Measurement of the [superscript 8]B solar neutrino flux with the KamLAND liquid scintillator detector

Abstract

We report a measurement of the neutrino-electron elastic scattering rate from [superscript 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 ± 0.14(stat) ± 0.17(syst) events per kton-day. Interpreted as due to a pure electron flavor flux with a [superscript 8]B neutrino spectrum, this corresponds to a spectrum integrated flux of 2.77 ± 0.26(stat) ± 0.32(syst) ×10[superscript 6] cm[superscript −2]s[superscript −1]. The analysis threshold is driven by [superscript 208]Tl present in the liquid scintillator, and the main source of systematic uncertainty is due to background from cosmogenic [superscript 11]Be. The measured rate is consistent with existing measurements and with standard solar model predictions which include matter-enhanced neutrino oscillation.