Measurement of the ${}^8$B solar neutrino flux with the KamLAND liquid scintillator detector

We report a measurement of the neutrino-electron elastic scattering rate from ${}^8$B solar neutrinos based on a 123 kton-day exposure of KamLAND. The background-subtracted electron recoil rate, above a 5.5-MeV analysis threshold is 1.49 $\pm$ 0.14(stat) $\pm$ 0.17(syst) events per kton-day. Interpreted as due to a pure electron flavor flux with a ${}^8$B neutrino spectrum, this corresponds to a spectrum integrated flux of 2.77 $\pm$ 0.26(stat) $\pm$ 0.32(syst) $\times$10${}^6$ cm${}^{-2}$s${}^{-1}$. The analysis threshold is driven by ${}^{208}$Tl present in the liquid scintillator, and the main source of systematic uncertainty is due to background from cosmogenic ${}^{11}$Be. The measured rate is consistent with existing measurements and with standard solar model predictions which include matter-enhanced neutrino oscillation.

Figure 1

Energy spectrum of ${}^8$B candidates with the best-fit spectrum and background components from the unbinned energy and rate analysis. The histograms display the results in bins of 0.5 MeV except for the last bin which due to limited statistics extends from 13.5 to 20 MeV.

Figure 2

Summary of measurements of the ${}^8$B flux using neutrino-electron scattering: this work KamLAND, Kamiokande II [11, 12], Super Kamiokande I [5], Super Kamiokande II [6], Super Kamiokande III [13], SNO D${}_2$O [7, 10], SNO Salt [8, 10], SNO NCD [9], and Borexino [14]. The SNO neutral current measurements [9, 10] are shown for reference with closed triangles.

Figure 3

Ratio of the measured (threshold corrected) solar neutrino-electron elastic-scattering rate and that predicted by the AGSS09-SSM as a function of energy (data points). The average of the water-Cherenkov detector results above 5 MeV is shown. The horizontal error bars indicate the range of energy over which the flux was measured. The central value indicates the flux averaged energy for that range. The solid curve (and $\pm$ 1$\sigma$ band) shows the ratio of AGSS09 with neutrino oscillation included to the prediction without oscillation. The discontinuities in the prediction arise as different neutrino sources become significant, the large uncertainty in the CNO neutrinos is evident at $\sim$1.2 MeV.