Precision Measurement of Neutrino Oscillation Parameters with KamLAND

Abstract

The KamLAND experiment has determined a precise value for the neutrino oscillation parameter \(\Delta m^{2}_{21}\) and stringent constraints on \(\theta_{12}\). The exposure to nuclear reactor anti-neutrinos is increased almost fourfold over previous results to 2.44\(\times10^{32}\) proton-yr due to longer livetime and an enlarged fiducial volume. An undistorted reactor \(\bar{\nu}_{e}\) energy spectrum is now rejected at >5\(\sigma\). Extending the analysis down to the inverse beta decay energy threshold, and incorporating geo-neutrinos, gives a best-fit at \(\Delta m^{2}_{21}\)= \(7.58^{+0.14}_{-0.13}(stat)^{+0.15}_{-0.15}(syst)\times10^{-5}\) eV\(^{2}\) and \(\tan^2 \theta_{12}\)=\(0.56^{+0.10}_{0.07}(stat)^{+0.10}_{-0.06}(syst)\). Local \(\Delta \chi^2\)-minima at higher and lower \(\Delta m^{2}_{21}\) are disfavored at >4\(\sigma\). Combining with solar neutrino data, we obtain \(\Delta m^{2}_{21}\)= \(7.59^{+0.21}_{-0.21}\times10^{-5}\) eV\(^{2}\) and \(\tan^2 \theta_{12}\)=\(0.47^{+0.06}_{-0.05}\).