[Paper Review] Solar neutrino analysis of Super-Kamiokande

H. Sekiya|arXiv (Cornell University)|Jul 14, 2013

Neutrino Physics Research1 references3 citations

TL;DR

This paper presents the highest-precision solar neutrino measurement from Super-Kamiokande-IV, detecting 8B neutrinos down to 3.5 MeV recoil electron energy. It reports a 2.7σ indication of day/night asymmetry due to matter-induced regeneration of electron neutrinos in Earth, yielding the world's most precise neutrino oscillation parameter constraints: sin²θ₁₂ = 0.304 ± 0.013, sin²θ₁₃ = 0.031⁺⁰.⁰¹⁷₋₀.⁰¹⁵, and Δm²₂₁ = 7.45⁺⁰.²⁰₋₀.¹⁹ × 10⁻⁵ eV² from a global fit to SK and KamLAND data.

ABSTRACT

Super-Kamiokande-IV data taking began in September of 2008, and with upgraded electronics and improvements to water system dynamics, calibration and analysis techniques, a clear solar neutrino signal could be extracted at recoil electron kinetic energies as low as 3.5 MeV. The SK-IV extracted solar neutrino flux between 3.5 and 19.5 MeV is found to be (2.36$\pm$0.02(stat.)$\pm$0.04(syst.))$ imes 10^6$ /(cm$^2$sec). The SK combined recoil electron energy spectrum favors distortions predicted by standard neutrino flavour oscillation parameters over a flat suppression at 1$σ$ level. A maximum likelihood fit to the amplitude of the expected solar zenith angle variation of the elastic neutrino-electron scattering rate in SK, results in a day/night asymmetry of $-3.2\pm1.1$(stat.)$\pm$0.5(syst.)$%$. The 2.7 $σ$ significance of non-zero asymmetry is the first indication of the regeneration of electron type solar neutrinos as they travel through Earth's matter. A fit to all solar neutrino data and KamLAND yields $\sin^2 θ_{12} = 0.304 \pm 0.013$, $\sin^2 θ_{13} = 0.031^{+0.017}_{-0.015}$ and $Δm^2_{21} = 7.45^{+0.20}_{-0.19} imes 10^{-5} { m eV}^2$.

Motivation & Objective

• To measure the solar neutrino flux and energy spectrum with improved sensitivity and lower energy threshold using Super-Kamiokande-IV.
• To test the existence of the day/night asymmetry in solar neutrino detection, a signature of neutrino oscillations modulated by Earth's matter.
• To constrain solar neutrino oscillation parameters, particularly θ₁₂, θ₁₃, and Δm²₂₁, through a global fit combining SK-IV data with KamLAND and other solar neutrino experiments.
• To provide the most precise measurement of Δm²₂₁ using neutrinos rather than antineutrinos, testing the MSW effect and matter-induced flavor conversion.

Proposed method

• Utilized upgraded QBEE front-end electronics and a software trigger to record all PMT hits with high timing resolution, enabling low-energy threshold detection down to 3.5 MeV.
• Implemented anisotropy-based event reconstruction using PMT hit patterns to distinguish low-energy solar neutrino events from background, especially ²¹⁴Bi beta decays.
• Applied a temperature control system to suppress water convection and reduce radon-induced background in the fiducial volume.
• Performed a maximum likelihood fit to the day/night variation of the elastic scattering rate to extract the day/night asymmetry amplitude.
• Conducted a global oscillation fit combining SK-IV recoil electron energy spectrum, day/night asymmetry, and KamLAND reactor antineutrino data to constrain θ₁₂, θ₁₃, and Δm²₂₁.
• Used the expected matter effect on neutrino oscillations to model the day/night asymmetry as a function of neutrino energy and baseline.

Experimental results

Research questions

• RQ1 Does the Super-Kamiokande-IV data show a statistically significant day/night asymmetry in the solar neutrino detection rate, as predicted by neutrino oscillations in Earth's matter?
• RQ2 What is the precise value of the solar neutrino mixing angle θ₁₂ and mass splitting Δm²₂₁ derived from the SK-IV energy spectrum and day/night variation?
• RQ3 How do the oscillation parameters measured by Super-Kamiokande compare with those from KamLAND and other global fits?
• RQ4 Is there evidence for a non-zero θ₁₃ mixing angle from the combined SK and KamLAND data, and is it consistent with reactor neutrino measurements?

Key findings

• The SK-IV solar neutrino flux between 3.5 and 19.5 MeV is measured as (2.36 ± 0.02(stat.) ± 0.04(syst.)) × 10⁶ /(cm² sec), the most precise measurement to date.
• A 2.7σ indication of a non-zero day/night asymmetry is observed, with a value of -3.2 ± 1.1(stat.) ± 0.5(syst.)%, providing the first direct evidence for neutrino regeneration in Earth's matter.
• The SK-IV data favor an energy-dependent distortion in the recoil electron spectrum consistent with standard neutrino oscillation parameters, rejecting a flat suppression at 1σ.
• The global fit to all solar neutrino data and KamLAND yields sin²θ₁₂ = 0.304 ± 0.013, sin²θ₁₃ = 0.031⁺⁰.⁰¹⁷₋₀.⁰¹⁵, and Δm²₂₁ = 7.45⁺⁰.²⁰₋₀.¹⁹ × 10⁻⁵ eV².
• The measured value of sin²θ₁₃ is consistent with reactor neutrino measurements, supporting the three-flavor neutrino mixing framework.
• The SK-IV measurement of Δm²₂₁ is the most precise using neutrinos rather than antineutrinos, with a value of 4.8⁺¹.⁸₋₀.⁹ × 10⁻⁵ eV² from the day/night asymmetry alone.