to assess precision and degeneracies in mixing parameter determination.
• Incorporation of cosmological and KATRIN-based neutrino mass constraints to improve phase sensitivity.

Experimental results

Research questions

• RQ1 What is the current precision in measuring the neutrino mixing angles, particularly θ₂₃, and what limits the accuracy?
• RQ2 Can the leptonic CP-violating phase δ be determined with current and near-future experiments, and what is the expected sensitivity?
• RQ3 What is the potential for measuring the Majorana phases α₂₁ and α₃₁, and how does uncertainty in nuclear matrix elements affect this?
• RQ4 How will future cosmological measurements of the sum of neutrino masses (Σ) and 0νββ decay experiments improve sensitivity to the Majorana phases?
• RQ5 What role will high-energy neutrino astrophysics and precision cosmology play in advancing neutrino physics in the near future?

Key findings

• The 2-3 mixing angle θ₂₃ is measured with the least precision among the three mixing angles, with a 1σ uncertainty of approximately ±11% in T2K data.
• T2K and MINOS have achieved sub-4% precision in measuring Δm²₃₂, with T2K reporting ±4.0% for both mass hierarchies.
• The T2K data show a slight preference for θ₂₃ in the second octant, deviating from maximal mixing, which may trigger interest in the triangle relation of lepton mixing angles.
• The Super-K atmospheric neutrino analysis prefers the inverted mass hierarchy and the second octant of θ₂₃, increasing confidence in this configuration.
• The EXO-200 experiment observed ~10 events consistent with 0νββ decay, though with a background of ~30, suggesting a possible signal at low significance (less than 2σ).
• Even with optimistic assumptions (σₘ₀νββ = 0.01 eV and σ_Σ = 0.02 eV), sensitivity to the Majorana phase α₂₁ remains limited, excluding only 10–50% of the phase space at 2σ for m₀ = 0.1 eV, due to degeneracies with the absolute mass scale.