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

Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. 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 ± 0.14(stat) ± 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 ± 0.26(stat) ± 0.32(syst) ×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. Neutrinos from 8 B β decay (Q ∼ 18 MeV) dominate the high-energy portion of the solar neutrino spectrum and have been a crucial source for solar neutrino experiments. Such experiments, together with reactor antineutrino experiments, have established flavor change through large mixing angle (LMA) neutrino oscillation with matter effects, introduced by Mikheev, Smirnov, and Wolfenstein (MSW) [1,2], as a consistent resolution of the solar neutrino problem [3–10]. Measurements of neutrino-electron scattering using water-Cherenkov techniques have been achieved in a number of An advantage of this detection technique is sensitivity to the neutrino direction of incidence, although analysis thresholds of ∼5 MeV are typically imposed. Recently, SNO has achieved a threshold of ∼3.5 MeV [10]. Liquid scintillator detectors may also be used to measure neutrino-electron elastic scattering; while insensitive to the neutrino direction, these detectors have better energy resolution and have the possibility of lower energy thresholds. A measurement with a threshold of ∼3.0 MeV, the lowest to date, has been reported by Borexino [14]. In this paper we report an independent, liquid-scintillator-based measurement of the 8 B flux with KamLAND. The KamLAND detector consists of 1 kton of liquid scintil-lator (LS) confined in a 6.5-m-radius, 135-μm-thick, spherical balloon. The LS is 80% dodecane, 20% pseudocumene, and 1.36 ± 0.03 g/l of PPO; the density is 0.78 g/cm 3. The balloon is suspended in purified mineral oil within a 9-m-radius …