[Paper Review] Unitarity Tests of the Neutrino Mixing Matrix

X. Qian, C. Zhang+2 more|arXiv (Cornell University)|Aug 26, 2013

Neutrino Physics Research17 citations

TL;DR

This paper proposes direct and indirect unitarity tests of the PMNS neutrino mixing matrix using reactor and solar neutrino experiments. By combining medium-baseline (JUNO) and short-baseline (Daya Bay) reactor antineutrino data with solar neutrino measurements (SNO), the authors demonstrate a model-independent test of |Uₑ₁|² + |Uₑ₂|² + |Uₑ₃|² = 1 at the 4% level at 68% confidence, while comparing sin²2θ₁₃ from reactor and accelerator experiments enables indirect constraints on new physics beyond the three-neutrino model.

ABSTRACT

We discuss unitarity tests of the neutrino mixing (PMNS) matrix. We show that the combination of solar neutrino experiments, medium-baseline and short-baseline reactor antineutrino experiments make it possible to perform the first direct unitarity test of the PMNS matrix. In particular, the measurements of Daya Bay and JUNO (a next generation medium-baseline reactor experiment) will lay the foundation of a precise unitarity test of $|U_{e1}|^2 + |U_{e2}|^2 + |U_{e3}|^2 = 1 $. Furthermore, the precision measurement of $\sin^22θ_{13}$ in both the $\barν_e$ disappearance and the $ν_e$ appearance (from a $ν_μ$ beam) channels will provide an indirect unitarity test of the PMNS matrix. Together with the search for appearance/disappearance at very short distances, these tests could provide important information about the possible new physics beyond the three-neutrino model.

Motivation & Objective

• To establish a direct, model-independent test of the unitarity of the PMNS matrix, specifically the sum of squares of the first-row elements: |Uₑ₁|² + |Uₑ₂|² + |Uₑ₃|² = 1.
• To assess the feasibility of combining medium-baseline (JUNO), short-baseline (Daya Bay), and solar neutrino (SNO) data for precision unitarity testing.
• To explore indirect unitarity tests via comparison of sin²2θ₁₃ measured in reactor disappearance and accelerator appearance channels.
• To constrain new physics beyond the three-neutrino model, such as sterile neutrinos or non-standard interactions, using precision measurements of oscillation parameters.

Proposed method

• Utilizes the SNO solar neutrino experiment as a proxy for measuring |Uₑ₂|²(|Uₑ₁|² + |Uₑ₂|²) + |Uₑ₃|⁴, approximating the first-row unitarity condition.
• Combines Daya Bay and JUNO reactor antineutrino disappearance data to constrain |Uₑ₁|² + |Uₑ₂|² + |Uₑ₃|² = 1 with high precision.
• Compares sin²2θ₁₃ values extracted from reactor antineutrino disappearance (Daya Bay) and accelerator neutrino appearance (T2K, LBNE) experiments to perform an indirect unitarity test.
• Analyzes the sensitivity of wide-band beam experiments like LBNE to detect spectral distortions from new physics, such as sterile neutrinos or non-standard interactions.
• Assesses the impact of matter effects and CP phase degeneracy on indirect unitarity tests, assuming negligible matter effects for simplicity.
• Evaluates the potential of very short-baseline experiments (e.g., ICARUS) to test unitarity via low-background νₑ appearance searches.

Experimental results

Research questions

• RQ1 Can the first-row unitarity condition |Uₑ₁|² + |Uₑ₂|² + |Uₑ₃|² = 1 be tested directly and model-independently using current and next-generation reactor and solar neutrino experiments?
• RQ2 What is the expected precision of the direct unitarity test when combining JUNO, Daya Bay, and SNO data?
• RQ3 How do discrepancies between sin²2θ₁₃ measured in reactor disappearance and accelerator appearance experiments constrain new physics beyond the three-neutrino model?
• RQ4 To what extent can wide-band beam experiments like LBNE enhance constraints on sterile neutrinos or non-standard interactions through spectral distortion analysis?
• RQ5 Can very short-baseline νₑ appearance experiments provide a sensitive probe of PMNS matrix unitarity?

Key findings

• The combination of JUNO, Daya Bay, and SNO experiments can test the first-row unitarity condition |Uₑ₁|² + |Uₑ₂|² + |Uₑ₃|² = 1 at the 4% level at 68% confidence, representing a direct, model-independent test.
• The precision of the direct unitarity test can be substantially improved with better constraints from solar neutrino measurements.
• A discrepancy of about 2σ between sin²2θ₁₃ values from reactor disappearance (Daya Bay) and accelerator appearance (T2K) experiments is currently observed, which could indicate new physics if confirmed with higher statistics.
• The comparison of sin²2θ₁₃ across reactor and accelerator experiments provides a powerful indirect unitarity test that constrains sterile neutrinos and non-standard interactions.
• Wide-band beam experiments like LBNE, with high-statistics spectral measurements, offer stringent constraints on new physics through spectral distortion analysis.
• Very short-baseline experiments such as ICARUS can effectively test unitarity by searching for low-background νₑ appearance, providing strong limits on sterile neutrino models.