Solar Neutrino Measurements in Super-Kamiokande-IV (1606.07538v1)

Abstract: Upgraded electronics, improved water system dynamics, better calibration and analysis techniques allowed Super-Kamiokande-IV to clearly observe very low-energy 8B solar neutrino interactions, with recoil electron kinetic energies as low as 3.49 MeV. Super-Kamiokande-IV data-taking began in September of 2008; this paper includes data until February 2014, a total livetime of 1664 days. The measured solar neutrino flux is (2.308+-0.020(stat.) + 0.039-0.040(syst.)) x 106/(cm2sec) assuming no oscillations. The observed recoil electron energy spectrum is consistent with no distortions due to neutrino oscillations. An extended maximum likelihood fit to the amplitude of the expected solar zenith angle variation of the neutrino-electron elastic scattering rate in SK-IV results in a day/night asymmetry of (-3.6+-1.6(stat.)+-0.6(syst.))%. The SK-IV solar neutrino data determine the solar mixing angle as sin2 theta_12 = 0.327+0.026-0.031, all SK solar data (SK-I, SK-II, SK III and SKIV) measures this angle to be sin2 theta_12 = 0.334+0.027-0.023, the determined mass-squared splitting is Delta m2_21 = 4.8+1.5-0.8 x10-5 eV2.

An Analysis of Solar Neutrino Measurements in Super-Kamiokande-IV

This paper presents an extensive paper of solar neutrino observations conducted during the fourth phase of the Super-Kamiokande (SK-IV) experiment. The primary focus is on the measurement of solar neutrino flux and the determination of neutrino oscillation parameters, facilitated by extensive upgrades in detector technology and data analysis techniques.

Measurement Techniques and Improvements

The SK-IV phase employs a water Cherenkov detector that utilizes physical properties such as scattering and absorption of Cherenkov light to detect solar neutrinos via elastic scattering with electrons. This paper reports on the data collected from October 2008 to February 2014, utilizing an improved detector with lower energy thresholds, enhanced electronics (QBEEs), and a refined water system to reduce background noise. As a result of these upgrades, SK-IV was able to detect recoil electrons down to kinetic energies of 3.49 MeV, surpassing the capabilities of the previous phases.

Key Measurements and Results

A significant achievement of the SK-IV phase is the measurement of the solar neutrino flux of $$(2.308 \pm 0.020(\text{stat.})^{+0.039}_{-0.040}(\text{syst.})) \times 10^6/\text{cm}^2/\text{sec}$$ assuming no oscillations. This result marks the most precise measurement of solar neutrinos by SK and underscores the efficacy of the current experimental setup.

The paper discusses the observed day/night asymmetry with values indicating terrestrial matter effects on solar neutrino oscillations. The day/night asymmetry observed was $$(-3.6 \pm 1.6(\text{stat.}) \pm 0.6(\text{syst.}))\%$$. The observed data is consistent with predictions from neutrino oscillation models, but the findings slightly favor lower values of $\Delta m^2_{21}$ than those obtained solely from KamLAND reactor antineutrino data.

Oscillation Parameters and Theoretical Implications

From the global analysis, including all phases of SK and other relevant solar neutrino experiments, the analysis yields oscillation parameters $\sin^2\theta_{12} = 0.334^{+0.027}_{-0.023}$ and $$\Delta m^2_{21} = 4.8^{+1.5}_{-0.8} \times 10^{-5} \text{eV}^2$$. Considering reactor constraints, $\theta_{13}$ was found to be $\sin^2 \theta_{13} = 0.028 \pm 0.015$, aligning with current observations from reactor neutrino experiments. These results provide robust support for the LMA-MSW model of solar neutrino oscillation, affirming that solar neutrinos undergo flavor transformation due to both vacuum oscillations and matter effects within the Sun.

Future Scope and Implications

The findings from SK-IV enhance the precision of solar neutrino flux measurements and enable the refinement of neutrino oscillation models. The results lay a foundation for further investigation into potential new physics beyond the standard three-flavor oscillation framework, such as the paper of non-standard interactions or variations of neutrino masses with fundamental particle properties.

The paper highlights the potential for future research using enhanced detector capabilities and methodologies to probe lower energy neutrinos and further explore the subtleties of neutrino interactions with terrestrial and solar matter. This work not only solidifies previous findings but also paves the way for additional studies using novel detection techniques in upcoming neutrino experiments.

These findings further underscore the necessity of interdisciplinary collaboration and ongoing development in both theoretical models and experimental techniques to unlock the remaining mysteries in neutrino physics and deepen our understanding of fundamental particle behavior in astrophysical processes.