Solar neutrino measurements in Super-Kamiokande-II

TL;DR

The paper reports the Solar neutrino measurements from Super-Kamiokande-II (SK-II), comparing them with the SK-I phase to verify consistency despite reduced PMT coverage and a higher energy threshold. It details detector upgrades, calibration (LINAC and $^{16}$N), and revised reconstruction, background rejection, and analysis methods, yielding a $^8$B flux of $2.38\times10^{6}$ cm$^{-2}$ s$^{-1}$ with no spectral distortion or solar-activity correlation, and a day-night asymmetry consistent with zero. Oscillation analyses—both SK-only and in combination with SNO and radiochemical data—favor the LMA region, with a global best-fit around $\tan^2\theta\approx0.40$ and $\Delta m^2\approx6.0\times10^{-5}$ eV$^2$, aligning with the contemporary neutrino-oscillation paradigm. By extending the solar neutrino time series to 9.5 years, SK-II reinforces the reliability of SK’s solar-neutrino program and strengthens constraints on neutrino oscillation parameters.

Abstract

The results of the second phase of the Super-Kamiokande solar neutrino measurement are presented and compared to the first phase. The solar neutrino flux spectrum and time-variation as well as oscillation results are statistically consistent with the first phase and do not show spectral distortion. The time-dependent flux measurement of the combined first and second phases coincides with the full period of solar cycle 23 and shows no correlation with solar activity. The measured boron 8 total flux is 2.38 +/-0.05(stat.) +0.16-0.15(sys.) X 10^6 cm^-2 sec^-1 and the day-night difference is found to be -6.3 +/-4.2(stat.) +/-3.7(sys.) %. There is no evidence of systematic tendencies between the first and second phases.

Figures (15)

• Figure 1: Wavelength dependence of the water parameter combined SK-II absorption-reflection coefficient.
• Figure 2: Vertex resolution (defined as 68.2% of reconstructed events which reconstruct inside a sphere of radius $\sigma$ from the correct vertex) of $^8$B Monte Carlo events as a function of total recoil electron energy.
• Figure 3: Directional resolution of Monte Carlo events as a function of recoil electron total energy.
• Figure 4: Upper figure shows the time variation of the measured water transparency (weighed by the Cherenkov spectrum) during SK-II. Lower figure shows the stability of the SK-II energy scale as a function of time. The absence of data points in late 2003 is from detector dead time due to an electronics upgrade.
• Figure 5: Energy resolution as a function of total recoil electron energy of MC events. The red dashed line is SK-I.