Electron Energy Spectra, Fluxes, and Day-Night Asymmetries of $^{8}$B Solar Neutrinos from the 391-Day Salt Phase SNO Data Set

TL;DR

This work presents a comprehensive analysis of the SNO salt-phase data set, enabling precise separation of charged-current and neutral-current solar neutrino events via enhanced neutron capture on $^{35}$Cl and event isotropy. By performing a detailed extended maximum-likelihood fit to multiple event observables, the authors extract the CC, NC, and ES fluxes, the CC energy spectrum, and day-night asymmetries, all with rigorous statistical and systematic uncertainties. The results support flavor transformation of solar neutrinos, compatible with the Large Mixing Angle (LMA) MSW solution, and yield a precise total active $^8$B flux in agreement with solar models. When combined with KamLAND reactor data, the analysis constrains the oscillation parameters to $ abla m^2 \,=\, (8.0^{+0.6}_{-0.4}) imes 10^{-5}\, ext{eV}^2$ and $ heta \,=\, 33.9^{+2.4}_{-2.2}$ degrees, illustrating the power of SNO's salt-phase measurements in probing neutrino properties and solar physics.

Abstract

Results are reported from the complete salt phase of the Sudbury Neutrino Observatory experiment in which NaCl was dissolved in the D$_2$O target. The addition of salt enhanced the signal from neutron capture, as compared to the pure D$_2$O detector. By making a statistical separation of charged-current events from other types based on event-isotropy criteria, the effective electron recoil energy spectrum has been extracted. In units of $ 10^6$ cm$^{-2}$ s$^{-1}$, the total flux of active-flavor neutrinos from $^8$B decay in the Sun is found to be $4.94^{+0.21}_{-0.21}{(stat)}^{+0.38}_{-0.34}{(syst)}$ and the integral flux of electron neutrinos for an undistorted $^8$B spectrum is $1.68^{+0.06}_{-0.06}{(stat)}^{+0.08}_{-0.09}{(syst)}$; the signal from ($ν_x$,e) elastic scattering is equivalent to an electron-neutrino flux of $2.35^{+0.22}_{-0.22}{(stat)}^{+0.15}_{-0.15}{(syst)}$. These results are consistent with those expected for neutrino oscillations with the so-called Large Mixing Angle parameters, and also with an undistorted spectrum. A search for matter-enhancement effects in the Earth through a possible day-night asymmetry in the charged-current integral rate is consistent with no asymmetry. Including results from other experiments, the best-fit values for two-neutrino mixing parameters are $Δm^2 = (8.0^{+0.6}_{-0.4}) \times 10^{-5}$ eV$^2$ and $θ= 33.9 ^{+2.4}_{-2.2}$ degrees.

Figures (36)

• Figure 1: Reduction of the data set as successive cuts are applied. The PMT instrumental, external light and pickup cuts remove instrumental backgrounds originating from the detector hardware. The high-level cuts (Sec. \ref{['sec:backgrounds']}) further reduce the instrumental backgrounds by rejecting events that do not possess the characteristics of Cherenkov light emission from single or multiple $\beta$s. The fiducial volume cut, which selects events reconstructed with $\rho<0.77$, removes most of the radioactive background events that originate outside the D$_2$O target.
• Figure 2: Signal loss as a function of energy for CC electrons assuming an undistorted $^8$B spectrum. Signal loss uncertainties are divided into two classes: uncorrelated (error bars) and correlated (error bands). Correlated uncertainties arise from systematic uncertainties in the measurement of signal loss and uncorrelated uncertainties arise from statistical uncertainties in the calibration data used in this measurement.
• Figure 3: Attenuation coefficients of (a) D$_2$O and (b) H$_2$O as a function of date for 369 and 420 nm pulsed laser scans. The data points are correlated by systematics that are common to each of the measurements. Note that the H$_2$O values are determined by subtracting the acrylic vessel ex-situ measured attenuation coefficients from the measured H$_2$O+acrylic values.
• Figure 4: Measured angular response curves for PMT-reflector assemblies from three optical scans taken at 385 nm during the salt phase of operation. The incident angle for photons originating from the fiducial volume ($\rho<0.77$) is confined to less than 35 degrees. The average response from all the laser scans is used as the input to the MC simulation. Note that the y axis zero is suppressed.
• Figure 5: PMT time-residual spectrum for a $^{16}$N run taken with the source positioned at the center of the detector. The time residual is calculated without any walk correction to the PMT hit time. Only hit PMTs with time residuals within the 20-ns "prompt window" centered at the "prompt peak" are used to estimate the energy of an event.